COM S 207IF Statement

Instructor: Ying Cai

Department of Computer ScienceIowa State Universityyingcai@iastate.edu

Used to Implement decisions

Two key elements condition body

amount <= balance

balance = balance - amount

true

false

condition

body

if (amount <= balance){ balance = balance – amount;}

if-then

Used to Implement decisions

Two key elements condition body amount <= balance

balance = balance - amount

true false

condition

body

balance = balance – OVERDRAFT_PENALTY

body

if (amount <= balance){ balance = balance – amount;}else{ balance = balance – OVERDRAFT_PENALTY;}

if-then-else

Example

if (originalPrice > 100){ discountPrice = originalPrice – 20;}else{ discountPrice = originalPrice – 10;}

originalPrice

95100105

discountPrice

859085

good to use braces

How about swapping the two statements

if (originalPrice < 100){ discountPrice = originalPrice – 10;}else{ discountPrice = originalPrice – 20;}

originalPrice

95100105

discountPrice

8510085

different when originalPrice is 100

Conditional Operator

condition ? value1 : value2

actualFloor = floor > 13 ? floor-1 : floor;

if (floor > 13) { actualFloor = floor – 1;}else{ actualFloor = floor;}

you will find this in many java programs, but not recommended for now

Relational Operators: Comparing Numbers

if (floor operator 13)

relationaloperators

> greater than< less than>= greater than or equal to<= less than or equal to== equal to!= not equal to

Expression Value

3 <= 4 true3 =< 4 ERROR3 > 4 false4 < 4 false4 <= 4 true3 == 5-2 true3 = 6/2 ERROR“10 > 5 ERROR

• use == for equality • cannot compare a string to a number

Comparing Floating-point Numbers

double r = Math.sqrt(2.0);

if (r * r == 2.0) { System.out.println(“Math.sqrt(2.0) squared is 2.0”);}else{ System.out.println(“Math.sqrt(2.0) squared is” + r * r);}

• Floating-point numbers have only a limited precision, and calculations can introduce round-off errors

In most cases, it does not make sense to compare to two reals exactly. Instead, we should test whether they are close enough: |x-y| < epsilon (where epsilon can be set to very small, say 10e-14)

Comparing Strings

String s1 = “Robert”;String s2 = s1.substring(0, 3); // s2 = “Rob”

if (s2 == “Rob”) // test is falseif (s2.compareTo(“Rob”) == 0) // test is true

• Use method “compareTo” instead of “==“

Nested Branches

if (condition1) { …}else if (condition 2){ …}else if (condition 3){ …}else{ …}

Richter ScaleValue Effect8 most structures fall7 many buildings destroyed6 many buildings damaged4.5 damaged to poorly constructed buildings

Nested Branches

if (richter >= 8.0){ System.out.println(“Most structures fall”);}else if (richter >= 7.0){ System.out.println(“Many buildings destroyed”);}else if (richter >= 6.0){ System.out.println(“many buildings damaged, some collapse”);}else if (richter >= 4.5){ System.out.println(“Damage to poorly constructed buildings”);}

Richter ScaleValue Effect8 most structures fall7 many buildings destroyed6 many buildings damaged4.5 damaged to poorly constructed buildings

Example: Federal Tax Rate Schedule

Single married

Taxable income rate Taxable income rate

<=$21,450 15% <=$35,800 15%

Over $21,450 but <= $51,900

28% Over $35,800 but <= $86,500 28%

Over $51,900 31% Over $86,500 31%

(1)Flow chart

(2)Test cases

(3)Code

your code

input

1. amount of tax due

1. marriage_status2. income

output

Read inputs: (married, income)

Start

single?

income <= 21450

income <= 51900

rate = 15%

rate = 28%

rate = 31%

income <= 35800

income <= 86500

rate = 15%

rate = 28%

rate = 31%

Yes No

Yes

Yes

Yes

Yes

No

No

No

No

Compute due tax: rate * income

End

Another Example

Write (1) test cases, (2) flowchart, and (3) code to prompt the user for three sides (of type INT) of a triangle. Then check if it is an equilateral triangle (i.e., all three sides are equal), else check if it is an isosceles triangle (i.e., two sides are equal). Print out if it is an equilateral, isosceles, or scalene triangle (i.e., no sides are equal).

One More Example

Horse Goat Monkey Rooster Dog Pig Mouse OX Tiger Rabbit Dragon Snake

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

1990 1991 1992 1993 1994  1995 1996 1997 1998 1999 2000 2001

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989

1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977

The celebration of the Chinese Lunar year goes back centuries.  There are 12 different creatures represented in the Chinese calendar.  A different animal is commemorated each year.  After 12 years the animals are repeated. If you were born in between 1966 and 2013, you can use the following table to find out your “year animal”. For examples, if you were born in 1990, your year animal is “Horse”; if you were born in 1991, your year animal is “Rabbit”.

Switch Statement

switch (variable) { case value1:

body1 break; case value2:

body2; break; case value3:

body3; break;

::: default: body; break;}