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CSCI 171 Presentation 5
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The while loop Executes a block as long as the condition is true general form: while (condition) { statement 1; statement 2; … statement n; }
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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
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Sample While Statement int main ( ) { int count; count = 1; while (count <=20) { printf (“\n%d”, count); count ++; } return 0; }
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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; }
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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++; }
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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; }
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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; }
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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
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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
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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."); }
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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); }
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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); }
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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; }
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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);
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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
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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); }
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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!!!"); }
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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
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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
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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
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Example of while within for int count1; int count2; for (count1 = 1; count1 < 10; count1++){ count2 = 0; while (count2 <3){ printf (“%d”, count2); count2 ++; }
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Illegally nested loops int count1, count2; for (count1 = 1; count1 < 10; count1 ++){ count2 = 1; do { printf (“%d”, count2); count2 ++; } }while (count2 < 3);
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