Python Mini-Course

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Day 2 Lesson 7Conditionals and Loops

• Python Mini-Course
• University of Oklahoma
• Department of Psychology

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Lesson objectives

1. Use conditional statements in Python, including
   the if, ifelse, and ifelif statements
2. Use Boolean logic to create complex conditionals
3. Use the for statement to create loops

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Conditional execution (if)

• Syntax
• if condition
• do_something
• Condition must be statement that evaluates to a
  boolean value (True or False)

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ExampleChecking user input

• x raw_input("X? ")
• if x.isdigit()
• print "You input a number"

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Alternative execution

• Syntax
• if condition
• do_something
• else
• do_alternative

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ExampleChecking user input

• x raw_input("x? ")
• if x.isdigit()
• print "You input a number"
• else
• print "Please input a number\
• next time"

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ExampleAvoiding division by zero

• x int(raw_input("x? "))
• y int(raw_input("y? "))
• if y ltgt 0
• print x / y
• else
• print "Attempted division by\
• zero"

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Chained conditionals

• Syntax
• if condition
• do_something
• elif condition
• do_alternative1
• else
• do_alternative2

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Example

• x int(raw_input("x? "))
• y int(raw_input("y? "))
• if x lt y
• print 'x is less than y'
• elif x gt y
• print 'x is greater than y'
• else
• print 'x and y are equal'

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Notes on conditionals

• You have to have at least one statement inside
  each branch but you can use the pass command
  while stubbing
• if x lt 0
• pass Handle neg values

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Notes on conditionals

• You can have as many elif branches as you need
• This replaces the select and switch commands in
  other languages
• Conditionals can be nested
• Example nested.py

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Complex condition statements

• Use Boolean logic operators and, or, and not to
  chain together simple conditions
• Example boolean.py

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The for loop

• Used to repeat a section of code a set number of
  times
• Syntax
• for target in sequence
• do_statements

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Example (power.py)

• def power(base, exponent)
• val base
• for x in range(1,exponent)
• val val base
• print val
• power(3,4)

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Example (power.py)

• def power(base, exponent)
• val base
• for x in range(1,exponent)
• print x, val
• val val base
• print val

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Example (power.py)

• def power(base, exponent)
• val base
• for x in range(1,exponent)
• print x, val
• val val base
• print val

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Exercises

1. Find a major bug in the power function and
   correct it
2. Add a docstring to the power function
3. Add type checking and value checking to the power
   function