CS 303E Class 10 Decision Making: if and ifelse statements

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CS 303E Class 10Decision Makingif and if-else
statements
As soon as questions of will or decision or
reason or choice of action arise, human science
is at a loss.Noam Chomsky
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Linear flow of control

• All we have seen so far is one statement after
  another
• The program must execute each statement in the
  order written. The methods we have created in
  our classes cant make decisions.
• A programming language that cannot branch is
  pretty limited in its usefulness.

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Branching making decisions

• If the condition is true, execute statement_1
• otherwise, execute statement_2
• The condition is an expression that can be true
  or false, such as x gt 0

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Branching example

• An example for a checking account
• The account gets interest or a penalty depending
  on whether the balance is over 0.

myBalance myBalance interest
true
myBalance gt 0
myBalance myBalance PENALTY
false
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Empty false branch

• If the condition is true, execute statement
• otherwise, do nothing
• Condition is an expression that can be true or
  false.

false
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Syntax -- if Statement

• if (condition) // one-way branch
• statement
• if (condition) // two-way branch
• statement
• else
• statement

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if Statement Examples

• if (sales gt SALES_FOR_BONUS)
• commission commission BONUS_RATE
• if (balance gt 0)
• balance balance (balance INTEREST_RATE)
• else
• balance balance - PENALTY

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Compound Statements

• Often, one statement isnt enough.
• So use a compound statement
• statement
• . . . zero or more statements
• statement between braces

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if with Compound Statements

• if (condition) if (condition)
• statement statement
• . . . . . .
• statement statement
• else
• statement
• . . .

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Example

• // Pay at payRate with time and one half for
• // overtime
• pay hoursWorked payRate
• if (hoursWorked gt FULL_TIME_HOURS)
• overtime hoursWorked - FULL_TIME_HOURS
• pay pay overtime (payRate / 2)

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Conditions for if statements

• Conditions are evaluated for the if statement
• The condition will either evaluate to true or
  false
• One form of condition is
• expression relop expression
• where relop is a relational operator.

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Boolean Relational Operators

• Math Name Java Java Example
• equal balance 0
• ? not equal ! answer ! "Yes"
• gt greater than gt expenses gt income
• ? greater than or equal gt points gt 60
• lt less than lt pressure lt max
• ? less than or equal lt expenses lt income

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Examples

• count 1 gt a b
• far - near gt 10
• Relational operators have lower precedence
• than arithmetic operators. The expressions are
  evaluated before the comparison.
• Invalid
• a lt b lt c // syntax error
• a b c // syntax error
• //but we can do these use logical operator
• What happens when you compare objects?

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Indentation Helps Programmers but Compilers
Ignore it.

• if (hours gt FULL_TIME_HOURS)
• over hours - FULL_TIME_HOURS
• pay pay over (rate / 2)

All simple statements within another statement
should be indented further to the right than the
outer statement. The two assignment statements
are within the if. All statements in a sequence
of statements in a compound statement should be
indented the same amount, as the two assignment
statements are.
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Multi-way Decisions

• Several possible paths

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Multi-way Decisions

• if (balance gt 0)
• System.out.println (Positive)
• else if (balance lt 0)
• System.out.println (Negative)
• else
• System.out.println (Zero)
• Compound statements can be used here too.

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Nested Decision Making
Figure 7 Income Tax Computation