Editing Python loops cheat sheet

== For loops ==

Use a <code>for</code> loop to do something to every element in a list.

 >>> names = ["Jessica", "Adam", "Liz"]
 >>> for name in names:
 ...     print(name)
 ...
 Jessica
 Adam
 Liz

 >>> names = ["Jessica", "Adam", "Liz"]
 >>> for name in names:
 ...     print("Hello " + name)
 ...
 Hello Jessica
 Hello Adam
 Hello Liz

=== <code>if</code> statements inside <code>for</code> loop ===

 >>> for name in ["Alice", "Bob", "Cassie", "Deb", "Ellen"]:
 ...     if name[0] in "AEIOU":
 ...         print(name + " starts with a vowel.")
 ... 
 Alice starts with a vowel.
 Ellen starts with a vowel.

=== Building up a list ===

Sometimes you want to build up a new list based on information about each element in an existing list. To do this, initialize an empty list before the <code>for</code> loop, and append elements to the new list inside the <code>for</code> loop:

 >>> vowel_names = []
 >>> for name in ["Alice", "Bob", "Cassie", "Deb", "Ellen"]:
 ...     if name[0] in "AEIOU":
 ...         vowel_names.append(name)
 ... 
 >>> print(vowel_names)
 ['Alice', 'Ellen']

=== Using a counter ===

Sometimes you want to keep track of the number of occurrences of something, or a running total, as you loop through a list. To do this, initialize a variable before the <code>for</code> loop that you update inside the <code>for</code> loop:

 >>> prices = [1.5, 2.35, 5.99, 16.49]
 >>> total = 0
 >>> for price in prices:
 ...     total = total + price
 ... 
 >>> total
 26.33

=== <code>for</code> loops inside <code>for</code> loops ===

You can put <code>for</code> loops inside <code>for</code> loops. The indentation dictates which <code>for</code> loop a line is in.

 >>> letters = ["a", "b", "c"]
 >>> numbers = [1, 2, 3]
 >>> for letter in letters:
 ...     for number in numbers:
 ...         print(letter * number)
 ...
 a
 aa
 aaa
 b
 bb
 bbb
 c
 cc
 ccc

The order of the <code>for</code> loops matters. Compare the above example with this one:

 >>> for number in numbers:
 ...     for letter in letters:
 ...         print(number * letter)
 ...
 a
 b
 c
 aa
 bb
 cc
 aaa
 bbb
 ccc

== Useful functions related to lists and for loops ==

==== sorting lists ====

Use <code>.sort()</code> to sort a list:

 >>> names = ["Eliza", "Joe", "Henry", "Harriet", "Wanda", "Pat"]
 >>> names.sort()
 >>> names
 ['Eliza', 'Harriet', 'Henry', 'Joe', 'Pat', 'Wanda']

==== Getting the maximum and minimum values from a list ====

 >>> numbers = [0, 3, 10, -1]
 >>> max(numbers)
 10
 >>> min(numbers)
 -1

==== Generating a list of numbers easily with <code>range()</code> ====

The <code>range()</code> function returns a list of numbers. This is handy for when you want to generate a list of numbers on the fly instead of creating the list yourself.

 >>> range(5)
 [0, 1, 2, 3, 4]

Use <code>range</code> when you want to loop over a bunch of numbers in a list:

 >>> numbers = range(5)
 >>> for number in numbers:
 ...     print(number * number)
 ...
 0
 1
 4
 9
 16

We could rewrite the above example like this:

 >>> for number in range(5):
 ...     print(number * number)
 ...
 0
 1
 4
 9
 16

=== Get user input with <code>input()</code> ===

 >>> for i in range(100):
 ...     my_input = input("Please type something> ")
 ...     if my_input == "Quit":
 ...         print("Goodbye!")
 ...         break
 ...     else:
 ...         print("You said: " + my_input)
 ... 
 Please type something> Hello
 You said: Hello
 Please type something> How are you?
 You said: How are you?
 Please type something> Quit
 Goodbye!
 >>>

[[Category:CDSW]]