Ranges and Lists

Before we come onto for loops we must understand ranges and lists.

1. List

A list in python is used to store multiple values in a single variable.

For example:

list1 = [1,2,3]

list2 = ["Citreon", "Ford", "Audi", "Mercedes"]

list3 = ["Citreon", "Ford", 67, True]

You can access an element in a list using indexing. Each member of the list has an index number starting from 0.

For example the code,

list3 = ["Citreon", "Ford", 67, True]
print(list3[0])
print(list3[2])

results in the output:

Citreon
67

Try creating your own list in the and printing some of its elements.

For now, we will not discuss more about lists until later in the course. It suffices to know that they store multiple values and are heterogeneous (a fancy way of saying they can store different types).

2. Ranges

A range allows us to create a sequence of numbers using the range() function.

range(10) # This will create a range of numbers 0,1,2,3,4,5,6,7,8,9

range(3, 10) # This will create a range of numbers 3,4,5,6,7,8,9

range(2, 10, 2) # This will create a range of numbers 2,4,6,8

Ranges can take either:

  • an end value range(end)
  • a start and end value range(start, end)
  • a start, end, and step value range(start, end, step)

In some ways a range behaves like a list, however, to save space, Python doesn't create the list.

If you type the following into the terminal, you will see that the object type of a range is a range!

a = range(10)
print(type(a)) # prints <class 'range'>

So think of it as a convenient way to store a list of numbers without actually storing them!

By default start is 0 and step is 1.

So range(10) is the same as both range(0,10) and range(0,10,1).

2.1 Convert a Range to a List

If you try and print a range you won't get the numbers. Try the following in the terminal.

a = range(10)
print(a)

and you will get the output:

range(0, 10)

You can convert the range to a list using the list() function.

a = list(range(2, 10)) # converts the range to a list [2,3,4,5,6,7,8,9]
print(a)

If you run the following code in the terminal or a Python file you will get the output:

[2,3,4,5,6,7,8,9]

The list() function is forcing Python to build the list in memory.

To prove this to ourselves let's check the size of range(10**6). Note 10**6 = 1000000

import sys
a = range(10**6)
print(sys.getsizeof(a))

This returns 48 which is 48 bytes of memory. Try a range of any size and you will always get 48.

Now let's convert it to a list and print the size:

import sys
a = list(range(10**6))
print(sys.getsizeof(a))

This returns a whopping 8000056 (8 million) bytes, which is approximately 166667 times as large!

=== TASK ===

Using the range() and list() functions, print out the odd numbers from 1 up to and including 99.

Your output should look like this:

[1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99]

HINT: Make sure you have read Section 2.1