1. ECE 103 Engineering Programming Chapter 19 Nested Loops
Herbert G. Mayer, PSU CS
Status 6/19/2015
Initial content copied verbatim from
ECE 103 material developed by
Professor Phillip PSU ECE

2. Syllabus
Nested while and for Loops
Tracing Through a Loop
Examples

3. Nested Loops
A loop (e.g., while, do-while, for) can be placed within the body of another loop
Nested loops can be multiple levels deep (e.g., loop within a loop within a loop, etc.)
For a nested loop, a break statement only exits out of the loop that contains the break

4. Nested while Loop: Typical 2-level nested while loop: while (expr1) {
Update expr2; }
update expr1;
← outer loop
← inner loop

5. Example: 2-level nested while loop
#include <stdio.h>
int main (void)
{ // main
int row, col;
row = 0;
while( row < 3 ) { // note: magic number!
col = 0;
while( col < 3 ) { // ditto: Magic number!
printf( "(%2d %2d) ", row, col );
col++;
} //end while
printf( "\n” );
row++;
return 0;
} //end main
( 0 0) ( 0 1) ( 0 2)
( 1 0) ( 1 1) ( 1 2)
( 2 0) ( 2 1) ( 2 2) 4

6. Nested for Loop: Typical 2-level nested for loop:
for ( expr1a; expr2a; expr3a ) { :
for ( expr1b; expr2b; expr3b ) }
← outer loop a
← inner loop b

7. Example: 3-level nested for loop
// level and R are defined
for( level = 1; level <= 3; level++ ) {
printf( "Level = %d\n", level );
for( R = 0; R < 3; R++ ) {
for( C = 0; C < 3; C++ )
printf( "%3d ", R*C*level );
printf( "\n” );
} //end for 6

8. Example: 3-level nested for loop
for( level = 1; level <= 3; level++ ) {
printf("Level = %d\n", level );
for( R = 0; R < 3; R++ ) {
for( C = 0; C < 3; C++ ) {
printf( "%3d ", R*C*level );
} //end for
printf( "\n” );
Level = 1
Level = 2
Level = 3 7

9. Example: for loop inside while loop
/* computes factorial( num ) */
#include <stdio.h>
int main( void )
{ // main
int num; /* Input value */
long prod = 0; /* Accumulator for partial product */
int i, Done = 0; /* Index and flag */
while( !Done ) {
printf( "Enter number: ” );
scanf( "%d", &num );
if( num < 0 ) { // Exit if user enters negative value
Done = 1;
}else{
prod = 1;
for( i = 1; i <= num; i++ ) {
prod *= i;
} //end for
printf( "factorial of %d = %d\n\n", num, prod );
} //end if
} //end main 8

10. Tracing Through a Loop
You can get a good understanding of loops by writing out a table of values
The table shows the current state at the start of each iteration
The columns of the table list the variables or calculations of interest at each iteration

11. for( m = 1; m <= 3; m++ ) { Example: s = m*m;
printf("%d %d\n", m, s); } //end for
Iteration # m
s = m*m
printf(…) 1
1 1 2 4
2 4 3 9
3 9 10

12. Suppose you have a 2-level nested loop
Suppose you have a 2-level nested loop. The outer loop performs m iterations, and the inner loop does n iterations:
This means:
For each iteration of the outer loop, the inner loop is executed n times
Total number of iterations is m×n

13. Example: for( m = 1; m <= 3; m++ ){ s = 0;
for( n = 1; n <= 3; n++ ) { // number iterations SAME
s += m*n;   printf( "%d %d %d\n", m, n, s );
} //end for } //end for
Iteration # m n
s += m*n
printf(…) 1 2 3 6 4 5 12 7 8 9 18 12

14. Example:
for( m = 1; m <= 3; m++ ) {
s = 0;
for( n = m; n <= 3; n++ ) {
s += m*n;   printf("%d %d %d\n", m, n, s);
} //end for } //end for
Here the number of iterations in the inner loop is controlled by the outer loop index
Iteration # m n
s += m*n
printf(…) 1 2 3 6 4 5 10 9 13