List Comprehension

List Comprehension is a concise method to create list in Python 3.

This technique is commonly used when:

• The list is a result of some operations applied to all its items

• It is a made from another sequence/iterable data

• The list is member of another list/sequence/iterable data that satisfies a certain condition

This is where the lambda function would be used, but… we will learn the other way for readability. We will definitely talk about lambda functions in our Functional Programming Unit

List Comprehension Example 1

We are to create a list which squares all the numbers from [0,10)

# Old Method
squares = []
for i in range(10):
    squares.append(i ** 2)

print('Our result: %s' % squares)

Our result: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# List Comprehension

squares = [i**2 for i in range(10)]

print('Our new result: %s' % squares)

Our new result: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

How Does it Work?

List comprehension consists of:

• A Square Bracket containing an expression that describes the list

• One or more For clause to explain its members

• Then a zero or more if clauses depending on the complexity of the list

Examine: [i**2 for i in range(10)]

-- i**2 for i in range(10) --> is the expression that describe the list
-- i**2 describes each item in the list
-- i is taken from the for clause
-- for i in range(10) describes where i comes from

List Comprehension Example 2

Create the list: [[1, 3], [1, 4], [2, 3], [2, 1], [2, 4], [3, 1], [3, 4]]
From
A = [1,2,3]
B = [3,1,4]

By using list comprehension

# Solution
a = [1,2,3]
b = [3,1,4]

result = [[x, y] for x in a for y in b if x != y]
print(result)

[[1, 3], [1, 4], [2, 3], [2, 1], [2, 4], [3, 1], [3, 4]]

Explanation:

Each item of our list has to be a list of two values; therefore, each item is described as [x, y]

There are two for clauses because we are create a list from two sources
-- x comes from a
-- y comes from b

There is a condition to our item --> x != y
-- Therefore, as long as the condition x != y is true, we will add the item described as [x,y] to our resulting list

List Comprehension Example 3

Use list comprehension to turn a 2D array called vec to a single list

vec = [[1,2,3], [4,5,6], [7,8,9]]
Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]

# Solution

vec = [[1,2,3], [4,5,6], [7,8,9]]

result = [value for row in vec for value in row]
print('Vec as a single list of values: %s' % result)

Vec as a single list of values: [1, 2, 3, 4, 5, 6, 7, 8, 9]

Explanation

Vec is an example of a matrix in Python 3 by using list of lists

To grab each value one by one from the rows we must do the following in order:
1. Explain what each item in the list comprehension is going to be ... in our case --> "value"
2. To now access where value is, define where it comes from ... in our case --> "row";
        therefore, for row in vec
3. Finally we construct our last for clause to denote that value comes from the row

With list comprehension, the order of your for clauses matter!