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Relational and Boolean OperatorsPowerPoint Presentation

Relational and Boolean Operators

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### Relational and Boolean Operators

CSIS 1595: Fundamentals of Programming and Problem Solving 1

Relational Operators

- Conditional expressions of form operand1 relationalOperator operand2
- Comparingoperand1 and operand2 in some way

- Python relational operators:
> greater than

< less than

<= less than or equal to

>= greater than or equal to

!= is not equal to

== is equal to

(not same as =)

Relational Operators Example

Relational and Arithmetic Operators

- Can combine in same conditionx = 3y = 7if x * 2 < y + 1: …
- Precedence: Arithmetic operators done first
- Evaluate expressions on either side of relational operator to get 2 values
- Evaluate relational operator in term of those 2 values to get either True or False

Relational Operators and Types

- Some operators only make sense for certain types
- Numbers: > < >= <= == !=
- Be careful with floats and ==

- Strings: == !=
- password = input(“Enter password:”)if password == “xyzzy”: …
- Can do > < >= <= but get strange results

- password = input(“Enter password:”)if password == “xyzzy”: …

- Numbers: > < >= <= == !=
- Can only compare similar types
“Fred” > 2 error

Equality and Float Type

- == checks whether operands are same number
- Can be problem with floats due to lack of precision
x = 5 * 2/3y = 2/3 * 5if x == y: … False

- Can be problem with floats due to lack of precision

Equality and Float Type

- Goal: determine whether x and y are “close enough”
- Within some “margin of error” of each other

- Method:
- Compute differencebetween the values
- Take absolute value
- Difference may be positive or negative
- Have built-in abs function

- Compare with “margin of error” using < instead of ==
x = 5 * 2/3y = 2/3 * 5if abs(x – y) < 0.000001: …

Boolean Logic

- Some problems involve multiple conditions
- “x must be between 1 and 10”
- “either a or b must not be 0”

- Can create using and, or, other boolean operators
- x must be greater than or equal to 1 and less than or equal to 10
- a must not be 0 orb must not be 0

Boolean Expressions

- Syntax: conditionalExprbooleanOpconditionalExpr
- Examples:
- if x >= 1 and x <= 10: …
- if a != 0 or b != 0: …

Boolean Operators

- and: true if both operands are true
- true and true true
- true and false false
- falseand true false
- falseand false false

- or: true if eitheroperands are true
- true and true true
- true and false true
- false and true true
- false and false false

not Operator

- not: Reversesboolean value of unary operand
- Syntax: not conditionalExpr
- not true false
- not false true

- Example: if not x > 0: …

- Syntax: not conditionalExpr
- Can often just use “opposite” relational operator
- if x <= 0: …

- Commonly used to reverse value of booleanfunction
- if not math.isfinite(x): …

Precedence and Boolean Operators

- Precedence:
- Evaluate arithmetic expressions in conditional expressions
- Evaluate conditional expressions to True or False
- Evaluate boolean operators using those values
- notevaluated first
- andevaluated next
- orevaluated last

- Can always use () if not sure!

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