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C0-1

INTRODUCTION TO PROGRAMMING IN C

MAE 4733Mechatronics Design

Gary E. Young

Professor

School of Mechanical and Aerospace Engineering

Oklahoma State University

Stillwater, Oklahoma

G. E. Young 1990 - 1999

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C1-1

Why C?

* A general purpose programming language

* Small and compact

* Powerful operators and control structures

* Natural interface with operating system

* Modular programming

* Low Level operators for real-time applications

* Portable

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C1-2

A First Program

Programming process:

EDIT - - -> COMPILE - - -> LINK - - -> EXECUTE

EDIT -- put program into a text file.

sea.c: (K&P, p. 6)

Note: K&P denotes Kelley and Pohl, A Book on C,4th Edition, Addison Wesley Longman, 1998.

#include

int main(void)

{

printf("from sea to shining C\n");

return 0;

}

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C1-3

Note: Kernighan & Ritchie in their book: The C

Programming Language (Prentice-Hall, 1

st

Ed. 1978,2nd Ed. 1988 (ANSI version), used the string

"hello, world\n" in the printf() function.

You may see references to this string in other

texts.

COMPILE and LINK -- The "cc" command can do both.

cc sea.c

EXECUTE -- Default executable file name is a.out.

Command: a.out

Output: from sea to shining C

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C1-4

Variables, Expressions, and Assignment (K&P, p. 11)

/* The distance of a marathon in kilometers. */

#include

int main(void)

{

int miles, yards;

float kilometers;

miles = 26;yards = 385;

kilometers = 1.609 * (miles + yards / 1760.0);

printf("\nA marathon is %f kilometers.\n\n",

kilometers);

return 0;}

The output of the program is

A marathon is 42.185970 kilometers.

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C1-5

The Use of #define and #include (K&P, p. 13)

The C compiler has a preprocessor built into it.

Lines starting with a # are called preprocessing

directives.

#define LIMIT 100

#define PI 3.14159

After the preprocessor stage and before the compile

stage, the following line

x = 2.0 * PI + LIMIT;

will look to the compiler like

x = 2.0 * 3.14159 + 100;

Capital letters (Caps) are used for defined parameters

to make the program easier to read and debug for

yourself and others.

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C1-6

#include is the same idea as #define only with lines of

code to be substituted.

#include "stdio.h"

will insert the header file (hence the '.h' extension)

stdio.h into this location.

The use of "stdio.h" or controls how the

operating system will look for this file. (You may have

more than one file with this name and you must be

careful to get the correct file.)

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C1-7

The Use of printf() and scanf()(K&P, p. 18)

Both printf() and scanf() are passed arguments which can

be thought of as control_string and other_arguments.

Look at a previous example.

printf("\nA marathon is %f kilometers.\n\n",kilometers);

The control_string must be between double quotes.

Here, the other_argument is the parameter to be printed.

The control_string is like a format statement.

The '%f' directs how the parameter is to be printed.

The '\' in '\n' indicates that the following character(here, an 'n') is a special character.

'\n' is interpreted as a carriage return (start a new

line, left justified).

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C1-8

The following three lines will produce the same output

(K&P, p. 18).

printf("abc");

printf("%s", "abc");

printf("%c%c%c", 'a', 'b', 'c');

printf() conversion characters

c character

d decimal integer

e floating point number in scientific notation

f floating point number

g in e- or -f format, whichever is shorter

s string (array of characters)

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C1-9

The other_arguments in the scanf() function are

addresses of the variables.

This will be confusing at first.

scanf("%d", &x);

will read from the standard input device (keyboard) a

number, interpret it as a decimal integer (%d directive)

and store it in a memory location associated with the

variable 'x'.

We don't know what the address of these variables are,so the '&' is an 'address of' operator and is used to

find this information.

Example:

#include

int main(void)

{

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C1-10

int a,b,c;

printf("%s","Enter two integers:");

scanf("%d%d", &a, &b);

c = a + b;

printf("\n\nThe sum is %d\n",c);

return 0;

}

scanf() conversion characters

c character

d decimal integer

f floating point number (float)

lf floating point number (double)

s string (array of characters)

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C1-11

Flow of Control (K&P, p. 21)

if, if-else, for, and while statements

if(expression)

statement

Example: if(x == 0)sum = 1;

if(expression)

statement1

elsestatement2

Example: if(x == 0)sum = 1;

elsesum = a + b;

while(expression)

statement

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C1-12

Example: while(i != 0)

index = index + 1;

for(expr1;expr2;expr3)

statement

equivalent to expr1;

while(expr2) {

statement

expr3;

}

Example: for(i = 1; i

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C1-13

Functions (K&P, p. 29)

Functions take arguments and return a value.

These arguments and returned value must be declared.

Example: double pow(double x, double y);

The arguments x and y are doubles (long floats)

and the pow function returns a double.

Example: Find the maximum and minimum values of a set of

numbers. (K&P, pp. 30 - 31)

In file maxmin.c

#include

float maximum(float x, float y);

float minimum(float x,float y);

void prn_info(void);

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C1-14

int main(void)

{

int i, n;

float max, min, x;

prn_info();

printf("Input n: ");

scanf("%d", &n);

printf("\nInput %d real numbers: ",n);

scanf("%f", &x);

max = min = x;

for(i=2; i

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C1-15

float maximum(float x, float y)

{

if(x > y)

return x;

else

return y;

}

float minimum(float x, float y)

{

if(x < y)

return x;

elsereturn y;

}

void prn_info(void)

{

printf("\n%s\n%s\n%s\n\n","This program reads an integer value for n, ",

"and then processes n real numbers to find ",

"max and min values.");

}

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C1-16

Non-ANSI C used formal parameters in function

declarations with no argument type checking.

float minimum(),maximum();

.

.

.

float minimum(x,y)

float x,y;

{

if(x

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C1-17

Arrays, Strings, and Pointers (K&P, p. 36)

#include

#define CLASS_SIZE 5

int main(void)

{

int i,j,score[CLASS_SIZE], sum = 0, tmp;

printf("Input %d scores: ",CLASS_SIZE);

for(i=0; i < CLASS_SIZE; ++i) {

scanf("%d",&score[i]);

sum += score[i];}

for(i=0; i < CLASS_SIZE - 1; ++i)

for(j=CLASS_SIZE - 1; j > i; --j)

if(score[j-1] < score[j]) {tmp = score[j-1];

score[j-1] = score[j];

score[j] = tmp;

}

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C1-18

printf("Ordered scores:\n\n");

for(i=0; i < CLASS_SIZE; ++i)

printf(" score[%d] =%5d\n",i,score[i]);

printf("\n%18d%s\n%18.1f%s\n\n",

sum," is the sum of all the scores",

(double) sum/CLASS_SIZE," is the average");

return 0;

}

Strings (K&P, p. 39)

A string is an array of characters.

A string is terminated by the null character '\0'.

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C1-19

Example: nice_day.c in (K&P, p. 39) modified.

#include

#include

#define MAXSTRING 100

int main(void)

{char c, name[MAXSTRING];

int i, sum = 0;

printf("\nHi! What is your name? ");

for(i = 0; (c=getchar()) != '\n'; ++i) {name[i] = c;

sum += 1;

}

name[i] = '\0' /* append '\0' to terminate string */

printf("\n%s%s%s","Nice to meet you ", name, ".");

printf("\nYou have %d letters in your name\n", sum);

return 0;

}

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C1-20

Pointers (K&P, p. 42)

A pointer is an address of an object in memory.

They are used because of their efficiency.

In function calls, a pointer can be used instead of

copying an array of data. Only one address is required!

An array name is a pointer to the first element in the

array by definition.

Example: file abc.c (K&P, p. 43)

#include

#include

#define MAXSTRING 100

int main(void)

{

char c = 'a', *p, s[MAXSTRING];

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C1-21

p = &c;

printf("%c%c%c ", *p, *p + 1, *p + 2);

strcpy(s,"ABC");

printf("%s %c%c%s\n", s, *s + 6, *s + 7, s+1);

strcpy(s, "she sells sea shells by the seashore");

p = s + 14;

for(; *p != '\0'; ++p) {

if(*p == 'e')*p = 'E';

if(*p == ' ')

*p = '\n';

}

printf("%s\n", s);return 0;

}

The output of this program is

abc ABC GHBCshe sells sea shElls

by

thE

sEashorE

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C1-22

Files (K&P, p. 47)

scanf() and printf() read and write to the standard

input/output devices (keyboard and monitor screen).

To read from and write to other devices use file pointers.

Example:

#include

int main(void)

{FILE *ifp, *ofp; /* FILE is defined in stdio.h */

/* ifp is an input file pointer */

/* ofp is an output file pointer */

. . .

ifp = fopen("in_file.dat", "r"); /* open for reading */ofp = fopen("out_file.dat", "w"); /* open for writing */

. . .

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C1-23

while(fscanf(ifp, "%d", &a) == 1)

sum += a;

fprintf(ofp, "The sum is %d.\n", sum);

. . .

while((c = getc(ifp)) != EOF) {do something}

. . . /* EOF stands for End of File */

/* Where might this be defined? */

fclose(ifp); /* Be sure to close your files */

fclose(ofp); /* before exiting your program. */

Three modes in which to open a file include the following.

"r" open text file for reading

"w" open text file for writing"a" open text file for appending

For a more complete list, see (K&P, p. 506).

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C1-24

* Given a program in a file called x.c:

how do you compile it?

how do you run it?

* What are the essential parts of a C program?

* How would you declare i to be an integer and set

it to the value 10 all in one statement?

* What is wrong with the following statements?

printf("hello/n");

b = a(3);

int i, c[10];

for(i=0; i

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C2-1

Keywords

auto do goto signed unsigned

break double if sizeof void

case else int static volatile

char enum long struct while

const extern register switch

continue float return typedef

default for short union

Some implementations may have additional keywords,

e.g., asm for in-line assembly language code.

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C2-2

Identifiers (K&P, p. 78)

Identifiers are made up of letters (first character is amust), numbers, and the underscore character.

No special characters are acceptable in the identifier.

Different compilers and systems will have differentconstraints on the number of recognizable characters.

Examples of acceptable identifiers:

k, _id, thisisvariable1, this_is_variable_1

Variables such as _sin and _id are usually used forsystem variables and should be avoided unless you arewriting a compiler or operating system.

Examples of non-acceptable identifiers:

not#me, 101_south, -plus, $$2u

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C2-3

Increment and Decrement Operators

(++ and --)

++ means "increment": adds 1 to a variable.

-- means "decrement": subtracts 1 from a variable.

"++i" increments i, THEN uses i's NEW value

"i++" uses i's OLD value, THEN increments i

preincrement and postincrement operators

--i; predecrement; i--; postdecrement

So now you can explain the origin of 'C++'.

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C2-4

What's the difference?

n = 3; n = 3;vs.

x = n++; x = ++n;

Example: (K&P, p.85)

int a,b,c=0;

a = ++c;

b = c++;

printf("%d %d %d\n",a,b,++c);

/* What gets printed? *//* Could it be 1 1 3? */

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C2-5

++i; and i++; are equivalent to i = i + 1;

but increment operators produce faster code.

Assignment Operators

=

+= -= *= /= %=

>>=

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C2-6

* What happens in the following piece of code?

while(c = ' ' || c == '\t' || c == '\n')

c = getc(fp);

/* getc(fp) gets next character from file */

/* pointed to by file pointer fp */

* while(1); /* same as for(;;); */

* What happens here?

if(x[i] > big);big = x[i];

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C2-7

* What gets printed on your system?

void main(void)

{

char c;

printf("sizeof(c) = %d\n",sizeof(c));

printf("sizeof('a') = %d\n",sizeof('a'));

printf("sizeof(c='a') = %d\n",sizeof(c='a');

} /* sizeof(x) returns size of 'x' in bytes */

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C3-1

Fundamental Data Types

char signed char unsigned char

short int long

unsigned short unsigned unsigned long

float double long double

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C3-2

Characters

* "char" is an integer data type.

* char values may be interpreted as integer numbers.

* char values may be interpreted as ASCII codes

(See Appendix D of K&P, pp. 703 - 704)

char c = 'a';

printf("%c",c); /* a is printed */

printf("%d",c); /* 97 is printed */

printf("%c%c%c", c, c+1, c+2); /* abc is printed*/

What is the result of:

char x;

int y;

x = 'r' + 1;

y = x + 1;

printf("x = %d\nx = %c\nx = %x\n

y = %d\n",x,x,x,y);

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C3-3

The escape character is used to represent special

characters. (See K&P, p. 113)

escape character is a backslash: \

What does this do:

printf("\n\n\t\007You goofed!!");

Hint: Octal numbers start with the escape

sequence \0. /* '0' not capital 'o' */

Hexadecimal numbers start with theescape sequence \x or \X.

Decimal numbers do not start with an

escape sequence.

To clear the screen on an ANSI terminal:

write(1, "\033[2J", 4);

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C3-4

* In addition to the provided data types, the Clanguage allows explicit association of a data type

with an identifier.

* This mechanism is called the typedef.

* Used most often when defining structures.

(K&P, Chapter 9).

Examples:

typedef char * string;

typedef int INCHES, FEET, YARDS;

typedef float vector[10];

INCHES length, width;

string s1 = "abc", s2 = "xyz";

vector x;

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C3-5

Mathematical Functions

Functions such as

sqrt() pow() exp() log() sin() cos() tan()

appear in a special math library.

To link to the math library when compiling:

cc program.c -lm (Unix)

Standard definitions are in themath.h header file.

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C3-6

Conversions and Casts

Some conversions are performed automatically

(See K&P, p. 132)

But sometimes conversions must be coerced by using

casts.

A cast takes the form: ()

Example:

double d, sqrt(double);

int i;

i = 4;

d = sqrt( (double) i);

Note: Due to associativity (float)i + 3 is

equivalent to ((float)i) + 3

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C3-7

Examples of Constants

int: 0 77 5013

long: 0 77L 5013L

double: 0.003 1.0 0.5013e-2

char: 'a' 'b' 'c'

octal: 00 0115 '\033'

hexadecimal: 0x0 0x4d '\xffff'

string: "this is a string constant"

* Note: 2l and 21 are not the same!

* Type suffixes are acceptable in either upper or

lower case. Be careful! In this example 2l can

be represented more clearly as 2L.

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C3-8

Hexadecimal and Octal Constants

Decimal Binary Hexadecimal Octal

0 00000000 00 000

1 00000001 01 001

2 00000010 02 002

3 00000011 03 003

...

31 00011111 1f 037

32 00100000 20 040

...

188 10111100 bc 274

...

254 11111110 fe 376255 11111111 ff 377

Conversion Examples:

0x123 = 1 x 162 + 2 x 161 + 3 x 160 = 291 (decimal)

0xabc = 10 x 162 + 11 x 161 + 12 x 160 = 2748 (decimal)

00164 = 1 x 82 + 6 * 81 + 4 x 80 = 116 (decimal)

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C3-9

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C4-1

Flow of Control

Relational, Equality, and Logical Operators

relational operators: less than: less than or equal: =

equality operators: equal: ==

not equal: !=

logical operators: (unary) negation: !logical and: &&logical or: ||

Logical values: "true" = non-zero"false" = 0

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C4-2

The Equality Operator

Example:

if (x == y)a = x + 500;elsea = 0;

What does this do?

y = 0;if (x = y)a = x + 500;elsea = 0;

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C4-3

The Compound Statement

Any group of statements (possibly includingdeclarations) enclosed in braces.

Example:if (a == b)

{x = 4;

}else{

x = 10;printf("a doesn't equal b");}

The Empty Statement

The empty statement is just a ';' and is used when wedon't want to do anything until something happens.

Example: while(x < y);

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C4-4

The if and the if-else Statements

if(expression)statement

Example: if(grade >= 90)

printf("Congratulations!\n");printf("Your grade is %d.\n", grade);

if(expression)

statement1elsestatement2

Example: if(c >= 'a' && c

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C4-5

while and for loops

while for

;while() { for(;;)

;

}

Examples:

sum = 0;i = 0; sum = 0;while(i < LIMIT) { for(i = 0; i < LIMIT; ++i)

sum += a[i]; sum += a[i];++i;

}

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C4-6

The Comma Operator

The comma operator in ,

* separates the two expressions

* guarantees that gets evaluatedbefore

* returns the value of and

* occurs most often in for loops

Example:

for(sum = 0, i = 0; i

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C4-7

The do Statement

The do statement is a variant of the while statement.

do

while();;

Example: (K&P, p. 173)

do {printf("Input a positive integer: ");scanf("%d",&n);if(error = (n

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C4-8

The break and continue Statements

Examples: (K&P, p. 180)

while (1) {

scanf("%lf", &x);if (x < 0.0)break; /* exit loop if x is negative */

}/* break jumps to here */

for (i = 0; i < TOTAL; ++i) {c = getchar();if (c >= '0' && c

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C4-9

The switch Statement


switch (c) {case 'a':

++a_cnt;

break;case 'b':case 'B':

++b_cnt;break;

default:

++other_cnt;}

/* expression in parentheses following thekeyword switch must be of integral type */

/* default is optional but should be used */

/* without break statement execution fallsthrough to next statement */

Th C diti l O t

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C4-10

The Conditional Operator

? :

expr_1 is evaluated first.

If it is true (nonzero) the value of expr_2 isthe value of the conditional expression as awhole.

If it is false (zero) the value of expr_3 isthe value of the conditional expression as awhole.

Examples:

x = (y < z)? y: z;

What is the result of the following line of code?

printf("%c",('A'

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C4-11

What does this print?

for (i = 0; i < 10; ++i)printf("\n%d",i);

What does this print?

x = 1;while (x < 6)

printf("\nx = %d",x);x += 1;

printf("\nx = %d",x);

What gets printed by the following code fragment ifcolor is equal to 2?

switch(color) {case 1: printf("red");case 2: printf("yellow");case 3: printf("blue");

}

F nctions

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C5-1

Functions

Earlier example: (K&P, p. 31)

int main(void)

{ ..

.max = maximum(max,x);

min = minimum(min,x);

.

.

.}

float maximum(float x, float y)

{

if(x > y)

return x;else

return y;

}

* In function definition the function name must be

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C5-2

* In function definition, the function name must be

preceded by a type if it returns a non-int value.

char firstchar(char *s)

* The returned value will be converted to this type

upon exit of the function.

float f(char a, char b, char c)

{

int i;

. . .

return (i); /* returned value converted */

}

* "return" need not return a value, i.e., return();

or return;. Function should be declared as void.

* "return" need not be present.

* Traditional function syntax should still compile

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C5-3

* Traditional function syntax should still compile

with an ANSI C compiler.

float minimum(x, y)

float x,y; /* These are formal parameters */

{

if(x < y)

return x;else

return y;

}

Note: This is different than

float minimum(x, y)

{

float x,y; /* These are not formal parameters */

if(x < y)

return x;else

return y;

}

Function Prototypes

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C5-4

Function Prototypes

Functions must be declared before they are used.

Function prototypes tell the compiler the number and

type of arguments that are to be passed and the type of

value to be returned.

Example:

#include

double sqrt(double);

Most times these function prototype declarations are

made in a header file.

#include

#include /* includes many more lines */

/* but it's easier this way */

Note:void f(char, int) is equivalent to void f(char c, int i)

Storage Classes

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C5-5

Storage Classes

* Function arguments are local variables.

* Any variables declared in the body of a function

are also local.

* By default, local variables have the storageclass automatic.

* Variables which are defined outside of any

function have the storage class external.

* Normally, external variables are available to all

functions.

(Exception: static external variables.)

A Summary of Storage Classes in C

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C5-6

A Summary of Storage Classes in C

* automatic

- local to a block

- discarded when the block is left

- Unless otherwise specified, variables declared

within a function or block are "automatic."

(Includes formal function arguments.)

* static

- retain their values throughout the execution of a

program.

- must be declared specifically with the "static"attribute.

- if declared inside a block or function they are

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C5-7

y

accessible only within the block or function in

which they are declared.

- When a block is exited then later reentered, they

can be reused without reinitializing them.

- When declared outside of any function they areexternal in the sense that they are accessible

within the source file in which they are declared,

but nowhere else.

* external

- are defined outside any function

- retain their values

- are accessible to many functions

- if declared static they are accessible only within

the file in which they are declared.

- other external variables are accessible to

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C5-8

functions in other files only if declared extern

within those other files

* register

- are automatic variables whose values are stored in

fast registers

- usually used for a small number of integer indices

* typedef

- not really a storage class

- lets you define identifiers which can be used

later as if they were data type keywords

- Example:

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C5-9

typedef struct {char name[20];

short age;

char employee_number[8];

long gross_pay;

char address[30];} PERSON_REC;

Henceforth, "PERSON_REC" can be treated as if it

were a data type, as in the declaration

PERSON_REC emp_rec;

Scope Rules

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C5-10

* A formal parameter is accessible only within thefunction in which it is declared.

* A variable declared within a block is accessible in

the block in which it is declared and in any blocks

which are nested within the block.

* If a block contains a declaration of a variable with

the same name as that of a variable declared in a

surrounding block, the variable declared in the outer

block is not accessible until the inner block is

left.

* If a function has an "extern" declaration of a

variable, there must be a corresponding external

"defining" declaration somewhere among the libraries

and files constituting the complete program.

The defining declaration does not contain the keyword

"extern".

Examples:

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C5-11

(K&P, p. 214)

{

int a = 2; /* outer block a */

printf("%d\n", a); /* 2 is printed */

{int a = 5;

printf("%d\n", a); /* 5 is printed */

}

printf("%d\n", ++a); /* 3 is printed */

}

(K&P, p. 218)

/* In file file1.c */

#include

int a = 1, b = 2, c = 3; /* external variables */

int f(void);

int main(void)

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C5-12

{

printf("%3d\n", f());printf("%3d%3d%3d\n", a, b, c);

return 0;

}

/* In file file2.c */

int f(void)

{

extern int a; /* Look for it elsewhere */

int b,c; /* Local variables */

a = b = c = 4;

return(a+b+c);

}

Scope Rules for Functions

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C5-13

* All C functions have storage class external.

This means that, generally speaking, every function

is accessible from anywhere in a program.

Exception: functions declared "static."

* Static functions can only be accessed from within the

file in which they are defined.

* All programs need function main().

Execution always begins with main().

All other functions are subordinate, and are called

directly or indirectly from main().

Example:

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C5-14

static int g(int x){

/* do something */

}

void f(void){

/* do something else */

b = g(a);

}

f() will call g() located in the same file

since g() is declared static.

Other g()'s could exist in other files and

would not be confused with this g().

Example of static Attribute:

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C5-15

void stfun(void)void main(void)

{

int i, x;

for (i=0; i < 10; ++i)

x = stfun();}

void stfun(void)

{ static int x = 0; /* x is static: it will retainits value from call to call.

x gets initialized only once. */

int y = 0; /* y is automatic: it will loseits value from call to call.

y gets re-initialized with every

call. */

x += 10;y += 10;

printf("x=%4d\ty=%4d\n", x, y);

}

Example of Scope Rules Across Files:

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C5-16

myprog.c: other.c:

int a = 2; void osub(int f)

static int b = 3; {

extern int a;void main(void) printf("%d\n",a + f);

{ }

int c = a + b;

msub(c);

}

void msub(int d)

{

int e = 7 * d;

osub(e);

}

int z; /* What gets printed *//* by this code fragment? */

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C5-17

/ by this code fragment? /

void f(int x){ x = 2;z += x;

}

void main(void){ z = 5;

f(z);printf("z = %d\n",z);

}

Arrays, Pointers, and Strings

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C6-1

Arrays

* C has only one-dimensional arrays, the size of

which must be fixed as a constant at the time of

compilation. Indexing of elements starts at 0.

* An element of an array may be an object of any

type including another array. This is how multi-

dimensional arrays are created.

* Only two things can be done to an array.

1. Determine its size.

2. Obtain a pointer to element 0 of the array.

All other operations are done with pointers.


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C6-2

#define N 100int a[N]; /* space for a[0] ... a[99] */

Initialization

Examples: (K&P, pp. 246 - 247)

float f[5] = {0.0, 1.0, 2.0, 3.0, 4.0};

int a[100] = {0}; /* initializes all elements to 0 */

The compiler can figure out the size of an array if itis initialized when it is defined.

The following declarations are equivalent.

int a[] = {2,3,5,-7}; and int a[4] = {2,3,5,-7};

char s[] = {"abc"}; and char s[] = {'a','b','c','\0'};

Multidimensional Arrays (K&P, pp. 277 - 282)

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C6-3

int a[100] one-dimensional array

int b[2][7] two-dimensional array

int c[5][3][2] three-dimensional array

Example:

int calendar[12][31]; /* calendar is an array of */

int *p; /* 12 arrays of 31 integer */

int i, j; /* elements each. */

Pointers

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C6-4

* A pointer points to a location in memory.

* Pointers are used to access memory locations and

manipulate addresses.

* Vital for coding real-time applications which

have fixed addresses for accessing the outside

world.

* When variables are passed as arguments to a

function, their values are copied to the

corresponding function parameters. This is termed

"call by value".

* To change the values of the variables directly,

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C6-5

pointers are used to provide a "call by reference"

mechanism.

Examples:

int a, c; /* a and c can hold integers */

int *b; /* b points to an integer variable */

a = 3;

b = &a; /* Assign b the address of a */

c = *b; /* Assign c the contents of the */

/* address pointed to by b */

Note: & is the address operator.

* is the dereference operator.

Example: (K&P, p. 252):

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C6-6

#include

void swap(int *, int *);

void main(void)

{

int i = 3, j = 5;

swap(&i, &j);

printf("%d %d\n",i,j); /* 5 3 is printed */

return 0;

}

void swap(int *p, int *q)

{

int tmp;

tmp = *p;*p = *q;

*q = tmp;

}

Array Names as Pointers

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C6-7

char str[20]; /* (1) allocates 20 bytes for *//* a char array and (2) makes */

/* str a pointer to the first */

/* element of the array. */

x = str[0]; /* stores the first element in x */

x = *str; /* stores the first element in x */

if( *str == 32 ) /* tests if the first element */

/*of the array is a space. */

if( *(str + 6) == 'A') /* tests if the 7th element */

{ do something } /* of the array is an 'A'. */

* Note: str is a pointer to the first element of

the array str[]


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C6-8

double a[2], *p, *q;

p = a; /* points to base of array */

q = p + 1; /* equivalent to q = &a[1] */

printf("%d\n", q - p); /* 1 is printed */

printf("%d\n", (int)q - (int)p); /* 8 is printed */

On most computers, a double is stored in 8 bytes.

p points to a double and q points to the next double.

The difference between p and q in terms of arrayelements is 1.

The difference between p and q in terms of memory

locations is 8.

Pointer arithmetic is not equal to memory arithmetic.

The compiler keeps track of this for us.

Strings

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C6-9

* A string is just an array of characters.

* A string is terminated with the NULL character '\0'.

Examples:

char r[100]; /* space for 100 char including '\0' */

char s[] = "abc"; is equivalent to

char s[] = {'a', 'b', 'c', '\0'};


char *p = "abc";

printf("%s %s\n",p, p+1); /* abc bc is printed */

Note: We could have used char s[] = "abc"; and 's,s+1'

Some String Functions

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C6-10

(See K&P, Appendix A.14, p. 670 - 675)

char b[50];

strcpy(b, "blue"); /* string copy */

if(strcmp(b, "green") == 0) /* string compare */

printf("It is green!\n");

strcat(b, " moon"); /* concatenation */

printf("b = %s\n",b); /* prints blue moon */

Example:

strcopy(char *t, char *s) /* copying a string */{

while( *t++ = *s++ );

}

Example:

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C6-11

unsigned strlen(const char *s){

register int n;

for(n = 0; *s != '\0'; ++s)

++n; /* finding the string length */

return n;

}

Arguments to main()

* main() can have two special arguments called

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C6-12

main() can have two special arguments called

argc: argument count and

argv: array of character strings which contain

the arguments.


/* Echoing the command line arguments in file my_echo */

#include

int main(int argc, char *argv[]){

int i;printf("argc = %d\n",argc);

for(i = 0; i < argc; ++i)printf("argv[%d] = %s\n",i, argv[i]);return 0;

}

* If the following command line is given

my echo a is for apple

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C6-13

my_echo a is for apple

the following is printed on the screen

argc = 5

argv[0] = my_echo

argv[1] = a

argv[2] = is

argv[3] = for

argv[4] = apple

What is wrong with the following pieces of code?

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C6-14

int i, j, a[10];

for(i = 1; i

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C7-1

logical operators:

(unary) bitwise complement: ~

bitwise and: &

bitwise inclusive or: |

bitwise exclusive or: ^

shift operators:

left shift: >

Bitwise Complement

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C7-2

The int a = 3243 (decimal) = 110010101011 (binary)

~a is the bitwise complement of a

~a = 001101010100 (binary)

Note: a + ~a = 111111111111 (binary), i.e., all 1's

Two's Complement

The two's complement of an integer is obtained by

taking the bitwise complement of its binaryrepresentation and adding 1 to it.

This is how to do subtraction (a + (-b))

Note: For a given word length, -1 is represented byall 1's. The carry bit is lost when 1 is added, so

the result is 0.

Bitwise Binary Logical Operators

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C7-3

Values of:

a b a & b a b a | b

0 0 0 0 0

1 0 0 1 1

0 1 0 1 1

1 1 1 0 1

Masks

i i i i i i

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C7-4

Use bitwise operators to mask individual bits.

Example:

0x8 is a mask for bit 3 (4th bit)

bit 7 6 5 4 3 2 1 0

0 1 0 1 1 0 1 1 (a number)

&

0 0 0 0 1 0 0 0 (mask)

0 0 0 0 1 0 0 0 (result)

Example:

#d fi MASK 0 8

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C7-5

#define MASK 0x8if(x & MASK)

{

/* do something if bit 3 (4th bit) is a '1'*/

}


For some int v, the following expression has the value

1 or 0 depending on the value of bit 2(3rd bit)

(v & (1

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C7-6

#include

void bit_print(int a)

{

int i,

int n = sizeof(int) * CHAR_BIT; /* in limits.h */

mask = 1

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C7-7

in declaring identifiers as elements of the set.


enum day {sun, mon, tue, wed, thu, fri, sat};

enum day d1, d2;

.

.

.

if( d1 == d2) /* do something */

/* By default, sun = 0, mon = 1, ..., sat = 6 */

/* These variables could have been explicitly

initialized using other integers */

enum day {sun = 0,mon,tue,wed = 5,thu,fri,sat = 9}

/* Note: mon = 1, tue = 2, thu = 6, fri = 7 */

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C7-8


enum suit {clubs = 1, diamonds, hearts, spades} a,b,c;

Note that diamonds = 2, hearts = 3, and spades = 4


/* Compute the next day with a cast */

enum day {sun, mon, tue, wed, thu, fri, sat};

typedef enum day day;

day find_next_day(day d)

{day next_day

return ((day)(((int)d + 1) % 7));

}

The Preprocessor

* The general sequence of events that takes place

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C8-1

during programming is

edit ---> compile ---> link ---> execute

* Preprocessing occurs between edit and compile.

* The preprocessor does not detect syntax errors.

e.g., the following substitution will be made

without complaint.

#define FORMULA (al+a2]*a3

* The symbol '#' usually must be in column 1.

The Use of #include

* Facilitates including code from other files.

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C8-2

* Allows common declarations and definitions to be

kept in separate files.

* These can then be used in many programs and by many

programmers.

* #include files can be nested.

* They commonly have the extension ".h", as in

"stdio.h". These are called header files.

Example:

What the preprocessor sees:

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C8-3

myprog.c defs.h

#include "defs.h" typedef struct {

char name[30];void main(void) int salary;

{ } EMP_REC;

EMP_REC employee;

char buffer[BUFFSIZE+1]; #define mod(x,y) \

. (x)%(y)

. #define BUFFSIZE 1000

.a = mod(a,c);

.

..}

What the preprocessor sees after the #include:

mprog.c

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C8-4

typedef struct {

char name[30];

int salary;

} EMP_REC;

#define mod(x,y) (x)%(y)

#define BUFFSIZE 1000

void main(void)

{EMP_REC employee;

char buffer[BUFFSIZE+1];

. . .a = mod(a,c);

. . .}

What the compiler sees (after preprocessing):

mprog.c

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C8-5

typedef struct {

char name[30];

int salary;

} EMP_REC;

void main(void)

{

EMP_REC employee;

char buffer[1000+1];

.

.

.a = (a)%(c);

.

..}

The Use of #define

* In effect, #define gives a name to a constant.

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C8-6

In effect, #define gives a name to a constant.

* It makes "magic numbers" easier to see.

* It can be used to change a value utilized many places

in your code by just changing one #define

statement.

Examples:

#define SECTORSIZE 512

#define BUFFERSIZE 3*SECTORSIZE

#define MESSAGE "error no. %d\n"

#define READWRITE 2

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C8-7

#define BASE_ADDRESS 0x2e

#define COMM_REG BASE_ADDRESS + 1

void main(void)

{

char filebuf[BUFFERSIZE + 1];

...printf(MESSAGE,n);

...open(filename, READWRITE);

...

}

Macros and Macro Arguments

* If a left parenthesis immediately follows the name,

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C8-8

p y ,

it is assumed to contain a list of "formal

arguments".

* If the macro definition includes formal arguments

the actual arguments are substituted for the

formal arguments.

* May look like a way to implement functions, but

remember that Macros are not functions!

Examples:

#define mod(x,y) (x) % (y) /* x and y are "arguments" */

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C8-9

...a = 17 + mod(42,max); /* expands to */

/*a = 17 + (42) % (max); */

(K&P, p. 368)

#define SQ(x) ((x) * (x))

...SQ(7 + w); /* expands to */

/* ((7 + w) * (7 + w)); */

* Why the extra parentheses?

/* Define using no parentheses. */

#define mod(x,y) x % y

...a = mod(42,max+1);

/* expands to a = 42 % max + 1;

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C8-10

* Since'%' has precedence over '+',the result is

* a = (42 % max) + 1;

*/

Similarly (K&P, p. 368),

#define SQ(x) x * x

SQ(a + b);

/* expands to a + b * a + b which because of

* precedence is not (a+b)2

*/

Conditional Compilation

#undef

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C8-11

* Makes the preprocessor forget the definition of

.

* Allows one to redefine , or to not have it

conflict with some other use of .

* Some compilers let you redefine a macro without first

using #undef.

* The conditional compilation commands allow lines

of source code to be included or not included

in a compilation.

#if...#else...#endif

* #if...#else...#endif tests whether some constant

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C8-12

integral expression is true (non-zero) or false

(zero).

#ifdef and #ifndef

* #ifdef and #ifndef tests whether or not some constant

or macro has been defined or not.

Example:

#define DEBUG 1 /* could also omit the '1' */

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C8-13

.

.

.#if DEBUG /*must be a "constant expression" */

printf(" %d iterations.\n", count++);

if (count > 100) return;

#else

if (count++ > 100) return;

#endif

Example:

#define SUN_4 1

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C8-14

.

.#ifdef SUN_4

{SUN_4-dependent code}

#endif#ifdef VAX

{VAX-dependent code}

#endif

#ifdef IBM_PC

{IBM_PC-dependent code}#endif

#ifndef MAXLENGTH

#define MAXLENGTH 100

#endif

#line

* This preprocessing directive causes the compiler

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C8-15

* This preprocessing directive causes the compiler

to renumber the source text so that the next

line is associated with the specified number.

Example:

/*Suppose the following line is number 25 */

#define BIGMAC lots and lots of stuff that takes \

up, oh, maybe eight or nine lines \

Since these lines are treated as \

one by the compiler,the compiler's\

error messages will all be off by \

9 lines which makes it really \

hard to connect error messages \

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C8-16

hard to connect error messages \

with the lines on which they \

actually occurred.

#line 35

b = min(x,y); /* This should be line 35 */

.

.

.

"I didn't mean THAT!"

* What is incorrect or dangerous about each of the

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C8-17

What is incorrect or dangerous about each of the

following code fragments?

#define MAX = 5

#define MAX 5;

#define GETHEAD #include

* ANSI C preprocessors are supposed to be smart

enough to not expand the following macro until

an 'out of memory core dump' message appears

on the console. How does your preprocessor

expand this macro?

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C8-18

expand this macro?

#define forever(x) x forever(x)

* Consider the following code fragment. What is the

value of b after execution of the following

statements?

#define SQUARE(x) (x) * (x)

a = 2;

b = SQUARE(a++);

* It is good to be reminded that macros are not

functions no matter how tempting it may be.

What is the effect of the following code?

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C8-19

What is the effect of the following code?

#define N 3

#define max(a,b) ((a)>(b)? (a): (b))

void main(void)

{ int i, biggest, x[N] = {2, 3, 1};

biggest = x[0];

i = 1;

while(i < N)

biggest = max(biggest, x[i++]);

printf("biggest = %d\n", biggest);

} /* Could it be possible that biggest = 1? */

Structures and Unions

Structures

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C9-1

* Provides a mechanism to aggregate variables of

different types.

* Although not required, it is good practice to

associate a tag name with the structure type.

* Can be used with typedef and placed in header files

to facilitate modularity and portability.

Example: (K&P, pp. 407 - 408)

Set up a structure to characterize playing cards.

struct card {

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C9-2

int pips;

char suit;

}

struct card c1, c2;

One could have done this at the 'same' time.

struct card {

int pips;

char suit;

} c1, c2;


Define a complex number structure using typedef.

typedef struct {

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C9-3

float re;

float im;

} complex;

complex a, b, c[100];

Accessing Members of a Structure

* Member access operators ( '.' and '->') are used

to access the individual members of a struct.

Example: (K&P, p. 411 - 412)

/* In file class_info.h */


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C9-4

struct student {

char *last_name;

int student_id;

char grade;

};

/* In 'main' program */

#include "class_info.h"

int main(void) {

struct student tmp, class[CLASS_SIZE];

. . .

tmp.grade = 'A';

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C9-5

tmp.last_name = "Casanova";

tmp.student_id = 910017; . . .

/* Function to count the number of failing grades */

int fail(struct student class[])

{ int i, cnt = 0;

for(i = 0; i < CLASS_SIZE; ++i)

cnt += class[i].grade == 'F';

return cnt;}

* C provides the member access operator '->' to access

the members of a structure via a pointer.

Example: (K&P, p. 413 - 414)

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C9-6

/* In file complex.h */

struct complex {

double re; /* real part */

double im; /* imaginary part */

};

typedef struct complex complex;

/* complex and complex in different name spaces */

/* better to use typedef directly */

/* In 'main' source file */

#include "complex.h"

void add(complex *a, complex *b, complex *c)

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C9-7

{

a -> re = b -> re + c -> re;

a -> im = b -> im + c -> im;

}

/* pointer_to_struct -> member_name

is equivalent to

(*pointer_to_struct).member_name */

Using Structures with Functions

* In ANSI C, structures can be passed as arguments

t f ti d b t d f th

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C9-8

to functions and can be returned from them.

* When a structure is passed as an argument to a

function, it is passed by value.

* Due to speed considerations, it may be

advantageous to pass pointers.

* A struct can have as one or more of its members

another struct.

Example: (K&P, pp. 416 - 417)

struct dept {

char dept_name[25];

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C9-9

int dept_no;

}

typedef struct {

char name[25];

int employee_id;

struct dept department;

struct home_address *a_ptr;

double salary;

...

} employee_data;

/* A function to update employee data */

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C9-10

employee_data update(employee_data e)

{ ...

printf("Input the department number: ");

scanf("%d", &n);

e.department.dept_no = n;

...

return e;

}

* Call by value requires that a copy of the data be

supplied to the function. This could take some

time if large structures are being passed so a

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C9-11

second approach using pointers follows.

void update(employee_data *p)

{ ...

printf("Input the department number: ");

scanf("%d", &n);

p->department.dept_no = n;

...}

* In main() or some other function one could write

for the first case

employee data e;

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C9-12

employee_data e;

e = update(e);

and in the second case

employee_data e;

update(&e);

* Note that in the first case

e.department.dept_no = (e.department).dept_no

and in the second case

p->department.dept_no = (p->department).dept_no

Unions

* Same as a structure except members share memory

storage.

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C9-13

* Size of union is size of largest member.

* Since data overlaps, the system will interpret the

same stored bit values according to which member

component is selected.

Example: (K&P, pp. 424 - 425)

typedef union int_or_float {

int i;

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C9-14

float f;

} number;

int main(void)

{

number n;

n.i = 4444;

printf("i: %10d f: %16.10e\n", n.i, n.f);

n.f = 4444.0;

printf("i: %10d f: %16.10e\n", n.i, n.f);

}

/* The output of this program is system dependent *//* One example follows */

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C9-15

/ p /

i: 4444 f: 0.6227370375e-41

i: 1166729216 f: 4.4440000000e+03

* What is the result of the following program?

#if(VAX || PC)

#define HEX0 0x6c6c6548

#define HEX1 0x77202c6f

#define HEX2 0x646c726f

#define HEX3 0x00000a21

#else

#define HEX0 0x48656c6c

#define HEX1 0x6f2c2077

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C9-16

#define HEX2 0x6f726c64

#define HEX3 0x210a0000

#endif

typedef union {

char what[16];

long cipher[4];

} mystery;

void main(void)

{

mystery x;

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C9-17

x.cipher[0] = HEX0;

x.cipher[1] = HEX1;

x.cipher[2] = HEX2;

x.cipher[3] = HEX3;

printf("%s", x.what);

}

* Given the following struct definition it appears

that the string defined as the first member is

of unknown size. Will the compiler complain?

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C9-18

If not, resolve this paradox.


struct student {

char *last_name;

int student_id;

char grade;

}

Input / Output

* read from and write to the standard I/O devices.

printf("control string", args to be printed);

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C11-1

scanf("control string", ptrs to args to be read);

* read from and write to general files or devices.

fprintf(file_ptr, "control string", arguments);

fscanf(file_ptr, "control string", ptrs to args);

* Three standard file pointers , stdin, stdout, and

stderr, are defined in stdio.h.


#include

int main(void){ int a, sum = 0; /* FILE defined in stdio.h */

ILE if f / S /

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C11-2

FILE *ifp, *ofp; /* See K&P, pp. 653 - 662 */

ifp = fopen("input.dat", "r");

ofp = fopen("output.dat", "w");

while(fscanf(ifp,"%d", &a) == 1)sum += a;

fprintf(ofp,"The sum is %d.\n", sum);

fclose(ifp);fclose(ofp);

return 0}

* fprintf(stdout,...); and printf(...); are equivalent.

* fscanf(stdin,...); and scanf(...); are equivalent.

* td i t t th l d fi d

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C11-3

* stderr points to the screen unless redefined.

Read/Write Mode Meaning

"r" open for reading

"w" open for writing

"a" open for appending

"r+" reading and writing

"w+" writing and reading

Some Conversion Characters

c char

d,i decimal int

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C11-4

u unsigned decimal int

o unsigned octal int

x,X unsigned hex int

e,E scientific notation (7.123e+00 or 7.123E+00)

f fixed floating point (7.123)

g,G e,E or f format, whichever is shorter

s string

MAE 4733 Notes1

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