Lexical Elements and Operators

Senem Kumova Metin 1

Lexical Elements and Operators


C like other languages, has an alphabet and rules for putting together words and punctuation to make correct, or legal programs

These rules are the syntax of the language and the compiler checks on the program syntax correctness


C Alphabet

lowercase letters : a b c ... z

uppercase letters : A B C ... Z

digits : 0 1 2 3 4 5 6 7 8 9

other characters : + * / = ( ) { } [ ] < > ' " ! # % & _ | ^ ~ \ . , ; : ?

white space characters : blank, newline \n, tab \t, etc.


The compiler firstly collects the characters of the program into tokens, which can be thought of as the basic vocabulary of the language (tokens are generated by lexical analyzer)

Next compiler checks syntax rules using

syntax analyzer according to Backus-Naur (BNF) Form (a kind of context-free grammar describing all valid constructions of the language)


6 kinds of Tokens in ANSI C

1. keywords 2. identifiers 3. constants 4. string constants 5. punctuators6. operators


1. Keywords

A keyword, also called a reserved word, has a strict meaning. There are 32 keywords in C They cannot be redefined!


2. Identifier (1/2)

A token that is composed of a sequence of letters, digits and the special character “_”

The programmer uses them chiefly to name variables and functions

They begin with a letter or underscore and are chosen to be meaningful to the human reader


C is case sensitive !!! int CS115; int Cs115; int cS115 are all

different !

float x, _id; char so_am_I; int cs115/* These are all valid */

int not#me; char c&&d; /* special characters not allowed */

char 123th; /* identifiers can not start with a digit

*/

2. Identifier (2/2)


3. Constantsinteger constants (e.g., 12, 790, however -32, +14 are considered constant integer expressions, and not integer constants)

int myint= 7; int yourint= 6;

floating constants (e.g., 1.0, 3.14, 2.72) float pi= 2.14 ; double c= 10.00002;

character constants written between single quotas (e.g., 'a', '+', '\n' (backslash n - newline character))

char mychar = ‘A’;

enumeration constants (e.g., enum day {su, mo, tu, we, th, fr, sa} – they are constants of type int, with values 0, 1, 2, ...)


4. String Constants(1/2)

A sequence of characters enclosed in a pair of double-quote marks

"abc" "123" "" (null string) " " (a string of blank characters)

NOTE: ‘a’ is not same with “a”


4. String Constants(2/2)

If the character " (double-quote) or \ (backslash) are to occur in a string constant, they must be preceded by a backslash character \

EXAMPLE:

#include<stdio.h>

main(){

printf( "a+b"); a+bprintf("\"a+b\""); "a+b" printf("a\\b"); a\b

}


5. Punctuators

Curly Brackets {} EXAMPLE: main() { /* …… */ }

Comma , EXAMPLE: int y, x,z;

Semicolon ; EXAMPLE: printf(“hi”);

Brackets (Paranthesis) () EXAMPLE: Group some expressions

2*(7+6)


6. Operators (1\2)Operators can be (arithmetic, logical, assignment, relational...)

() /* immediately following the name of function */

[] /* Used for arrays*/ -> . /* Access members of structures */

+ /* addition operator*/ - /*subtraction operator */ * /*multiplication operator */ / /*division operator */ % /* integer modulo division: 7 % 3 = 1 */

&& || ! /* logic AND , OR and NOT */ ^ /* bitwise XOR */ ~ /* bitwise negation */ & | /* bitwise AND and OR */


6. Operators (2\2)

sizeof /* size of an object */

?: /* e.g., (n > 0) ? f : n */

* & /* indirection through a pointer and address of an object */

>> << /* shift right and shift left */

< > <= >= == != /* relational operators */

++ -- /* increment and decrement operator*/

= += -= *= /= %= >>= <<= &= ^=


Precedence and Associativity of Operators

Category Operator Associativity

Postfix () [] -> . ++ - - Left to right

Unary + - ! ~ ++ - - (type) * & sizeof Right to left

Multiplicative * / % Left to right Additive + - Left to right Shift << >> Left to right Relational < <= > >= Left to right Equality == != Left to right Bitwise AND & Left to right Bitwise XOR ^ Left to right Bitwise OR | Left to right Logical AND && Left to right Logical OR || Left to right Conditional ?: Right to left

Assignment = += -= *= /= %= >>= <<= &= ^= |= Right to left

Comma , Left to right


Increment and Decrement Operators (1/4)

cnt++ or ++cnt cnt-- or --cnt

/* cnt ++ or cnt -- ( postfix) ++cnt or -- cnt (prefix) */

Side effect : These operators not only yield a value they also changes the stored value of a variable in memory !!

++45 /* constants cannot be incremented */

++(a*b+1) /* ordinary expressions cannot be incremented */


Increment and Decrement Operators (2/4)#include<stdio.h>main() { int a1 = 1, a2 = 1;

int b1 = 0, b2 = 0;

b1= ++a1; printf(“ a1 = %d b1 =%d\n”, a1, b1);

b2 =a2++;printf(“ a2= %d b2=%d\n”, a2,b2); }

OUTPUT

a1= 2 b1= 2a2= 2 b2= 1



#include<stdio.h>main() { int a1 = 1, a2 = 1;

int b1 = 0, b2 = 0;

b1= ++a1; // a1=a1+1 then b1=a1 printf(“ a1= %d b1=%d\n”, a1, b1);

b2 =a2++; // b2=a2 then a2=a2+1printf(“ a1= %d b1=%d\n”, a1,b1);

}



#include<stdio.h>main() {

int a = 0, b = 0, c=0;

a= ++c;b=c++;

printf(“ %d %d %d \n”,a,c,++c);}


Assignment Operators (1/2)

Changes the value of a variable !! Unlike other languages C treats = as an operator!!

Variable = right_side Examples :

int a =0, b= 4; a = b // Values of a,b??

int a=0, b=0, c=0;

a=(b=2)+(c=3); // Values of a,b,c??

Assigment operators= +

=-= *= /= %= >>= <<= &= ^= \=


#include<stdio.h>main() { int a = 1; int b= 3;

a= b ; // now a is 3 and b is 3a+=2; // means a= a+2a+=b; // means a= a+ba*=3; // means a= a*3a*=b;// means a= a*b

}

= +=

-= *= /= %= >>= <<= &= ^= \=

Assignment Operators (2/2)


Assignment Compability

int x= 3; // a decimal integerdouble y= 5.234; // a floating number x = y; // truncates y! x is 5 y = x; // it is OK.x = 5 + 3.2; // truncates the result!y = 5 + 3.2; // it is OKx = 10/4; // x is 2y = 10/4; // y is 2.0y = 10/4.0; // y is 2.5

/* CHECK IT IN YOUR PROGRAM */


Integer Modulo Operator

#include<stdio.h>main(){ int y= 0; int x;

x= 35; y= x%5; // y = mod5(x) = 0y= x%3 // y = mod3(x) = 2 y= x%-3 // y = mod-3(x) = 2

x= -35;y= x%5; // y = mod5(x) = 0y= x%3 // y = mod3(x) = -2 y= x%-3 // y = mod-3(x) = -2

}


Relational Operators (1\2)

#include<stdio.h>main(){ int y; int x;

y=0; x= 35if( x == y) // If x is equal to y ?

printf(“ x equals to y \n”);else

printf(“ x does not equal to y\n ”);

x=y;if( x != y) // If x is not equal to y ?

printf(“ x does not equal to y \n”);else

printf(“ x equals to y\n ”); }


Relational Operators (2\2)

#include<stdio.h>main(){ int y; int x;

y=35; x= 35if( x <= y) // If x is less than or equal to y ?

printf(“ x is less than or equal to y \n”);else

printf(“ x is greater than y\n ”);

x=34;if( !(x <= y)) // NOT + If x is less than or equal to

y ?// If x is NOT less than or equal to y?// If x is greater than y?

printf(“ x is greater than y \n”);else

printf(“ x is not greater than y\n ”); }


Logical Operators (AND + OR ) (1\2)

#include<stdio.h>main(){ int x=0; int y= 1;

if( x< 1 && y<0) printf(“ x is less than 1 AND y is less than 1

“); else printf(“ x >= 1 OR y>=0 \n OR both of

them”);

if( x ==1 || y==1)) printf(“ x equals to 1 OR y equals to 1”);

else printf(“ Neither of x or y is not equla to 1 \n

”);}


Logical Operators: && and || (2\2)

AND OPERATOR : &&

First Second Statement && Statement RESULT True && True = True True && False = False False && True = False False && False = False

SOME EXAMPLES :

int X=-1, Y=-1;

• X>0 && Y< 0 F && T FALSE • X!=0 && Y!=10 T && T TRUE• !(X>0 && Y<0) !( F&&T )

!( F )TRUE

OR OPERATOR : ||

First Second Statement || Statement RESULT True || True = True True || False = True False || True = True False || False = False

SOME EXAMPLES :

int X=-1, Y=-1;

• X>0 || Y< 0 F || T TRUE • X!=0 || Y!=10 T || T TRUE• !(X>0 || Y<0) !( F||T )

!( T )FALSE


Conditional Operator ?:

int x, y;

if( x>0) y=3;else y=5;

int x, y;y=x>0 ? 3 : 5;

CAN BE CONVERTED TO

expression1 ? expression2 : expression3

1. Convert first operand to boolean ( FALSE / TRUE)2. If first operand = True (1), Evaluate the second operand3. If first operand = False (0), Evaluate the third operand

Lexical Elements and Operators

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