142 H -1

ConditionalsEssential feature for a computer language

Conditional Statement:allows the computer to choose betweenseveral paths depending on the value of a variable or expression:

e.g. if today is my birthday, add 1 to my age

if the income is less than $30,000, take for the tax rate 10%else take 15%

C Syntax:

if (temperature > 98.6) printf("you have a fever.\n");

if (condition) statement;

executed if and only ifthe condition is true

142H -2

Flow ChartIn terms of a flow chart,

if (x < 100) x = x+1;

translates intox = x+1

True x< 100

False

Writing a conditional expression:

what follows the if

• Also called logical or boolean expressions

• Made of variables, constants, arithmetic expressions and relational operators

> for greater than< for less than<= for less than or equal to

!= for not equal to== for equal to

>= for greater than or equal to

142 H -3

• Use boolean operators to write conditions

&& for AND|| for OR! for NOT

Recall the logical table:

FFTT

TFFF

TTTF

P, Q are conditionsT and F stand for True and False

P Q ! P P && Q P || QT TT FF TF F

Examples

temperature > 90.0 && humidity > 50.0

!(salary<30000 || exemptions<4)

number_of_children==3 && age>35

Check the precedence order (inside cover of text)

to be safe (and clear), use parentheses

142 H-4

Value of conditional expressions

Recall that expressions have a value.

What is the value of a conditional expression?

Can think of it as TRUE or FALSE

(however, there is no such type in C… but there is in C++)

In Reality: it is an integer

in C, FALSE is 0

TRUE is non zero(frequently 1, and always 1 if the result of the relational operator)

Try these (but don’t use them!)

if (0) printf(“0 is true”);if (!0) printf(“0 is false”);if (0.9) printf(“is printf executed?”);

Syntax of the if statement (1)

142 H-5

Examples:

temperature = 99.5;

if (temperature>98.6) printf(“You have a fever”);

pulse = 80.;

controlflow

temperature = 98.1;

if (temperature>98.6) printf(“You have a fever”);

pulse = 80.;

controlflow

(printf isnot executed)

if (temperature>98.6){ printf(“You have a fever”); aspirin = aspirin - 2;}

A block can contain• any number of statements•any kind of statements

Can have no statements in a block:

if (x>100) {} /* OK */

Can also have more than 1 statementuse a compound statement (also called “block”)

142 H-6

Syntax of the if statement (2)

block

Some Examples (1)

142 H-7

_ An ATM program

if (balance >= withdrawal){ balance = balance - withdrawal; dispense_funds(withdrawal);}

function called to give themoney to the ATM user.

Can’t omit theparentheses (compilation error)

What happens if we omit the braces {}?

dispense_funds is executed even whenbalance is less than withdrawal!

/* one version */if (x>=0) abs=x;if (x<0) abs=-x;

/* another version */abs = x;if (x<0) abs=-x;

double abs(double x){ if (x<0) x=-x; return x;}

As a function:

local variable.Nothing is changed inmain

_ Computing an absolute value

Some Examples (2)

142 H-8

142 H-9

if - else statement

to compute the absolute value of x:

if (x>=0) abs = x;else abs = -x;

printf(“Absolute value of %.2f is %.2f”,x,abs);

TrueFalse

Control Flow:

get here whether the condition istrue or false

x >= 0

abs = xabs = -x

printf...

142 H-10

Formatting if statements

Style: if (condition){ statement1; statement2; ...}else{ statement3; statement4; ...}

e.g. #define BILL_SIZE 20...if (balance >= withdrawal){ balance = balance - withdrawal; dispense_funds(withdrawal);}else{ if (balance >= BILL_SIZE) printf(“Try a smaller amount”); else printf(“Sorry, you’re really broke!”);}

142 H-11

if statement and { } (1)

Consider:

if (x==5) if (y==5) printf(“Both are 5\n”); else printf(“x is 5, but y is not\n”);else if (y==5) printf(“y is 5, but x is not\n”); else printf(“Neither is 5\n”);

OK

if statement and { } (2)

However:

if (x==5) if (y==5) printf(“Both are 5\n”);else printf(“Is anybody 5?\n”);

Which ifis it associatedto?

if (x==5){ if (y==5) printf(“Both are 5\n”);}else printf(“Is anybody 5?\n”);

Write

142 H-12

This one!

142 H-13

Matching elses with ifs

• Each else matches some if in the same block

if { if if else { if if else else} else } else

• Within the same block, read the ifs and elses left to right, matching each else to the closest unmatched if

• Some ifs may not be matched to any else

if if if else else

if if else if

code within { }

unmatched

unmatched