CSCI 130

Advanced Program Control

Chapter 8

Program Controls so far

• for loop

• while loop

• do…while loop

The break statement

• Can be placed inside the body of a loop

• When break is encountered, loop is exited (regardless of the condition)

• Execution passes to first statement outside the loop

• A break inside a nested loop only causes exiting of the innermost loop

break example

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

{

if (count == 5)

break;

}

• Without break loop iterates 11 times

• With break, loop stops during 6th iteration

Another break example

Write a for loop that will search a 10 element array for the value 12.

for (i = 0; i < 10; i ++) { //for written with no break

if (arrayName[i] = 12) //this is less efficient

foundFlag = ‘Yes’; //Entire array always

} //searched

-----------------------------------------------------------------------------

for (i = 0; i < 10; i++) { //for written with break

if (arrayName[i] = 12) { //more efficient

foundFlag = ‘Yes’; //if 12 is first element

break; //than loop only entered

} //once

}

The continue statement

• Can be placed within the body of a loop

• When continue encountered, control passes to the next iteration of the loop

• Statements between continue and end of loop not executed

• Significantly different than break

Example of continue

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

{

if (count == 5)

continue;

printf(“%d ”, count);

}

• Loop iterates 11 times, with or without continue statement

• Output: 1 2 3 4 6 7 8 9 10– 5 is skipped

Another continue example

Write the code to check an array of 100 elements. Write out only those elements in the array that are not prime

for (i = 0; i < 100; i++) {

if (numIsPrime(arrayName[i]))

continue;

printf(“\n%d”, arrayName[i]);

}

Note: numIsPrime will return a positive number for primes

The goto statement

• C’s unconditional branching statement

• goto and target statement must be in the same function

• goto can always be performed using better structures (do…while, etc.)

• NEVER use a goto

Example of a goto

void main ( ) { int count= 3; printf("Before any goto %d\n", count);

goto location0; printf("This will not print out\n");

location0: ; printf("At location 0\n");

location1: ; printf("At location 1\n"); }

Strong suggestion about goto

NEVER

USE

A

GOTO

Infinite Loops

• Condition will never be evaluated as false

• Theoretically would run forever

• Avoid infinite loops

Examples of an infinite loop

for (i = 0; i < 10; i+1) //i+1 does not change

printf(“%d”, i); //the value of i

---------------------------------

while ( i < 10) { //Programmer thinks

printf(“%d”, i); //i always starts greater

i-=1; //than 10

}

----------------------------------

for (i = 1; i !=10; i +=2) //Printing out odd ints

printf(“%d”, i); //up to 10?

The switch statement

• Most flexible program control statement

• Program control based on an expression with more than 2 possible values

Referencing elements in an array

• General form of switch statement: switch(expression)

{

case template1:

statements;

case template2:

statements;

…

case templaten:

statements;

default:

statements;

}

Concrete example of switch

switch(i) { case 1: printf(”The number is 1”);

case 2: printf(“The number is 2”);

case 3: printf(”The number is 3”);

default: printf(”The number is not 1, 2, or 3"); }

Evaluation of a switch statement

• If expression matches a template, control passes to first statement within that template

• If no match, control passes to first statement within default

• If no match and no default, control passed to first statement after switch structure

Output of switch statement switch(i) { case 1: printf(”The number is 1\n”); case 2: printf(“The number is 2\n”); case 3: printf(”The number is 3\n”); default: printf(”The number is not 1, 2, or 3\n"); }

If i = 1 Output: The number is 1 The number is 2 The number is 3 The number is not 1, 2, or 3

Correct way to code a switch

switch(i) { case 1: printf(”The number is 1\n”); break; case 2: printf(“The number is 2\n”); break; case 3: printf(”The number is 3\n”); break; default: printf(”The number is not 1, 2, or 3\n"); }

System functions

• All within stdlib.h file (must be included)

• exit( )– terminates execution

• atexit( )– performs functions at program termination

• system( )– executes operating system commands

exit ( ) function

• Terminates program execution #include <stdio.h> #include <stdlib.h>

void main ( ) { char i; exit(0);

printf("Enter a character"); //These statements will scanf("%c", &i); //not be executed }

exit ( ) function continued

• If 0 is passed into function it means program executed normally

• If 1 is passed into function it means program abnormally terminated

• stdlib.h has two symbolic constanst:– #define EXIT_SUCCESS 0– #define EXIT_FAILURE 1

• can call exit(EXIT_SUCCESS)• can call exit(EXIT_FAILURE)

atexit ( ) function

• Specifies one (or more) functions that are automatically executed at termination time

• Up to 32 functions can be registered in this way

• Executed in reverse order

atexit( ) function continued

#include <stdio.h> #include <stdlib.h>

void cleanup(); void cleanupLast();

void main ( ) { char i; atexit(cleanupLast); atexit(cleanup); printf("Enter a character"); scanf("%c", &i); }

system ( ) function

• Executes operating system commands• Example:

system(“c:\dir *.exe”);

• Must capture any output in a file– will not open up another console– will not print to current console

• Can execute any command line system(“c:\winnt\system32\notepad.exe”);

system( ) does not work in CW

• system( ) function does not work in Code Warrior

• Help states:

“The system() function is an empty function that is included in the Metrowerks stdlib.h to conform to the ANSI C Standard Library specification.”