AEEE 195 – Repetition Structures: Part B

Spring semester 2011

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Outline Repetition structures definition

“while” loop

“do/while” loop

Inifite loop

Arithmetic operators

Examples

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Repetition Structures

Are structures that allow you to repeat execution of the same code

That is, you can execute particular statements more than once in a controlled fashion

Statements are executed as long as some condition remains true

Also known as “loops” Sometimes referred to as “iterations”

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“while” Repetition Structure

Actions will be performed as long as some condition is true

For example (pseudocode),

while (something is true){ do this (again and again...)

}

Can have various kinds of conditions tested

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Parts of a While Loop

Every while loop will always contain three main elements: Priming: initialize your variables. Testing: test against some known condition. Updating: update the variable that is tested.

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Simple While Loop

#include <stdio.h>#define MAX 10main (){

int index =1;  while (index <= MAX) {

printf ("Index: %d\n", index);index = index + 1;

}}

OUTPUT:

Index: 1

Index: 2

Index: 3

Index: 4

Index: 5

Index: 6

Index: 7

Index: 8

Index: 9

Index: 10

1. Priming1. Priming

2. Test 2. Test ConditionCondition

3. Update3. Update

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While Loop Flowchart

1. Priming1. Priming

Set index=1Set index=1

2. Test2. Test

index index

<= 10<= 10

3a. print index3a. print index

3b. Update index = index 3b. Update index = index + 1;+ 1;

TRUETRUE

FALSEFALSE

next iterationnext iteration

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Infinite Loop

Infinite Loop: A loop that never ends. Generally, you want to avoid these! There are special cases, however, when you do

want to create infinite loops on purpose.

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Infinite Loop: Example #1

#include <stdio.h>

#define MAX 10

main ()

{

int index =1;

  while (index <= MAX)

{

printf ("Index: %d\n", index);

}

} Here, part 3 is deleted : the index = index + 1 statement.Here, part 3 is deleted : the index = index + 1 statement.

Index: 1

Index: 1

Index: 1

Index: 1

Index: 1

… [forever]

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Infinite Loop: Example #2

#include <stdio.h>/*no MAX here*/ main (){

int index = 1;  while (index > 0)

{printf ("Index: %d\n", index);index = index + 1;

}}

Here, part 2 has changed : Here, part 2 has changed : the test condition.the test condition.

Index: 1

Index: 2

Index: 3

Index: 4

Index: 5

… [forever]

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“do/while” Repetition Structure Similar to the while structure But instead of testing the condition at the top,

test is at the bottom of the loop Used when you want to execute the loop AT

LEAST ONCE

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do/while pseudocode

do

{

something

} while (condition is true) ;

Note the semicolon at the end

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do/while example

int iCounter = 1; do { printf( “%d “, iCounter ); iCounter = iCounter + 1;

} while ( iCounter <= 10 ) ;

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do/while loop Flowchart1. Priming

Set index=1;

3. Test

index

<= 10

2a. print index

2b. Update index = index + 1;

TRUETRUE

FALSEFALSE

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Example: while vs do-while structures

int num;

printf(“Give a positive number: );

scanf(“%d”, &num);

while(num <=0)

{

printf(“Give a positive number: );

scanf(“%d”, &num);

}

printf(“The number is %d\n”, num);

int num;

do

{

printf(“Give a positive number: );

scanf(“%d”, &num);

} while(num <=0);

printf(“The number is %d\n”, num);

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Shortcuts

C provides abbreviations for some common operations Assignment operators Increment/Decrement operators

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Assignment Operators

Abbreviations are provided for the basic binary operations Addition Subtraction Multiplication Division Modulus (%)

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Addition Assignment Operator Instead of writing

iCount = iCount + 1; You can write

iCount += 1 ; Adds the value of the expression on the right

to the variable on the left and stores the new total in the variable on the left

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Subtraction Assignment Operator Instead of writing

iCount = iCount - 1; You can write

iCount -= 1 ; Subtracts the value of the expression on the

right from the variable on the left and stores the new total in the variable on the left

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Multiplication Assignment Operator Instead of writing iCount = iCount * 2; You can write iCount *= 2; Multiplies the variable on the left by the value

of the expression on the right and stores the new total in the variable on the left

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Division Assignment Operator Instead of writing

iCount = iCount / 2; You can write

iCount /= 2; Divides the variable on the left by the value of

the expression on the right and stores the new total in the variable on the left

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Modulus Assignment Operator Instead of writing iSeconds = iSeconds % 60; You can write iSeconds %= 60; Takes the remainder of dividing the variable

on the left by the value of the expression on the right, and stores the new total in the variable on the left

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Examples

Assume variable int c = 10;

3 to cc = c % 7;c %= 7;10%=

2 to cc = c / 5;c /= 5;10/=

60 to cc = c * 6;c *= 6;10*=

7 to cc = c – 3;c -= 3;10-=

17 to cc = c + 7;c += 7;10+=

AssignsMeaningSample

Initial Value Operator

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Increment and Decrement Operators C provides unary increment operators ++ and

decrement operators –- (no spaces between them)

Increment operators add 1 Decrement operators subtract 1 Not for other assignment operators (*, /, %)

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Increment Operator ++

Instead of writing iCount = iCount + 1; or

iCount += 1; You can write

iCount++ ; (post-increment) or

++iCount ; (pre-increment)

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Post-increment

The ++ operator is after the variable

Causes the initial value of the variable to be used in the expression where it appears, and THEN adds the 1 to the variable

For example,int iCount = 5;printf( “%d\n”, iCount++ );

Would print 5… but iCount is incremented to 6 after the statement

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Pre-increment

The ++ operator is before the variable

Adds 1 to the initial value of the variable BEFORE it is used in the expression where it appears

For example,int iCount = 5;printf( “%d\n”, ++iCount);

Would print 6… and iCount is incremented to 6 after the statement

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More examples ++post-increment vs. pre-increment

int iTotal = 0;int iCount = 5;

iTotal = iCount++ * 2;printf( “%d\n”, iTotal);printf( “%d\n”, iCount);

Would print 10 for iTotal and then 6 for iCount

iTotal = ++iCount * 2;printf( “%d\n”, iTotal);printf( “%d\n”, iCount);

Would print 14 for iTotal and then 7 for iCount

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Decrement Operator --

Similar to increment in syntax/operation Instead of writing iCount = iCount - 1; or

iCount -= 1; You can write

iCount-- ; or --iCount ;

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Post-decrement

The -- operator is after the variable Causes the initial value of the variable to be used in

the expression where it appears, and THEN subtracts the 1 from the variable

For example,int iCount = 5;printf( “%d\n”, iCount-- );

Would print 5… but iCount is decremented to 4 after the statement

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Pre-decrement

The -- operator is before the variable Subtracts 1 from the initial value of the variable

BEFORE it is used in the expression where it appears

For example,int iCount = 5;printf( “%d\n”, --iCount);

Would print 4… and iCount is decremented to 4 after the statement

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More examples --post-decrement vs. pre-decrement

int iTotal = 0;int iCount = 5;

iTotal = iCount-- * 2;printf( “%d\n”, iTotal);printf( “%d\n”, iCount);

Would print 10 for iTotal and then 4 for iCount

iTotal = --iCount * 2;printf( “%d\n”, iTotal);printf( “%d\n”, iCount);

Would print 6 for iTotal and then 3 for iCount

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Summary Table

Subtract 1 from c THEN use new value of c in

expression

--c--

Use value of c in expression THEN subtract 1 from c

c----

Add 1 to c THEN use the new value of c in expression

++c++

Use value of c in expression THEN add 1 to c

c++++

ExplanationSampleOperator