1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Fall 2013
Lecture 8:
Loops: while, do-while

2. ECE Application Programming: Lecture 7
Lecture outline
Announcements/reminders
Program 3 posted; due 9/25
Review
Conditional statements: switch
Today’s lecture
Loops: while, do-while
5/23/2018
ECE Application Programming: Lecture 7

3. Review: switch statements
When checking multiple exact values for expression, more sense to use switch statement
switch (<expr>) {
case <val1> :
...
break;
case <val2> :
default: }
break allows you to exit switch statement after completing code
Otherwise, program will continue to run through cases until finding break
default covers any values without specific case
5/23/2018
ECE Application Programming: Lecture 4

4. ECE Application Programming: Lecture 4
Repetition
Say we have a program to print squares of numbers between 0 and 10:
void main() {
int i; // Number to square
int iSquared; // Square of the number
printf(" i i^2\n"); // Column headings
// Compute and display the squares of numbers 0 through 10
i = 0;
iSquared = i * i;
printf("%2d%10d\n", i, iSquared);
... // Code for i = 1, 2, ... 8, 9
i = 10; }
5/23/2018
ECE Application Programming: Lecture 4

5. ECE Application Programming: Lecture 4
while loops
Previous program does same thing 11 times
Repetitive code can be captured in a loop
Much less code to do same amount of work
Simplest form: while loop
while (<expression>)
<statement>  loop body
Loop body will repeat as long as <expression> is true
Loop body must therefore change expression
<statement> may be one or more lines
If multiple lines, need { } to denote block
5/23/2018
ECE Application Programming: Lecture 4

6. ECE Application Programming: Lecture 4
while loops - example
x = 7; while ( x < 10 ) { printf("%d ",x); x = x + 1; }
OUTPUT:
7 8 9
5/23/2018
ECE Application Programming: Lecture 4

7. ECE Application Programming: Lecture 4
while loops - example
x = 7; while ( x < 3 ) { printf("%d ",x); x = x + 1; }
OUTPUT:
(no output)
Possible to have while loop body that never executes!
5/23/2018
ECE Application Programming: Lecture 4

8. Repetition with while loop
Rewriting previous program with loop
int main() {
int i; // Number to square
int iSquared; // Square of the number
printf(" i i^2\n"); // Column headings
// Compute and display the squares of numbers 0 to 10
i = 0; // Initialize i
while (i <= 10) { // Loop until i > 10
iSquared = i * i;
printf("%2d%10d\n", i, iSquared);
i = i + 1; // Increment i }
return 0;
5/23/2018
ECE Application Programming: Lecture 4

9. Application: loop with flexible limit
Could determine loop limit based on variable
Result of calculation
Input value
See while2.c for an example
Program to calculate average grade
First reads # of grades to enter, then list of grades
Keeps running sum of all grades entered
Calculates average at end
Loop:
while (gradeCount < numGrades) {
scanf("%lf", &grade); // Read grade
gradeSum = gradeSum + grade; // Add to sum
gradeCount = gradeCount + 1; // Inc. count }
5/23/2018
ECE Application Programming: Lecture 4

10. Application: sentinel value
Common to read input until a certain value(sentinel) is entered
May be predetermined (i.e., run program until user enters ‘q’ for “quit”)
Run until invalid value entered
In file input, will often run until end of file
See while3.c for an example
Refined version of average grade program
Core of program:
// Prompt for and read first grade
printf("Enter grade: ");
scanf("%lf", &grade);
/* Continue reading/accumulating grades until invalid
value entered */
while ((grade >= 0.0) && (grade <= 100.0)) {
gradeSum = gradeSum + grade; // Accumulate grade
gradeCount = gradeCount + 1; // Increment grade count
printf("Enter grade: "); // Prompt for and
scanf("%lf", &grade); // read next grade }
5/23/2018
ECE Application Programming: Lecture 4

11. ECE Application Programming: Lecture 4
do-while loops
while loop is pre-tested
Check condition at start; if false, don’t enter loop
To guarantee at least one iteration, use post-tested loop: do-while
Checks condition at end of loop
do { <statements> } while ( <expression> );
Don’t forget semicolon!
5/23/2018
ECE Application Programming: Lecture 4

12. While vs. do-while: flowcharts
while: pre-tested loop
Check condition, then execute loop body
do-while: post-tested loop
Execute loop body, then check condition
A = 0?
Loop body
FALSE
TRUE
A = 0?
TRUE
Loop body
FALSE
All loops characterized by conditional test, backwards arrows indicating repetition of code
5/23/2018
ECE Application Programming: Lecture 4

13. comparison while vs do-while
x = 7; do { printf("%d ",x); x = x + 1; } while ( x < 10 );
OUTPUT:
7 8 9
x = 7; while ( x < 10 ) { printf("%d",x); x = x + 1; }
OUTPUT:
7 8 9
5/23/2018
ECE Application Programming: Lecture 4

14. comparison while vs do-while
x = 7; do { printf("%d",x); x = x + 1; } while ( x < 3 );
OUTPUT: 7
x = 7; while ( x < 3 ) { printf("%d",x); x = x + 1; }
OUTPUT:
(no output)
5/23/2018
ECE Application Programming: Lecture 4

15. Application: sentinel value
Core of program demonstrating while loop
// Prompt for and read first grade
printf("Enter grade: ");
scanf("%lf", &grade);
/* Continue reading/accumulating grades until invalid
value entered */
while ((grade >= 0.0) && (grade <= 100.0)) {
gradeSum = gradeSum + grade; // Accumulate grade
gradeCount = gradeCount + 1; // Increment grade count
printf("Enter grade: "); // Prompt for and
scanf("%lf", &grade); // read next grade }
5/23/2018
ECE Application Programming: Lecture 4

16. Application: sentinel value
Rewrite grade average program to ensure at least one grade is read
Change core of program (shown previously):
/* Prompt for and read grades until invalid
value entered */
do {
printf("Enter grade: "); // Prompt for and
scanf("%lf", &grade); // read grade
if ((grade >= 0.0) && (grade <= 100.0)) {
gradeSum = gradeSum + grade; // Accumulate grade
gradeCount = gradeCount + 1; // Inc. grade count }
} while ((grade >= 0.0) && (grade <= 100.0));
5/23/2018
ECE Application Programming: Lecture 4

17. ECE Application Programming: Lecture 7
Final notes
Next time
Loops: for
Reminders:
Program 3 due 9/25
5/23/2018
ECE Application Programming: Lecture 7