CSCI 171 Presentation 5

The while loop Executes 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 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 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 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 10) { printf(“Invalid data – please enter a number between 1 and 10); scanf(“%d”, &nbr); }

Sample program 5.4 #include 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 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 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 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 loop Executes 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 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 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);