1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Summer 2012
Lecture 5
Loops

2. ECE Application Programming: Lecture 5
Lecture outline
Announcements/reminders
Program 4 due today
Program 5 posted; due Tuesday, 7/31
Exams to be returned during tomorrow’s lecture
Will catch up on program grading ASAP
Exam 2 next Thursday, 8/2
Today’s lecture: loops
While loops
Do-while loops
For loops
Break/continue
1/28/2018
ECE Application Programming: Lecture 5

3. ECE Application Programming: Lecture 5
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; }
1/28/2018
ECE Application Programming: Lecture 5

4. ECE Application Programming: Lecture 5
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
1/28/2018
ECE Application Programming: Lecture 5

5. ECE Application Programming: Lecture 5
while loops - example
x = 7; while ( x < 10 ) { printf("%d ",x); x = x + 1; }
OUTPUT:
7 8 9
1/28/2018
ECE Application Programming: Lecture 5

6. ECE Application Programming: Lecture 5
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!
1/28/2018
ECE Application Programming: Lecture 5

7. 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;
1/28/2018
ECE Application Programming: Lecture 5

8. 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 }
1/28/2018
ECE Application Programming: Lecture 5

9. 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 }
1/28/2018
ECE Application Programming: Lecture 5

10. ECE Application Programming: Lecture 5
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!
1/28/2018
ECE Application Programming: Lecture 5

11. 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
1/28/2018
ECE Application Programming: Lecture 5

12. 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
1/28/2018
ECE Application Programming: Lecture 5

13. 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)
1/28/2018
ECE Application Programming: Lecture 5

14. 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 }
1/28/2018
ECE Application Programming: Lecture 5

15. 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));
1/28/2018
ECE Application Programming: Lecture 5

16. Example: while/do-while loops
What do each of the following print?
int i = 0;
while (i < 30) {
if ((i % 3) == 0)
printf("%d\n", i);
i = i + 2; }
Assume input is:
do {
scanf("%d",&x);
printf("x = %d, x/2 = %d\n", x, x/2);
} while ((x > 2) || (x < 0));
Assume input is: b c d e f g h
char c = 'a';
while (c != 'h') {
printf("%c", c);
scanf("%c", &c);
1/28/2018
ECE Application Programming: Lecture 5

17. ECE Application Programming: Lecture 5
Example solution
int i = 0;
while (i < 30) {
if ((i % 3) == 0)
printf(“%d\n”, i);
i = i + 2; }
Output: 6 12 18 24
1/28/2018
ECE Application Programming: Lecture 5

18. Example solution (cont.)
Assume input is:
do {
scanf(“%d”,&x);
printf(“x = %d, x/2 = %d\n”, x, x/2);
} while ((x > 2) || (x < 0));
Output:
x = 3, x/2 = 1
x = 8, x/2 = 4
x = -4, x/2 = -2
x = -7, x/2 = -3
x = 0, x/2 = 0
1/28/2018
ECE Application Programming: Lecture 5

19. Example solution (cont.)
Assume input is: b c d e f g h
char c = ‘a’;
while (c != ‘h’) {
printf(“%c”, c);
scanf(“%c”, &c); }
Output:
ab c d e f g
1/28/2018
ECE Application Programming: Lecture 5

20. ECE Application Programming: Lecture 5
Justifying for loops
Common loop structure
Initialize variable
Loop until that variable reaches certain limit
At end of each iteration, change variable by fixed amount
Example: squares program
i = 0;
while (i <= 10) {
iSquared = i * i;
printf("%2d%10d\n", i, iSquared);
i = i + 1; }
1/28/2018
ECE Application Programming: Lecture 5

21. ECE Application Programming: Lecture 5
for loops
for loops include all three aspects in one construct
Form:
for (<init var>; <test>; <change var>)
<statements>
<init var> is basic assignment
<test> same type of condition as if, while
<change var> change variable by fixed amount
Example: for (i = 0; i < 20; i++)
… You may be wondering what i++ means
1/28/2018
ECE Application Programming: Lecture 5

22. ECE Application Programming: Lecture 5
Changing variables
Can do operation + assignment w/one operator
Simply adding/subtracting 1:
x++  x = x + 1 (post-increment)
x--  x = x – 1 (post-decrement)
++x  x = x + 1 (pre-increment)
--x  x = x – 1 (pre-decrement)
Augmented assignment: change variable by amount other than 1
x += y  x = x + y
x -= y  x = x – y
x *= y  x = x * y
x /= y  x = x / y
1/28/2018
ECE Application Programming: Lecture 5

23. Pre- vs. post-increment/decrement
Pre-increment/decrement: perform increment/ decrement, then evaluate expression
Post-increment/decrement: evaluate expression, then perform increment/decrement
Example: what does the following print?
int n = 5;
printf(“n = %d\n”, ++n);
printf(“Now, n = %d\n”, n++);
printf(“Finally, n = %d\n”, n);
1/28/2018
ECE Application Programming: Lecture 5

24. ECE Application Programming: Lecture 5
Example soln.
int n = 5;
printf(“n = %d\n”, ++n);
printf(“Now, n = %d\n”, n++);
printf(“Finally, n = %d\n”, n);
Output:
n = 6 (n pre-incremented)
Now, n = 6 (n post-incremented)
Finally, n = 7 (Shows effect of n++)
1/28/2018
ECE Application Programming: Lecture 5

25. Simple usage of for loop
ECE Intro to Computer Engineering I
03/07/2005
Simple usage of for loop
Common sequence of code:
x=0; while (x<12) { printf("%d ",x); x++; }
Output:
1/28/2018
ECE Application Programming: Lecture 5
(c) 2005, P. H. Viall

26. ECE Application Programming: Lecture 5
Intro to for loops
Common sequence of code:
x=0; while (x<12) { printf("%d ",x); x++; }
Equivalent for construct
for (x=0 ; x<12 ; x++ ) { printf("%d ",x); }
Initial value
Test condition
Body of loop (may be 0, 1, or several statements)
End of loop change
1/28/2018
ECE Application Programming: Lecture 5

27. Repetition with for loop
Rewriting squares 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
for (i = 0; i <= 10; i++) { // Loop until i > 10
iSquared = i * i;
printf("%2d%10d\n", i, iSquared); }
return 0;
1/28/2018
ECE Application Programming: Lecture 5

28. Repetition with for loop (cont.)
Generalizing program:
int main() {
int i; // Number to square
int iSquared; // Square of the number
int iStart; // Initial value
int iStop; // Last value
int iStep; // Increment
printf(“Enter start, stop, and increment: “);
scanf(“%d %d %d”, &iStart, &iStop, &iStep);
printf(" i i^2\n"); // Column headings
// Compute and display the squares of numbers iStart to iStop
// with increment iStep
for (i = iStart; i <= iStop; i += iStep) {
iSquared = i * i;
printf("%2d%10d\n", i, iSquared); }
return 0;
1/28/2018
ECE Application Programming: Lecture 5

29. ECE Application Programming: Lecture 5
Example: for loops
What does each of the following print?
for (i=5; i<40; i+=8) { printf("%d ", i); }
for (i=10; i<=100; i=i+10) { if (i%20) printf("%d ", i); }
for (i=5; i<10; i+=i%2) { printf("%d ", i++); }
for (i=-5; i<-10; i--) { printf("%d ", i); }
1/28/2018
ECE Application Programming: Lecture 5

30. ECE Application Programming: Lecture 5
Example solution
What does each of the following print?
for (i=5; i<40; i+=8) { printf("%d ", i); }
OUTPUT:
for (i=10; i<=100; i=i+10) { if (i%20) printf("%d ", i); }
OUTPUT:
for (i=5; i<10; i+=i%2) { printf("%d ", i++); }
OUTPUT: 5 6 8
for (i=-5; i<-10; i--) { printf("%d ", i); }
OUTPUT: No output
1/28/2018
ECE Application Programming: Lecture 5

31. ECE Application Programming: Lecture 5
Justifying for loops
Common loop structure
Initialize variable
Loop until that variable reaches certain limit
At end of each iteration, change variable by fixed amount
Example: squares program
i = 0;
while (i <= 10) {
iSquared = i * i;
printf("%2d%10d\n", i, iSquared);
i = i + 1; }
1/28/2018
ECE Application Programming: Lecture 5

32. ECE Application Programming: Lecture 5
for loops
for loops include all three aspects in one construct
Form:
for (<init var>; <test>; <change var>)
<statements>
<init var> is basic assignment
<test> same type of condition as if, while
<change var> change variable by fixed amount
Example: for (i = 0; i < 20; i++)
… You may be wondering what i++ means
1/28/2018
ECE Application Programming: Lecture 5

33. ECE Application Programming: Lecture 5
Changing variables
Can do operation + assignment w/one operator
Simply adding/subtracting 1:
x++  x = x + 1 (post-increment)
x--  x = x – 1 (post-decrement)
++x  x = x + 1 (pre-increment)
--x  x = x – 1 (pre-decrement)
Augmented assignment: change variable by amount other than 1
x += y  x = x + y
x -= y  x = x – y
x *= y  x = x * y
x /= y  x = x / y
1/28/2018
ECE Application Programming: Lecture 5

34. Pre- vs. post-increment/decrement
Pre-increment/decrement: perform increment/ decrement, then evaluate expression
Post-increment/decrement: evaluate expression, then perform increment/decrement
Example: what does the following print?
int n = 5;
printf("n = %d\n", ++n);
printf("Now, n = %d\n", n++);
printf("Finally, n = %d\n", n);
1/28/2018
ECE Application Programming: Lecture 5

35. ECE Application Programming: Lecture 5
Example soln.
int n = 5;
printf("n = %d\n", ++n);
printf("Now, n = %d\n", n++);
printf("Finally, n = %d\n", n);
Output:
n = 6 (n pre-incremented)
Now, n = 6 (n post-incremented)
Finally, n = 7 (Shows effect of n++)
1/28/2018
ECE Application Programming: Lecture 5

36. Simple usage of for loop
ECE Intro to Computer Engineering I
03/07/2005
Simple usage of for loop
Common sequence of code:
x=0; while (x<12) { printf("%d ",x); x++; }
Output:
1/28/2018
ECE Application Programming: Lecture 5
(c) 2005, P. H. Viall

37. ECE Application Programming: Lecture 5
Intro to for loops
Common sequence of code:
x=0; while (x<12) { printf("%d ",x); x++; }
Equivalent for construct
for (x=0 ; x<12 ; x++ ) { printf("%d ",x); }
Initial value
Test condition
Body of loop (may be 0, 1, or several statements)
End of loop change
1/28/2018
ECE Application Programming: Lecture 5

38. Repetition with for loop
Rewriting squares 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
for (i = 0; i <= 10; i++) { // Loop until i > 10
iSquared = i * i;
printf("%2d%10d\n", i, iSquared); }
return 0;
1/28/2018
ECE Application Programming: Lecture 5

39. Repetition with for loop (cont.)
Generalizing program:
int main() {
int i; // Number to square
int iSquared; // Square of the number
int iStart; // Initial value
int iStop; // Last value
int iStep; // Increment
printf(“Enter start, stop, and increment: “);
scanf(“%d %d %d”, &iStart, &iStop, &iStep);
printf(" i i^2\n"); // Column headings
// Compute and display the squares of numbers iStart to iStop
// with increment iStep
for (i = iStart; i <= iStop; i += iStep) {
iSquared = i * i;
printf("%2d%10d\n", i, iSquared); }
return 0;
1/28/2018
ECE Application Programming: Lecture 5

40. ECE Application Programming: Lecture 5
Example: for loops
What does each of the following print?
for (i=5; i<40; i+=8) { printf("%d ", i); }
for (i=10; i<=100; i=i+10) { if (i%20) printf("%d ", i); }
for (i=5; i<10; i+=i%2) { printf("%d ", i++); }
for (i=-5; i<-10; i--) { printf("%d ", i); }
1/28/2018
ECE Application Programming: Lecture 5

41. ECE Application Programming: Lecture 5
Example solution
What does each of the following print?
for (i=5; i<40; i+=8) { printf("%d ", i); }
OUTPUT:
for (i=10; i<=100; i=i+10) { if (i%20) printf("%d ", i); }
OUTPUT:
for (i=5; i<10; i+=i%2) { printf("%d ", i++); }
OUTPUT: 5 6 8
for (i=-5; i<-10; i--) { printf("%d ", i); }
OUTPUT: No output
1/28/2018
ECE Application Programming: Lecture 5

42. ECE Application Programming: Lecture 5
Early loop exits
May want to
Exit loop early
Skip rest of loop body but run more iterations
Early loop termination: break
Stops loop immediately and goes to statement after loop end
Skipping loop body: continue
Stops current iteration, but will execute following iteration if loop is not done
In for loop, statement at end of loop still executed
E.g. i++ always executed in: for (i = 0; i < 10; i++)
1/28/2018
ECE Application Programming: Lecture 5

43. ECE Application Programming: Lecture 5
Using break with loops
The keyword break will exit a loop immediately
Similar to its use with switch/case
Modifying grade averaging program:
// 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))
break;
// Grade must be valid if you reach this point
gradeSum = gradeSum + grade; // Accumulate grade
gradeCount = gradeCount + 1; // Inc. grade count
} while (1); // Condition is unnecessary
// Use “infinite” loop
1/28/2018
ECE Application Programming: Lecture 5

44. Using continue with loops
Want to leave loop body but execute more iterations
Use continue—will end loop and then evaluate condition
Example: Read and sum 10 non-zero numbers
n = 0;
sum = 0;
while (n < 10) {
scanf(“%d”, &i);
if (i == 0)
continue;
sum = sum + i;
n++;
/* continue jumps to this point */ }
1/28/2018
ECE Application Programming: Lecture 5

45. ECE Application Programming: Lecture 5
Next time
PE3 (Loops)
Functions
Exam 1 Review
Reminders
Program 4 due today
Program 5 due Tuesday, 7/31
Exam 2 Thursday, 8/2
1/28/2018
ECE Application Programming: Lecture 5