CSCI 171

Presentation 5

The while loopExecutes a block as long as the condition is true

general form: while (condition) { statement 1; statement 2; … statement n; }

The While Statement

Execution of a while loop

• Condition evaluated– If condition evaluates to false

• loop terminates – execution passes to first line of code outside loop

– If condition evaluates to true• Body of loop is executed

• Process is repeated from beginning

Sample While Statement

int main ( ) { int count; count = 1; while (count <=20) {

printf (“\n%d”, count); count ++;

}

return 0; }

Sample Program 5.1#include <stdio.h>

int main( void ) {int increment = 0;int counter = 0;

printf("Please enter the increment: ");scanf("%d", &increment);printf("\n\n");

while (counter < 20){

printf("%d ", counter+=increment);}

return 0;}

Compare the for Loop & while int count; for (count=1; count <=20; count++) { printf (“\n%d”, count); } ---------------------------------------------------- int count = 1; while (count <=20) { printf (“\n%d”, count); count++; }

Writing a while loop

Write the C code to print the odd numbers less than 20 using a while loop

void main () { int count = 1; while (count < 20) { printf (“\n%d”, count); count += 2; } }

Sample Program 5.2

//Rewrite this using a while loop

#include <stdio.h>

int main( void ) {int counter = 0, sign_change = 1;

for (; counter < 20; counter++) {printf("%d\n", counter*sign_change);sign_change *= -1;

}

return 0;}

Uses of the while loop

• Iterating until the a flag is set indicated the process should be terminated– sentinel

• Iterating until the user enters valid data– data validation

The while loop and a sentinel

printf (“Enter a test score (0 to finish)”);

printf (“The average will be calculated”);

scanf (“%d”, &score);

count = 0;

while (score != 0) {

count += 1;

total += score;

printf (“Enter next score”);

scanf (“%d”, &score);

}

//Calculate average – make sure count > 0

Sample Program 5.3#include <stdio.h>void main () {

int count = 0, total = 0, score = 0;float average = 0;printf("Enter a test score (0 to finish)");printf("\nThe average will be calculated");printf("\n\nEnter first score: ");scanf ("%d", &score);while (score != 0) {

total += score;printf ("Enter next score: ");scanf ("%d", &score);count += 1;

}if (count) {

average = (float)total / count;printf ("\nThe average score is %.2f", average);

}else

printf("\nNo scores entered.");}

The while loop and data validation

//Allow the user to enter a number between 1 and 10 //and validate their entry

printf(“Enter a number between 1 and 10: “); scanf(“%d”, &nbr);

while (nbr < 1 || nbr > 10) {printf(“Invalid data – please enter a number between 1 and 10);scanf(“%d”, &nbr);

}

Sample program 5.4#include <stdio.h>

void main () {int ctr = 0, nbr = 0, array[5];printf("The program will allow the user to enter 5 integers.");printf("\nThe numbers must be between 1 and 10.");printf("\nThe sum of the numbers will then be printed.");while (ctr < 5) {

printf("\n\nEnter number %d (between 1 & 10): ", ctr + 1);scanf("%d", &nbr);while (nbr < 1 || nbr > 10) {printf("\nSorry, the number must be between 1 & 10.");printf("\nEnter number %d (between 1 & 10): ", ctr + 1);scanf("%d", &nbr);}array [ctr] = nbr;ctr ++;

}nbr = 0;for (ctr = 0; ctr < 5; ctr++)

nbr += array[ctr];printf("\n\nThe sum is: %d", nbr);

}

Sample Program 5.5#include <stdio.h>

int main() {int option = 0;

printf("1. Find the largest value");printf("\n2. Find the smallest value");printf("\n3. Find the average");

printf("\n\nPlease make selection: ");scanf("%d", &option);

while ((option < 1) || (option >3)) {printf("Invalid selection, please re-enter (valid values are 1, 2, and 3): ");scanf("%d", &option);

}

printf("\nCongratulations, you entered a valid menu option!!!");

return 0;}

The do … while loopExecutes a block as long as the condition is true

general form: do { statement 1; statement 2; … statement n; } while (condition);

The do … while loop

Execution of a do…while loop

• Body of loop is executed

• Condition is executed– If condition evaluates to false

• loop terminates – execution passes to first line of code outside loop

– If condition evaluates to true• Process is repeated from beginning

Sample Program 5.6#include <stdio.h>void main ( ) {

float average = 0;int count = 0;int score = 0;int total = 0;printf ("Enter a test score: ");scanf ("%d", &score);do {

total += score;count += 1;printf ("Enter next score (0 to finish): ");scanf ("%d", &score);

} while (score != 0);average = (float)total/count;printf ("The average is %.2f", average);

}

Sample Program 5.7//Run the code and fix any errors//Would a while loop work better here than a do…while?#include <stdio.h>void main ( ) {

int choice = 0;

printf ("1 - Add a record");printf ("\n2 - Delete a record");printf ("\n3 - Change a record");printf ("\n\n Enter a selection: ");scanf("%d", &choice);do {

printf("Invalid selection, please re-enter (valid values are 1, 2, and 3): ");scanf("%d", &choice);

} while (choice != 1 || choice != 2 || choice != 3);printf("\nCongratulations, you entered a valid menu option!!!");

}

Comparing loops

• Tasks accomplished with for loop can be accomplished with while or do..while loop.

• For loop has initialization condition, & increment in one spot– increased readability

– generally easier to work with

• If # repetitions cannot be determined before iterations begin, must use while

Comparing loops

• Top-checking loops– Condition is evaluated PRIOR to first iteration

• Loop may never execute

• for loops

• while loops

• Bottom-checking loops– Condition is not evaluated until AFTER the first

iteration• Guaranteed at least one iteration

• do…while loops

Nested loops

• Any loop can be nested within any other loops• for within while

• while within for

• for within do … while

• do … while within for, within while,

• etc.

• Loops must be completely nested within each other

Example of while within for

int count1;

int count2;

for (count1 = 1; count1 < 10; count1++){

count2 = 0;

while (count2 <3) {

printf (“%d”, count2);

count2 ++;

}

}

Illegally nested loops

int count1, count2;

for (count1 = 1; count1 < 10; count1 ++) {

count2 = 1;

do

{

printf (“%d”, count2);

count2 ++;

}

} while (count2 < 3);