lecture #6 selection control structures cs602 (1)

8/10/2019 Lecture #6 Selection Control Structures CS602 (1)

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Lecture 6Selection Control Structures

C Programming

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Lecture 6 Topics

Nested Structures

Switch statement

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Multi-alternative Selection is also called multi-way branching , and can beaccomplished by using NESTED if statements.

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Nested if Statements if ( Expression1 )

Statement 1

else if ( Expression2 ) Statement 2

.

.

.

else if ( ExpressionN )Statement N

else Statement N+1

EXACTLY 1 of these statements will be executed. 4


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Nested if StatementsEach Expression is evaluated in sequence, until some Expression is found that is true .

Only the specific Statement following that particulartrue Expression is executed.

If no Expression is true , the Statement following thefinal else is executed.

Actually, the final else and final Statement areoptional . If omitted, and no Expression is true, thenno Statement is executed.

AN EXAMPLE . . .5


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Multi-way Branchingif ( creditsEarned >= 90 )

printf ( Fourth year student ) ;

else if ( creditsEarned >= 60 )

printf ( Third year student ) ;

else if ( creditsEarned >= 30 )

printf ( Second year student ) ;else

printf ( First year student ) ;

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Example 1Display one word to describe the int value of

number as Positive , Negative , or Zero

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Answer

if (number > 0)printf ( Positive );

else if (number < 0)

printf ( Negative );

else

printf ( Zero ) ;

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Example 2Your city classifies a pollution index:-

less than 35 as Pleasant ,35 through 60 as Unpleasant , and above 60 as Health Hazard.

Display the correct description of the pollution

index value.

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Answer if ( index < 35 )

printf ( Pleasant );

else if ( index


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Example 3Every Sunday thru Thursday you go toclass. When it is raining you take an umbrella.

But on the weekend, what you do dependson the weather.

If it is raining you read in bed.Otherwise, you have fun outdoors.

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// program tells how to spend your day #include < stdio.h >

void main ( ){ int day;

char raining;

printf( Enter day (use 1 for Sunday) ) ;scanf( %d , &day ) ;

printf( Is it raining? (Y/N) ) ;scanf( %c , &raining );

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if ( ( day == 6) || (day == 7) ) /* Fri or Sat */ { if (raining == Y)

printf( Read in bed );

elseprintf( Have fun outdoors );

}

else{ printf( Go to class );if (raining == Y)

printf( Take an umbrella );}

} /* End of Program */

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In the absence of braces, an else is alwayspaired with the closest preceding if that doesntalready have an else paired with it.

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Note on pairing else with if


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Bad Example has output: FAIL float average;

average = 100.0;

if ( average >= 60.0 )

if ( average < 70.0 )printf( Marginal PASS );else

printf( FAIL );

WHY? The compiler ignores indentation and pairsthe else with the second if.

average

100 0

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To correct the problem, use braces

float average;

average = 100.0;

if ( average >= 60.0 )

{if ( average < 70.0 )

printf ( Marginal PASS );

}else

printf ( FAIL ) ;

average

100 0

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Switch statement

Used to select one of several alternatives

BASED on

the

value

of

a single

variable.

This variable may be an int or a char but NOT a float ( or double).

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The switch Statementswitch (expression) { case constantl : statement 1; break ; case constant2:

statement2;

break; case constant3: statement3; break; case constantX: statementX; break; default : default statement; }

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Multiple cases with same statements are acceptedIf break is omitted from a case, and the program is executing its statements , itwill continue to execute other cases until a break is metdefault can appear in any order


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Examplechar grade ;

printf(Enter your letter grade: );

scanf(%c, &grade);

switch ( grade )

{ case A : printf( Excellent Job);

break;

case B : printf ( Very Good );

break; case C : printf( Not bad );

break;

case F

: printf(Faiing);

break;

default : printf( Wrong Input );

}

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Light bulbs

Write a program to ask the user for the

brightness of a light bulb (in Watts), and print

out the expected lifetime:

Brightness Lifetime in hours 25 2500

40, 60 1000 75, 100 750

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int bright ;printf(Enter the bulb brightness: );scanf(%d, &bright);switch ( bright ) {

case 25 : printf( Expected Lifetime is 2500 hours);break;case 40 :case 60 : printf ( Expected Lifetime is 1000 hours );

break;case 75 :case 100 : printf(Expected Lifetime is 750 hours );

break;default : printf(Wrong Input );

}

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lightBulb.c

Hint :case 25+2 is accepted also bur case 25+x is not valid


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break vs

return

break means exit the switch statement and continue on with the rest of the program.

return means exit the whole program.

They could both be used anywhere in the program.

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Testing Selection Control Structures

to test a program with branches, use enoughdata sets so that every branch is executed atleast once

this is called minimum complete coverage

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How to Test a Program

Design and implement a test plan A test plan is a document that specifies the test

cases to try, the reason for each, and theexpected output

implement the test plan by verifying that theprogram outputs the predicted results

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lecture #6 selection control structures cs602 (1)

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