04_lists-and-iteration.md

Lists and Iteration

Intro to lists

Lists are yet another extremely useful tool in Python. You might know them in other languages as vectors or arrays. They are pretty self-explanatory, simply an ordered sequence of objects enclosed in square brackets ([]). The "objects" here can be just about anything: strings, numerics, other lists, variables pointing to objects...I actually don't know of any type that could not be in a list.

Of note is the fact that lists do not have to be comprised of all one type (in other languages, you have to limit it to one type). This makes lists seriously versatile.

Let's start with a list called pets that we will use in later steps:

pets = ["dogs", "cats", "guinea pigs"]

Indexing and list methods

Members of a list can be accessed by their index, a number referring to their position in the list. Importantly, Python is known as a zero-indexed language, meaning that it starts counting from 0, not 1. That means:

print(pets[0])

prints 'dogs'.

To add an item to a list, use the list.append() method:

pets.append("fish")
print(pets)

prints ['dogs', 'cats', 'guinea pigs', 'fish'].

If you want to know how many items are in a list, use the len() function:

print(len(pets))

prints 4.

Try it yourself: Lists

Make a new file called lists.py. Define a list called fruits that has members "apple", "banana", "orange", "grape".

append a pineapple to your list.

And print the third member of the list.

Iteration: for loops

Since I keep saying it, I figured I would write a program to tell you how important and useful each section is.

for section in tutorial:
    rachael.say(section + " is a really important and useful tool!")

And that's a for loop!

A for loop takes each member of a list (with the syntax: for member in list) one at a time, and does something to it that you get to define. This process of doing something to each member of a list is known as iteration.

Let's say you wanted to take a list of numbers and double them, and print the results to the screen:

numbers = [1, 2, 23, 634563.75, 23.0, 1212**2]
for n in numbers:
    double_n = n*2
    print(double_n)

Notice that statements that are part of the for loop are indented. This is actually meaningful, because it tells the interpreter that the statements should be thought of as nested within the loop and part of the iteration, not separate. See the following example:

for pet in pets:
    print(pet + " are great!")
print("All pets are great!")

The last line is only printed once, because it is outside the loop. If I had indented it, it would have been printed with every iteration.

Try it yourself: for loop

Go back to lists.py. Write a loop that would print the following:

I need to get more apples.
I need to get more bananas.
I need to get more oranges.
I need to get more grapes.
I need to get more pineapples.
Time for a grocery run!

Advanced topic: while loops and list comprehension

I won't go into detail about these for the sake of time, but you should know about while loops and list comprehension. Instead, I will leave it as a challenge. See if you can figure out what these code snippets are doing and why. I'd be happy to work through them with you if you have questions about them later.

while loop: (hint: the += operator is called "recursive addition" and it's---get this---really useful)

x = 1
y = []
while x <= 5:
    y.append(2**x)
    x += 1
print(y)

list comprehension:

x = [1, 2, 3, 4, 5]
y = [2**i for i in x]
print(y)

Up next: control flow

See 05_control-flow.md to get started with that.