CSCI 171

1. CSCI 171 Presentation 5

2. The while loop Executes a block as long as the condition is true general form: while (condition) { statement 1; statement 2; … statement n; }

3. 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

4. Sample While Statement • int main ( ) { • int count; • count = 1; • while (count <=20) { • printf (“\n%d”, count); • count ++; • } • return 0; • }

5. 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; }

6. 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++; • }

7. 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; • } • }

8. 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; }

9. 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

10. 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

11. 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."); }

12. 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); }

13. 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); }

14. 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; }

15. 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);

16. 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

17. 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); }

18. 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!!!"); }

19. 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

20. 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

21. 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

22. Example of while within for int count1; int count2; for (count1 = 1; count1 < 10; count1++) { count2 = 0; while (count2 <3) { printf (“%d”, count2); count2 ++; } }

23. Illegally nested loops int count1, count2; for (count1 = 1; count1 < 10; count1 ++) { count2 = 1; do { printf (“%d”, count2); count2 ++; } } while (count2 < 3);