ekt120 week02 selection

8/12/2019 Ekt120 Week02 Selection

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UniMAP Sem II-09/10 EKT120: Computer Programming 1

Week 3Selection Structures


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Outline Recall selection control structure Types of selection

One-way selection Two-way selection Multi-selection Compound statement

Nested if Conditional operator Switch structure


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Recall..

Selection Structure Used to choose among alternative

courses of action

C has three types: if, if..else, andswitch


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The ifselection structure

ifstructure is a single-entry/single-exit

structure

true

false

grade >= 60 print Pass

If students grade is greater than or equal to 60

Print Pass


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The if..elseselection

structure Specifies an action to be performed

both when the condition is trueand

when it is false

truefalse

print Fail print Pass

grade >= 60

If students grade is greater than

or equal to 60

print Pass

else

print Fail


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Selection Statements Used to control the flow of a program

Also called as decision orbranches

Branches are conditions or choices usedto enable selection of program flow


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Types of selection One-way selection = if

Two-way selection = if..else

Multi-selection

Nested if

Switch structure =switch


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One-way Selection = if

In C, a conditionis represented by a logical (Boolean)expression

trueand falseare logical (Boolean) values

The syntax of one-way selection is:

if (expression) statement;

If the value of the expression is true, statement is

executed; if false, statement is not executed and the computer

goes on to the next statement in the program.


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true

false

grade >= 60 print Pass

If students grade is greater than or equal to 60

Print Pass


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..

if(grade >= 60)

printf(Pass);

..

..


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Another example:

char grade;

if(markah>= 90)

grade = 'A';

...

printf(Grade is : %c\n, grade);


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Another example:

if (temperature is greater than 70 degree

and it is not raining) recommended activity is golfing

bool rain=false;

if((temp > 70) && !(rain))

printf(recommended activity is golfing);


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Common Errors

if score >= 90 //no parentheses

grade = 'A';

if(score >= 90);//; not here

grade = 'A';


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Two-way Selection =if..else

The syntax of two-way selection is:

if (expression)

statement1;

else

statement2;

If the value of the expression is true,statement1 is executed;

if false, statement2 is executed


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truefalse

print

Fail

print

Pass

grade >=

60

If students grade is greater than or equal

to 60

print Pass

else

print Fail


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if(grade >=60)

printf(Pass);else

printf(Fail);


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Another example:if (hour > 40.0) //Line 1

wages = 40.0 * rate +1.5 * rate * (hour - 40.0); //Line 2

else //Line 3

wages = hour * rate; //Line 4

If hour is 50, then the statement at Line 2is

executed If hour is 30, then the statement at Line 4is

executed


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Multi-selection = if-else if

The syntax is:

if(exp1)

stmt1;else if(exp2)

stmt2;

else if(exp3)

stmt3;

else

stmt n;

An if-else if control structureshifts program control, step by

step, through a series of

statement blocks.


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E.g.

temp display

>30 0c hot

20-30 0c mild

10-20 0c cold

30 Print hottrue

false

temp > 20 Print mildtrue

temp >10 Print cold

Print very cold

true

false

false


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if(temp > 30)printf( hot\n);

else if((temp >=20) && (temp=10) && (temp < 20))

printf(cold\n);else

printf( very cold\n);


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Compound (Block of) Statement A compound statement (also called a

block of statements) takes the form of { statement 1; statement 2; . . .

statement n; }

It is considered a single statement


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Compound (Block of) Statement

Example:if (age > 18){

printf("Eligible to vote\n);printf("No longer a minor\n);

}else{

printf("Not eligible to vote\n);printf(Still a minor\n);

}


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Nested if

When one control statement is within another, it issaid to be nested

if(exp1)

if(exp2)statement1; OR

if(exp1)

{

statement1;if(exp2)

statement2;

}


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Nested if

Example:if (temperature >= 50)

{

if (temperature >= 80)

printf( "Good day for swimming.\n);

else

printf( "Good day for golfing.\n);

}

else

printf("Good day to play tennis.\n);


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Nested if

Another example


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The Conditional Operator (? :)

The syntax of using the conditional operatoris: expression1 ? expression2 : expression3;

This is called a conditional expression. The statement:

if (a >= b)max = a;

else

max = b; Is equivalent to the statement:

max = (a >= b) ? a : b;


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switch Structures

Similar to if-else if control structure The general form (syntax):

switch (expression){

case value1: statements1; break;case value2: statements2; break;..

.case valuen: statementsn; break;default: statements;

}


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switch Structures

The breakstatement has a specialmeaning and may or may not appearafter each statement.

In C, switch, case, break, and defaultare reserved words.

In a switchstructure, first the

expression is evaluated. The value ofthe expression is then used to performthe corresponding action.


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switch Structures

The expression is usually an identifier.

The value of the expression can be only integral.

The expression is sometimes called the selector. Itsvalue determines which statement is selected forexecution.

A particular casevalue should appear only once.

One or more statements may follow a case label, so

you do not need to use braces to turn multiplestatements into a single compound statement.

The breakstatement may or may not appear aftereach statement.


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switch Structures

Example:switch (grade){

case 'A': printf("The grade is A.); break;case 'B': printf("The grade is B.); break;case 'C': printf("The grade is C.); break;case 'D': printf("The grade is D.); break;case 'F': printf("The grade is F.); break;default: printf("The grade is invalid.);

} where, grade is a variable of the type char. If the

value of grade is, say 'A', the output isThe grade is A.


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switch Structures

The switch statement executes according to thefollowing rules: When the value of the expression is matched against a case

value (also called a label), the statements execute untileither a break statement is found or the end of the switchstructure is reached.

If the value of the expression does not match any of thecase values, the statements following the default labelexecute. If the switch structure has no default label, and if

the value of the expression does not match any of the casevalues, the entire switch statement is skipped.

A break statement causes an immediate exit from theswitch structure


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Whats wrong??


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End Week 2

Q & A!

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