Selection Making Decisions

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Logical Data in C

Data is called logical if it conveys avalue of true or false.

If a data value is zero, it is consideredfalse. If a value is non-zero, it is

considered true.

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Logical operators

The relational operators are the Logical AND, LogicalOR and and the NOT operator.

Value 1 Value2 && | | ! (Value 1)

T T T T F

T F F T F

F T F T TF F F F T


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Logical Operator Examples

int a = 7, b=0;

a && b // prints 0

a || b // prints 1

!a && b // prints 0!a || b // prints 0

a && !b // prints 1

!(a || b) // prints 0

!(a && b) // prints 1

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Relational operators

Six relational operators support logical relationships.They are all binary operators that accept twooperands and compare them. The value of an

expression involving a relational operator is eithertrue or false.

The operators are:< less than

greater than the other two

>= greater than or equal

== equal

!= not equal


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Examples

x = 1;

y = 4;

z = 14;

x 1 0

!( x = 1 && y == 3 || z < 14 0


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Simplifying Expressions

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Two-way Selection

The basic decision statement in the computer is thetwo-way selection. The decision is described to thecomputer as a conditional statement that can be

answered either true or false. If the answer is true,one or more action statements is executed. If theanswer is false, then a different action or set ofactions is executed. Regardless of which set of

actions is executed, the program continues with thenext statement, after the selection.


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ifstatement

if is a mechanism for executing a statement/sconditionally.

If expression

action 1

else

action 2

In this case only action 1 or action 2 is executed,

depending on whether the expression is true or false.


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ifstatement cont.

Usually the expression in an ifstatement is a relational, equality or

logical expression.if j < k

let min = j

if j < kprint j is smaller than k\n


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ifstatement cont.

if j < k

then

let min = j

print j is smaller than k

elseprint j is larger than k


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Nested ifstatements

if expression 1

then

action 1if expression 2

then

action 2

elseaction 3

else

action 4


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Dangling else problem

Once you start nesting ifelse statements, you canencounter the dangling else problem. This problem

is created when there is no matching else for every if.

In some programming languages:

an else is always paired with the most recent unpaired if

This might result in your program not matching youractual intent.


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Dangling else problem

Write the code for the following pseudocode:

if x is greater to 5

if x is less than 10print that the value is between 5 and 10

else

print that the value is less than 5

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Range Checks

Range CheckCompare a variable to aseries of values between limits

To perform a range check, makecomparisons using either the lowest orhighest value in each range of values

you are using

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Common Errors with Range

Checks Ask one question too many

Path is dead or unreachable

Asking unnecessary questions

Never ask a question is there is only onepossible answer or outcome

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The case construct

Case structuresimply provides a convenientalternative to using a series of decisions when youmust make choices based on the value stored in a

single variable

switch ( expression )

case case-label-1 :

statement-list

case case-label-2 :

statement-list

...

default :

statement-list


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The switch statement cont.

Switch works by evaluating expression, passingcontrol to the case labeled with its value (or thedefault) and executing the cases statement-list.

The case-labelshave to have expressions that thecompiler can evaluate to an integer constant, and allthe case-labelshave to be unique.


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The switch statement cont.switch ( operator )

case '+' :// when the value of operator is +

result = op1 + op 2

end +

case '-' :// when the value of operator is -

result = op1 - op 2

end -case '*' :// when the value of operator is *

result = op1 * op 2

end *

case '/' :// when the value of operator is /

if ( op2 != 0.0 )

result = op1 / op 2;end /

default :// when the value of operator is not any of the above

print Unknown operator

result = 0.0

end default

end switch


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Summary Every decision you make in a compute program involves

evaluating a Boolean expression For any two values that are the same type, you can use the

logical comparison operators to decide whether the two valuesare equal, the first is greater than the second, or the first is lessthan the second

An AND decision occurs when two conditions must be true inorder for a resulting action to take place

In an AND decision, first ask the question that is less likely to betrue

Most programming languages allow you to ask two or morequestions in a single comparison by using a logical AND

operator


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Summary

When you must satisfy two or more criteria to initiate an event ina program, you must make sure that the second decision ismade entirely within the first decision

An OR decision occurs when you want to take action when one

or the other of two conditions are true In an OR decision, first ask the question that is more likely to be

true Most programming languages allow you to ask two or more

questions in a single comparison To perform a range check, make comparisons with either the

lowest or highest value in each range of values you are using

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Summary

Common errors that occur when programmers perform rangechecks include asking unnecessary and previously answeredquestions

The case structure provides a convenient alternative to using a

series of decisions when you must make choices based on thevalue stored in a single variable

A decision table is a problem-analysis tool that consists ofconditions, possible combinations of Boolean values for theconditions, possible actions based on the conditions, and the

action that corresponds to each Boolean value of each condition

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Selection Making Decisions

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