python list generation usage

List Comprehensions: List Comprehensions is a very simple but most commonly used function in python.

You can know from the name that the list generation should return a list type, which can generate the required list in the simplest and most understandable way.

Example: I need to get a list composed of the squares of all the numbers in the list 1-100. At this time, you can use a for loop:

Python code

a = []  
for value in range(1, 101):  
    a.append(value * value)  
  
print(a)

The a obtained at this time is an array composed of the square of each number in 1-100. This method is simple, but using list generation is even simpler.

Python code

a = [value * value for value in range(1,101)]  
print(a)

The a obtained is exactly the same as the a in the previous method.

In a = [value * value for value in range(1,101)], value * value is an expression. The number value comes from the for loop behind the expression. Each time the for loop loops, the expression is calculated. , and finally save the calculation results of each cycle in the for loop in a list. Finally assign it to a.

In list generation, you can also use multiple loops. For example:

Python code

a = [x * y for x in range(1,3) for y in range(3,5)]  
print(a)

The generated result is:

Terminal code

[3, 4, 6, 8]

range(1,3) is [1, 2], range(3,5 ) is [3, 4], x comes from range (1, 3), y comes from range (3, 5)

The results are: 1*3, 1*4, 2*3, 2*4

In addition, You can add conditional judgments to the list generation:

Python code

a = [value * value for value in range(1, 11) if value % 2 == 0]  
print(a)

#The result is:

[4, 16, 36, 64, 100]

Add conditional selection to the value after the for loop. This example is to calculate the square of an even number from 1 to 10