Conditional Statements: The `if` statement

The if Statement

• Control Structure is a logical design that controls the order in which statements execute
• Sequence structures are sets of statements that execute in the order they appear
• Decision structures will perform a specific action only if a condition exists, also known as selection structure.
• The flowchart diamond represents a true or false condition that must be tested
• Actions are conditionally executed only when the condition is true
• Single alternative decision structure provides only one alternative path of execution
• The if clause includes the keyword if followed by the condition that can be true or false
• When the if statement executes, if the condition is true, the block statements are executed, otherwise the block statements are skipped

Boolean Expressions and Relational Operators

• Boolean expressions are tested by if to determine if it is true or false, e.g. a > b is true if a is greater than b, otherwise it is false
• Relational Operators determine whether a specific relationship exists between two values, e.g. >, >=, <=, ==, !=
• >= and <= operators test more than one relationship, and it is enough for one of the relationships to exist for the expression to be true
• == determines whether the two operands are equal to one another and must not be confused with the assignment operator =
• != operators determines whether the two operands are not equal
• Relational operators can be used in a decision block, e.g. if balance == 0 or if payment != balance
• A block can be inside another block, e.g. an if statement inside a function, statements in inner block must be indented with respect to the outer block

The if-else Statement

• Dual alternative decision structure has two possible paths of execution
• One path is taken if the condition is true, and the other if the condition is false
• Syntax:

if condition:
   statements
else:
   other statements

• the if and else clauses must be aligned
• statements must be consistently indented

Comparing Strings

• Strings can be compared using the == and != operators
• String comparisons are case sensitive
• Strings can be compared using >, <, >=, and <= by comparing character by character based on the ASCII values for each character
• If a word is a substring of a longer word, the longer word is greater than the shorter word

Logical Operators

• Logical operators create complex Boolean expressions
• and and or operators are binary operators that connect two Boolean expressions into a compound Boolean expression
• The not operator is a unary operator, that reverses the truth of its Boolean operand

The and Operator

• Takes two Boolean expressions as operands
• Creates a compound Boolean expression that is true only when both sub expressions are true
• Truth Table
  • false and false = false
  • false and true = false
  • true and false = false
  • true and true = true
• Can be use to simplify nested decision structures

The or Operator

• Takes two Boolean expressions as operands
• Creates compound Boolean expression that is true when either of the sub expressions is true
• Truth Table
  • false or false = false
  • false or true = true
  • true or false = true
  • true or true = true
• Can be use to simplify nested decision structures

Short Circuit Evaluation

• Short Circuit evaluation is when value of a compound Boolean expression is decided after evaluating only one sub expression
• The or and and operators perform this function
  • For the or operator, when the left operand is true, the compound expression is true. Otherwise, the right operand is evaluated
  • For the and operator, when the left operand is false, the compound expression is false. Otherwise, the right operand is evaluated

The not Operator

• The not operator takes one Boolean expression and reverses its logical value
• Truth Table
  • not false = true
  • not true = false
• Use parentheses around an expression to clarify to what the not operator is being applied

Boolean Variables

• Boolean variables reference one of two values, True or False and are represented by the bool data type
• Booleans are commonly used as flags
  • A Flag is a variable that signals when some condition exists in a program
  • A Flag set to False means the condition does not exist
  • A Flag set to True means the condition does exists

Checking Numeric Ranges with Logical Operators

• To determine whether a numeric value is within a specific range of values, use and, e.g. x >= 10 and x <= 20
• To determine whether a numeric value is outside of a specific range of values, use or, e.g. x < 10 or x > 20

The in Operator

• The in operator determines whether a given value is a constituent element of a sequence such as a string, e.g. "s" in "sebastian" is true

The not in Operator

• The not in membership operator evaluates to true if it cannot find a variable in the specified sequence, and false otherwise, e.g. print("sss" not in "sebastian")