Agenda Perform Quiz #1 (20 minutes) Loops –Introduction / Purpose –while loops Structure / Examples involving a while loop –do/while loops Structure / Examples involving a do/while loop –Homework Question

Loops –Loops are used for repetition when running a program. –To make sure the loop ends, we need a condition to control the loop; otherwise, the loop will continue forever! Perform action

Types of loops while and do/while Loops –The “while” and “do/while” loops are often called indeterminate loops because they are best used when the number of repetitions are not known in advance. for Loops –The “for” loop is often called a determinate loop since it is best used when the number of repetitions is already known.

In Class Exercise #1 Which are the best types of loops for the following situations: determinate or indeterminate? –ask user to enter a group of marks until they are finished. –print a multiplication table (1 - 12) –indicate that user has inputted an invalid selection, and needs to try again (error checking)

while loop Test condition Commands if TRUE true false The “while” loop test the condition first (referred to as a pre- test). Therefore, if the pre- test condition is FALSE, the loop is NOT executed.

while loop - Example 1 main () { int i = 0; while ( i < 3 ) { printf (“i is: %d\n”, i); i = i + 1; } OUTPUT: i is: 0 i is: 1 i is: 2 Declare and assign variable i (counter) Pretest condition involving variable i Perform action in while loop if test is TRUE, advance the value of variable i (or “counter”) by 1 and return to pretest Tip: the short-form of i = i + 1; is i++; i = I - 1; is i--;

while loop - Example 2 main () { int i ; printf (“\nEnter a positive integer: “); scanf (“%d”, &i); while ( i < 0 ) { printf (“Oops! Enter a positive integer: ”); scanf (“%d”, &i); } printf (“\n”); } Declare and assign variable i (used to store selection, NOT used as a counter!) Prompt & scan value of i Notice NO semicolon “;” when performing pretest and notice braces (code block) Perform action if condition is TRUE (error-checking procedure). User is informed of mistake and must re-enter until a positive integer is finally entered

while loop - Example 2 main () { int i ; printf (“\nEnter a positive integer: “); scanf (“%d”, &i); while ( i < 0 ) { printf (“Oops! Enter a positive integer!: ”); scanf (“%d”, &i); } printf (“\n”); } It is OK to give very simple variable names like i, j, x or y when using them for looping purposes

In Class Exercise #2 Can you think of how both programs above could be modified to create “infinite loops?”

while loop - Example 1 main () { int i = 0; while ( i != 3 ) { printf (“i is: %d\n”, i); i = i + 2; } OUTPUT: i is: 0 i is: 2 i is: , etc. etc. etc ---> forever! Test condition will only be FALSE (thus stop the loop) if i is not equal to 3, but counter can skip 3 and go beyond 3! So you have an infinate loop!

while loop - Example 2 main () { int i ; printf (“\nEnter a positive integer: “); scanf (“%d”, &i); while ( i < 0 ) { printf (“Oops! Enter a positive integer: ”); } printf (“\n”); } WARNING: If you do NOT include an opportunity to allow user to re-enter again, you will encounter an “infinite loop”!!

do/while loop Test condition Commands true false The do statement performs an action first, and then is followed by the while statement to test condition (referred to as a “post test”) Therefore, action is performed at least once, even if condition is FALSE

do/while loop - Example 1 main () { int i = 10; do { printf (“i is: %d\n”, i); i--; } while (i > 0); } Declare and assign variable i (counter) Perform action first: Prompt & scan value of i After action is performed once, then perform post test) Notice while condition immediately after closing brace for do statement, and semi-colon “;” after while statement!

do/while loop - Example 2 main () { int num; do { printf (“\nGuess the secret number: ”, num); scanf (“%d”, &num); } while (num != 5); printf (“That’s right!\n”); printf (“The secret number is 5.\n\n”); } When post test is false (i.e. num is equal to 5, program continues, and prints the message “That’s right! The secret number is 5.” Declare variable num Prompt user for number, and then test and repeat loop while num is not equal to 5

Homework Question Create a program to prompt the user for a group of integers. If the user enters a negative integer, then the program should display the average of the entered integers. –What loop should you use? –Plan the program –Create & compile the source code