CS 161Introduction to Programming

and Problem Solving

Chapter 16Iterative Statements in C++

Herbert G. Mayer, PSUStatus 10/30/2014

Initial content copied verbatim fromECE 103 material by prof. Phillip Wong @ PSU

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

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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

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Setup

expression

post-code

StatementsTRUE

FALSE

Typical loops require these operations:

• Setup

• Test expression evaluation

• Update

Update

pre-code

Exit loop

Stay in loop

Each single pass through a loop is called an iteration

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A loop consists of several parts: Setup

Before entering the loop, any values referenced by the test expression should be initialized.

Test expression evaluationAn expression is tested to see if it is true or false.

true : Repeat the body of the loop false : Exit the loop

BodyThe body contains the code that is to be repeated. It can be a single statement or a statement block.

UpdateWithin the loop, there should be a way to update the test expression so that it will fail eventually.

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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

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Loops in C++

General loop types: Conditional loops

Statements are repeated for as long as a test expression remains true.→ while, do-while

Counted loopsStatements are repeated a specified number of times.→ for

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while

pre-code;

while ( expression )Statement// ; included in statement

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

a) Execute pre-codeb) At the loop head, evaluate expressionc) If expression is true

Execute Statement Include user code back to Step (b)else Exit the loop and continue at Step (d)

d) Execute post-code

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Example:k = 0;while( k < 5 )

k = k + 1;// not recommended style of while

k = 0;while( k < 5 ) {

k = k + 1;printf( "k = %d\n", k );

} //end while

num = 0;while( num < 5 ) {

printf( "num = %d\n", num++ );} //end while

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;

elsesum += num;

// not recommended style of if} //end while

 printf( "sum = %d\n", sum );

return 0;} //end main

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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( ’S’ == ch ) { // ‘S’ ch = ’$’;} //end ifprintf( "%c", ch );

} //end whileprintf( "\n” );return 0;

} //end main

My sister is funny!MY $I$TER I$ FUNNY!

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do-while

pre-code;

doStatement// ; included in 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

a) Execute pre-codeb) Enter the loop and execute Statementc) At end of loop, evaluate expressiond) If expression is true

Goto Step (b)else Exit the loop and goto Step (e)

e) Execute post-code

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Example:k = 0;do

k = k + 1;while( k < 5 );

k = 0;do {

k = k + 1;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

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for

pre-code;

for (expr1; expr2; expr3)Statement;

post-code;

Often used if the number of iterations is known ahead of time. Loop body may be executed zero or more times

Execution Sequence

expr1 → setup expressionexpr2 → test expressionexpr3 → update expression

a)Execute pre-codeb)At start of loop, evaluate expr1c)If expr2 is true Execute Statement Evaluate expr3 Goto Step (c)else Exit the loop and goto Step (d)d)Execute post-code

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Mapping between simple for and while loops:

for( expr1; expr2; expr3 ){

Statements;}

// only required:for( expr1; expr2; expr3 )

Statement;

expr1; /* Setup */

while( expr2 ) /* Test */{

Statements;

expr3; /* Update */}

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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!

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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

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Example:for( n = 1; n < 5; n++ )

printf( "n = %d\n", n );

for( n = 1; n < 5; ++n )printf( "n = %d\n", n );

for( n = 1; n < 5; )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 / 180.0 );

prod = 1;for( a = 1; a <= 3; a++ )

prod = prod * a;

#1 : a → 1prod = 1 * 1 → 1

#2 : a → 2prod = 1 * 2 → 2

#3 : a → 3prod = 2 * 3 → 6

for( ch='A'; ch<='Z'; ch++ )printf( "%c", ch );

/* infinite loop */for( ;; )

printf( "Hello\n” );

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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

/* C90 version */#include <stdio.h>

int main( void ){ // main

int k;

for( k = 0; k < 3; k++ )printf("%d\n", k);

// end forprintf( "%d\n", k ); /* Works

*/

return 0;} //end main

// C99 version#include <stdio.h>

int main( void ){ // main

// This is legal in C99for( int k = 0; k < 3; k++ )

printf("%d\n", k);// end for// Compiler fails on this kprintf( "%d\n", k );

return 0;} //end main

C99

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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, anddo-while loops

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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 1sum = 0;

while( TRUE ) {printf( "x? ” );scanf( "%d", &x );sum = sum + x;

if( sum > 1000 )break;

} //end while

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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

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Example:#include <stdio.h>#define TRUE 1

int main (void){ // main

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 > 250.0 ) /* Check if sum limit is reached */

break;} //end whileprintf( "sum = %f\n", sum );return 0;

} //end main