Relational and Boolean Operators

CSIS 1595: Fundamentals of Programming and Problem Solving 1

Relational Operators• Conditional expressions of form

operand1 relationalOperator operand2– Comparing operand1 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 first1. Evaluate expressions on either side of relational operator

to get 2 values2. 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

• 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

Equality and Float Type

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

• Method:– Compute difference between the values– Take absolute value

• Difference may be positive or negative• Have built-in fabs function

– Compare with “margin of error” using < instead of ==x = 5 * 2/3y = 2/3 * 5if math.fabs(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 or b must not be 0

Boolean Expressions

• Syntax: conditionalExpr booleanOp conditionalExpr

• 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– false and true false– false and false false

• or: true if either operands are true– true or true true– true or false true– false or true true– false or false false

not Operator

• not: Reverses boolean value of unary operand – Syntax: not conditionalExpr• not true false• not false true

– Example: if not x > 0: …• Can often just use “opposite” relational operator– if x <= 0: …

• Commonly used to reverse value of boolean function– if not math.isfinite(x): …

Precedence and Boolean Operators

• Precedence:1. Evaluate arithmetic expressions in conditional expressions2. Evaluate conditional expressions to True or False3. Evaluate boolean operators using those values

1. not evaluated first2. and evaluated next3. or evaluated last

– Can always use () if not sure!