Boolean Tricks

Expressions in Conditions

• Condition must evaluate to Boolean but can be arbitrarily complex

Expressions in Conditions

• Condition must evaluate to Boolean but can be arbitrarily complex

• Just because you can doesn't mean you should. Better:

%

• Check if x is a multiple of y:x % y == 0

Even/Odd

• x % 2 is:– 0 even– 1 odd

Comparing Floating Points

• According to C++:3.02 != (3 + .01 + .01)

• Do NOT compare floats directly with == or !=

Comparing Floating Points

• Compare if two floating points are "close enough"|num1 – num2| < threshold

Comparing Floating Points

• Compare if two floating points are "close enough"|num1 – num2| < threshold

• absolute value: abs(expression) –in <cmath>

• threshold–depends on problem, have 15 significant digits to work with

Comparing Floating Points

Naked Booleans

• Boolean variable can be used as condition:

• Same as:

!

• ! is "not"

• Preferred to:

Problem

• Want to read in temperature and print "Temp is good" if it is between 68 and 72, or warning if it is too hot/cold.

Boolean Flags

• Flag variable : Boolean used to track condition– Start as true or false– Modify based on eveidence

Nested Ifs

Nested ifs

• Conditionals can be "nested"

If/Else Matching

• This code has a logic error:

If/Else Matching

• This code has a logic error:

If/Else Matching

• What computer saw:

If/Else Matching

• { } make ending of if/else explicit

Editor will show brace matching

Use indentation as visual clue

If/Else

• If can be the body of an else:

If/Else

• if… else if… else if… else form:– More compact/legible– Only one option is chosen

Bad If/Else

• A chain of plain if's not mutually exclusive!

Example: Computing TaxesThe US federal personal income tax is calculated based on the filing status and taxable income. There are four filing statuses: single filers, married filing jointly, married filing separately, and head of household.

Read in someone's status and income and determine their tax:

Conditions

• 2 Inputs– Marital status– $ earned

• 1 Output: tax

What happens?

• No matter what:– Get status– Get income– Print tax

• Conditionally:– What math to do

Step 1

• Distinguish based on status

Step 2

• Refine each category based on earnings: