1. CMPT 102 Introduction to Scientific Computer Programming
Control Structures
while Loops
continue; and break; statements

2. Control Structures Three methods of processing a program
In sequence
Branching
Looping
Branch: Altering the flow of program execution by making a selection or choice
Loop: Altering the flow of program execution by repetition of a particular block of statement(s)
© Janice Regan, CMPT 102, Sept. 2006

3. Basic Loops
When one action is to be repeated a number of times a loop is used. Loops are repetition structures
There are two common types of loops
while loop or do...while loop
Used to continue repetition while a condition holds
Can also be used (along with a counter variable) to repeat a particular number of times
for loop
Specifically designed to repeat a particular number of times
© Janice Regan, CMPT 102, Sept. 2006

4. A while Loop in C while ( condition ) {
/* Series of actions to be taken */
/* each time the loop is executed */
/* loop is executed when condition is True */
action 1;
action 2; }
/* When condition is false execute following actions */
actions;
© Janice Regan, CMPT 102, Sept. 2006

5. Structure of while loop
condition
Statement 1;
Statement n;
© Janice Regan, CMPT 102, Sept. 2006

6. Counting while statement
Initial statements
start = n
end = m
loopIndex = start;
Repeats Statements
m-n times
Loop condition
loopIndex < end F
loopIndex += step; T
Update statement
First Statement:
…..
Last Statement;
© Janice Regan, CMPT 102, Sept. 2006

7. Example while Loop in C /* Sum the Integers from 1 to 25 inclusive */
while ( X <= 25 ) {
Sum += X;
X++; }
printf(“%d\n”, Sum);
© Janice Regan, CMPT 102, Sept. 2006

8. do…while Loop in C do { /*Series of actions to be taken */
/*each time the loop is executed */
/*Always executed on first pass */
/* Subsequently executed if condition is True */
action 1;
action 2; }
while ( condition );
/* When condition is false execute following actions */
actions;
© Janice Regan, CMPT 102, Sept. 2006

9. Structure of do…while Loop
condition
Statement 1;
Statement n;
Loop condition
© Janice Regan, CMPT 102, Sept. 2006

10. Counting do…while statement
Initial statement
start = n
end = m
loopIndex = start;
Repeats Statements
m-n times
First Statement:
…..
Last Statement;
Update statement
loopIndex += step;
loopIndex < end T F
Loop condition
© Janice Regan, CMPT 102, Sept. 2006

11. Example do…while Loop X = 1; sum = 0; do { sum += X; X++; }
/* Sum the Integers from 1 to 25 inclusive */
X = 1;
sum = 0; do {
sum += X;
X++; }
while ( X <= 25 );
printf(“%d”, sum);
© Janice Regan, CMPT 102, Sept. 2006

12. Differences do vs do…while
/* Sum Integers from n to m */
n = 4:
m = 3:
sum = 0; do {
sum += n;
n++; }
while ( n <= m );
printf(“%d”, sum);
/* Sum Integers from n to m */
n = 4:
m = 3:
sum = 0;
while ( n <= m ) {
sum += n;
n++; }
printf(“%d”, sum);
After code sum = 4
Body of loop executes once
After code sum = 0
Body of loop does not execute
© Janice Regan, CMPT 102, Sept. 2006

13. Infinite Loops
When using while or do..while loops it is possible that the loop condition is always true.
If the variable or expression tested in the loop condition is not modified in the action statements within the loop, then if the condition is true for the first loop test it remains true for ever
Even if the variable or expression changes it may satisfy the test condition for ever
An infinite loop will cause your program to execute forever
If your program is caught in an infinite loop you can terminate it by typing <CNTRL>C into your linux or cygwin command window
© Janice Regan, CMPT 102, Sept. 2006

14. Uses of an infinite loop
To make an infinite loop useful, you must have a way to exit from it
It is possible to exit from a loop by using a break; statement (discussed below)
If you use an infinite loop intentionally it is usually a loop you intend to execute many times until
Some condition is met OR
The loop has executed the maximum number of times
If you use an infinite loop intentionally you should always exit after a maximum number of times through the loop, just in case the condition you expect to take you out of the loop never occurs.
© Janice Regan, CMPT 102, Sept. 2006

15. The Break Statement: Loops
Sometimes you wish to exit from a loop even if the loop condition is still true
For example if your loop is reading and processing data and an error occurs that can be recognized but cannot be corrected, you may wish to print an error message and exit the loop immediately
break;
A C statement that causes you to exit the loop you are in
Program execution will continue at the statement following the loop
© Janice Regan, CMPT 102, Sept. 2006

16. break statement Loop condition Condition F T Statement 1; ….. T
Statement n;
© Janice Regan, CMPT 102, Sept. 2006

17. The Continue Statement:
Sometimes you wish to execute only some of the statements in the body of the loop for a particular value of the loop index
For example you may wish to skip the last half of the body of the loop if (loopindex/14)= = 1
continue;
A C statement that causes you to go from anywhere in the loop to the update statement and loop condition
Program execution will continue at the update statement of the loop
© Janice Regan, CMPT 102, Sept. 2006

18. continue statement Condition F T Statement 1; ….. T condition2
Statement n;
© Janice Regan, CMPT 102, Sept. 2006

19. continue in Counting while
Start = n
End = m
loopIndex = start;
Repeats Statements
m-n times
Initial statement
Loop condition
loopIndex < end F
loopIndex += step; T
First Statement:
…..
Update statement T
condition2
continue; F
…..
Last Statement;
© Janice Regan, CMPT 102, Sept. 2006

20. Example infinite Loop in C
/* The user inputs a positive integer */
/* the routine prints the square of the number, */
/* to terminate the user enters */
while ( 1 ) {
printf("enter a positive integer " );
scanf("%d", &positiveIntNum);
if(positiveIntNum < 0)
if( positiveIntNum == -999) {break};
printf("error*** negative input ***\n");
continue; }
printf("%d\n", positiveIntNum*positiveIntNum);
© Janice Regan, CMPT 102, Sept. 2006