1. ECE 103 Engineering Programming Chapter 18 Iteration
Herbert G. Mayer, PSU CS
Status 7/6/2016
Initial content copied verbatim from
ECE 103 material developed by
Professor Phillip PSU ECE

2. Syllabus
Basic Loops
while
do-while
for
break, continue
Examples

3. Basic Loops
Loops are another method of changing the execution sequence of a program, by iterating
A loop is a section of code that is executed repeatedly as long as a condition remains true
Once the expression becomes false (fails), the program ends the loop
In C one can “hand-manufacture” loops via Goto Statements, viewed as sinfully evil; that is how a computer executes loops, by so-called “branch” or “jump” instructions

4. Typical loops require these operations:
pre-code
Setup
Test expression evaluation
Update
Setup
Stay in loop
TRUE
Statements
expression
FALSE
Update
Each single pass through a loop is called an iteration
Exit loop
post-code

5. A loop consists of several parts:
Setup Before entering the loop, any values referenced by the test expression should be initialized.
Test expression evaluation An expression is tested to see if it is true or false.
true : Repeat the body of the loop
false : Exit the loop
Body The body contains the code that is to be repeated. It can be a single statement or a statement block.
Update Within the loop, there should be a way to update the test expression so that it will fail eventually.

6. Test expression must evaluate to either true (non-zero) or false (zero)
Test expression is evaluated every iteration – either at the start or end of the loop
The test is also called the termination condition
An infinite loop occurs if the termination condition is never satisfied

7. Loop Statements in C General loop types: Conditional loops
Statements are repeated for as long as a test expression remains true.
→ while, do-while
Counted loops
Statements are repeated a specified number of times.
→ for 6

8. while pre-code; while (expression) Statement; post-code;
Use if the number of iterations is not known ahead of time
Variables in expression must be initialized before entering the loop
Loop body may be executed 0 or more times
Note that the Statement may be a compound statement { }
Execution Sequence
Execute pre-code
At start of loop, evaluate expression
If expression is true Execute Statement Goto Step (b) else Exit the loop and goto Step (d)
Execute post-code

9. Example: k = 0; while( k < 5 ) k = k + 1;
// not recommended style of while
while( k < 5 ) {
printf( "k = %d\n", k );
} //end while
num = 0;
while( num < 5 ) {
printf( "num = %d\n", num++ );
z = 5;
while( z-- )
printf( "z = %d\n", z );
// not recommended style, no { }
#include <stdio.h>
int main (void)
{ // main
int num; /* Input from user */
int sum = 0; /* Running sum */
int Done = 0; /* Flag */
/* This adds up numbers > 0 */
while( !Done ) {
printf( "Enter next number: ” );
scanf("%d", &num);
if ( num < 1 )
Done = 1;
else
sum += num;
// not recommended style of if
} //end while
printf( "sum = %d\n", sum );
return 0;
} //end main 8

10. Example: // Echo an input stream ( capitalized, 'S' replaced by '$’ )
#include <stdio.h>
#include <ctype.h>
int main( void )
{ // main
int ch; /* Holds input character */
/* Read input stream until end-of-line is detected */
while( ( ch = getchar() ) != '\n' ) {
ch = toupper( ch );
if( ch == ‘S' ) {
ch = '$';
} //end if
printf( "%c", ch );
} //end while
printf( "\n” );
return 0;
} //end main
My sister is funny!
MY $I$TER I$ FUNNY! 9

11. do-while pre-code; do Statement; while (expression); post-code;
The do-while loop tests the expression at the end
Hence will be done at least once!
Note semicolon after ending parenthesis, as that is the end of the do-while statement
Execution Sequence
Execute pre-code
Enter the loop and execute Statement
At end of loop, evaluate expression
If expression is true Goto Step (b) else Exit the loop and goto Step (e)
Execute post-code

12. Example: k = 0; do k = k + 1; while( k < 5 ); do {
printf( "k = %d\n", k );
} while( k < 5 );
#include <stdio.h>
int main( void )
{ // main
int num; /* Input from user */
int sum = 0; /* Running sum */
int Done = 0; /* Flag */
/* This adds up numbers > 0 */
do {
printf( "Enter next number: ” );
scanf( "%d", &num );
if( num < 1 ) // not recommended!
Done = 1;
else
sum += num;
} while( !Done );
printf( "sum = %d\n", sum );
return 0;
} //end main 11

13. for pre-code; for( expr1; expr2; expr3 ) Statement; post-code;
Used if the number of iterations is computable, or known ahead of time.
Loop body may be executed zero or more times
Execution Sequence
expr1 → setup expression
expr2 → test expression
expr3 → update expression
Execute pre-code
At start of loop, evaluate expr1
If expr2 is true Execute Statement Evaluate expr Goto Step (c) else Exit the loop and goto Step (d)
Execute post-code

14. Mapping between simple for and while loops:
for( expr1; expr2; expr3) {
Statements; }
expr1; /* Setup */
while( expr2 ) /* Test */ {
Statements;
expr3; /* Update */ }

15. → Infinite loop: expr2 is never false!
Example:
for( k=0; k<=3; k++ ) ...
→ 4 iterations, values of k : 0, 1, 2, 3
for( t=1; t<11; t+=2 ) ...
→ 5 iterations, values of t : 1, 3, 5, 7, 9
for( x=1.0; x<=2.0; x+=0.5 ) ...
→ 3 iterations, values of x : 1.0, 1.5, 2.0 // careful “float”
for( cnt = 2; cnt >= -1; cnt-- ) ...
→ 4 iterations, values of cnt : 2, 1, 0, -1
for( ; ; )
→ Infinite loop: expr2 is never false! 14

16. Be careful counting iterations, especially when starting at 0, or using < versus <= in expressions!
Example:
for( k=1; k<=5; k++ ) ...
→ 5 iterations, values of k : 1, 2, 3, 4, 5
for( k=1; k<5; k++ ) ...
→ 4 iterations, values of k : 1, 2, 3, 4
for( k=0; k<=5; k++ ) ...
→ 6 iterations, values of k : 0, 1, 2, 3, 4, 5
for( k=0; k<5; k++ ) .. .
→ 5 iterations, values of k : 0, 1, 2, 3, 4 15

17. Example: for( n = 1; n < 5; n++ ) printf( "n = %d\n", n );
s_idx = 2;
e_idx = 4;
for( n = s_idx; n <= e_idx; n++ ) {
x = exp( n );
printf( "%d %f\n", n, x );
} //end for
for( T = 10; T >= 0; T-- )
x = sin( T*pi / );
prod = 1;
for( a = 1; a <= 3; a++ )
prod = prod * a;
#1 : a → 1
prod = 1 * 1 → 1
#2 : a → 2
prod = 1 * 2 → 2
#3 : a → 3
prod = 2 * 3 → 6
for( ch='A'; ch<='Z'; ch++ )
printf( "%c", ch );
/* infinite loop */
for( ;; )
printf( "Hello\n” ); 16

18. Variable Declarations in for Loops
Variable declarations are allowed in the first expression of the for loop header. Typically used to declare the iteration variable
Note: NOT in old versions of C!
The scope of the variable extends only to the body of the loop
C99
/* C90 version */
#include <stdio.h>
int main( void )
{ // main
int k;
for( k = 0; k < 3; k++ )
printf("%d\n", k);
// end for
printf( "%d\n", k ); /* Works */
return 0;
} //end main
// C99 version
#include <stdio.h>
int main( void )
{ // main
// This is legal in C99
for( int k = 0; k < 3; k++ )
printf("%d\n", k);
// end for
// Compiler fails on this k
printf( "%d\n", k );
return 0;
} //end main 17

19. break Statement
break causes an immediate exit from the loop that contains it; note: innermost loop!
Once a break is encountered, execution goes to the first statement following the loop body
break statements work with for, while, and do-while loops

20. Example: /* Use a state variable */ done = 0; /* State */ sum = 0;
while( !done ) {
printf( "x? ” );
scanf( "%d", &x );
sum = sum + x;
if( sum > 1000 )
done = 1;
} //end while
/* Use a break statement */
#define TRUE 1
// assuming sum, x declared
sum = 0;
while( TRUE ) {
printf( "x? ” );
scanf( "%d", &x );
sum = sum + x;
if( sum > 1000 )
break;
} //end while 19

21. continue Statement The continue statement restarts a loop
Once a continue is encountered, execution proceeds to the start of the loop
continue statements work with for, while, and do-while loops.
If continue is used in a for statement, the setup expression is not executed again

22. Example: #include <stdio.h> #define TRUE 1 int main (void)
float x; /* Input value */
float sum = 0.0; /* Accumulates partial sums */
while( TRUE ) {
scanf( "%f\n", &x );
if( x < 10.0 || x > 95.0 ) // Check for outside range
continue;
sum += x;
if( sum > ) /* Check if sum limit is reached */
break;
} //end while
printf( "sum = %f\n", sum );
return 0;
} //end main 21