ID : 325

viewed : 61

Tags : PythonPython List

94

This tutorial demonstrates how to perform the list subtraction, or in other words, list minus list in Python.

As defined by the set theory in mathematics, the difference of two sets refers to the elements from one set that do not exist in the other set.

For example, if we declare these two lists:

`list1 = [1, 2, 4] list2 = [2, 3] `

The difference of `list1 - list2`

would be `[1, 4]`

, while `list2 - list1`

would be `[3]`

.

`set`

to Perform List Subtraction in PythonSet theory operations are supported in Python. However, only the `set`

data type support these operations. Therefore, to use the `set`

operation, lists have to be converted into sets. This is possible by wrapping a list around the function `set()`

.

Note: Converting a list to a set will remove any type of order and remove duplicate values from the list.

`listA = [1, 2, 4, 7, 9, 11, 11, 14, 14] listB = [2, 3, 7, 8, 11, 13, 13, 16] setA = set(listA) setB = set(listB) print('A - B = ', setA - setB) `

Output:

`A - B = {1, 4, 9, 14} `

The result outputs the difference between the two sets and removes the duplicate values.

We can use the function `list()`

to convert the result from a `set`

to a list.

`listA = [1, 2, 4, 7, 9, 11, 11, 14, 14] listB = [2, 3, 7, 8, 11, 13, 13, 16] setA = set(listA) setB = set(listB) list_diff = list(setA - setB) print('A - B: ', list_diff) `

Output:

`A - B: [1, 4, 9, 14] `

List comprehension can be used to check if an element exists only in the first list but does not exist in the second list. This solution makes it possible to perform the difference operation without converting the list to a set.

`listA = [1, 2, 4, 7, 9, 11, 11, 14, 14] listB = [2, 3, 7, 8, 11, 13, 13, 16] listSub = [elem for elem in listA if elem not in listB] print('A - B =', listSub) `

Output:

`A - B = [1, 4, 9, 14, 14] `

This solution does not mess with the order of the list and removes duplicates.

However, the value `11`

is repeated twice in `listA`

, and both iterations of `11`

are removed from the result of `A - B`

since `11`

exists in both sets. This behavior is as expected.