What You Will Learn:

  • How to get input from a user
  • Understand the syntax of conditional statements
  • Use conditional statements
  • Understand what a boolean is

Why This is Important:

Sometimes we only want our code to run if certain conditions are in place. If a condition is met, then python should run the code otherwise something else should happen.

User Input

In python, if you want to get text from a user, you can use the raw_input() function. Let’s try it out!

>>> name = raw_input('Enter your name: ')
Enter your name: Georgia
>>> name
'Georgia'

The raw_input() method allows the user to input any answer. The input() function will only allow the user to input a string.

Conditionals

Conditional statements are a type of control flow: They can control which parts of code get executed, and which do not. The basic conditional statement is the if statement:

if condition_1:
    run this block of code

Here is an example with two variables:

yourAnimal = raw_input('What is your favorite animal? ')
myAnimal = "dog"

if yourAnimal == myAnimal:
  print "That's my favorite animal, too!"

Comparison Operator

  • ”==” means “are these two things equal?”.
  • ”=”, means assignment - you’re setting a variable equal to a value.

Boolean Values

False and True are special values called booleans. Booleans can be assigned to variables. For example:

x = (3 == 5)
print x

First, we calculate (3 == 5). This is a question: “Is 3 equal to 5?”. The answer to that question is False: 3 is not equal to 5.

So, when we say x = (3==5), this is equivalent to x = False. We’re setting X to the boolean value False. Now, when we print x, we just print out False.

Here are some important boolean operators:

Else and Elif conditional statements

There are a few other conditional statements that work together with if. For example, you often want to do something if the condition failed:

yourAnimal = raw_input('What is your favorite animal? ')
myAnimal = "dog"
if yourAnimal == myAnimal:
  print "That's my favorite animal, too!"
else:
  print "I don't think you understand how cool dogs are."

There’s also “elif”. This is the “catch-all” branch, if a condition is not met in the prior branches, then the code in the elif branch will be executed.

if x > 500:
  print "x is really big"
elif x > 50:
  print "x is sort of big"
elif x < 0:
  print "x is negative!"
else:
  print "x is not very interesting"

Student Practice

In your practice.py try writing a conditional statement with an if condition, elif condition and else condition

  • Pick a Number: Write a program that makes the user guess a number 1-10. If their guess is correct, tell them they’ve won. If their guess is too high or too low, give the user a hint. you will need to use python’s input() method.

  • userGuess = input('Guess a number between 1-10')

  • Stretch: Phone Number Validator: Write a program that checks if a string is formatted correctly as a phone number (312)867-5309.
  • Check if there are 10 integers
  • Check if the first three integers are between two parenthesis
  • Check to make sure that a dash separates the 6th and 7th integer.