{"id":113670,"date":"2020-06-01T11:47:57","date_gmt":"2020-06-01T11:47:57","guid":{"rendered":"https:\/\/blog.finxter.com\/?p=9161"},"modified":"2020-06-01T11:47:57","modified_gmt":"2020-06-01T11:47:57","slug":"how-to-join-a-list-of-lists-you-dont-you-flatten-it","status":"publish","type":"post","link":"https:\/\/sickgaming.net\/blog\/2020\/06\/01\/how-to-join-a-list-of-lists-you-dont-you-flatten-it\/","title":{"rendered":"How to Join a List of Lists? You Don\u2019t. You Flatten It!"},"content":{"rendered":"

Short answer: To flatten your list of list lst<\/code>, use the nested list comprehension [x for l in lst for x in l]<\/code> to iterate over all nested lists (for l in lst<\/code>) and over all their elements (for x in l<\/code>). Each element x<\/code>, you just place in the outer list, unchanged.<\/strong><\/p>\n

When browsing StackOverflow<\/a>, I stumble upon this question from time to time: “How to Join a List of Lists?<\/strong><\/em>“. My first thought is that the asking person is referring to the join() function<\/a> that converts an iterable (such as a list<\/a>) to a string by concatenating <\/a>its elements. <\/p>\n

But nothing can be further from the truth! The question is usually about “flattening” the list: to transform a nested list of lists<\/a> such as [[1, 2, 3], [4, 5, 6]]<\/code> to a flat list [1, 2, 3, 4, 5, 6]<\/code>. So, the real question is:<\/p>\n

How to Flatten a List of Lists in Python?<\/h2>\n

Problem<\/strong>: Given a list of lists<\/a>. How to flatten the list of lists by getting rid of the inner lists—and keeping their elements?<\/p>\n

Example<\/strong>: You want to transform a given list into a flat list like here:<\/p>\n

lst = [[2, 2], [4], [1, 2, 3, 4], [1, 2, 3]] # ... Flatten the list here ... print(lst)\n# [2, 2, 4, 1, 2, 3, 4, 1, 2, 3]<\/pre>\n
\"Flatten<\/figure>\n
Solution<\/strong>: Use a nested list comprehension<\/a> statement [x for l in lst for x in l]<\/code> to flatten the list. <\/p>\n
lst = [[2, 2], [4], [1, 2, 3, 4], [1, 2, 3]] # ... Flatten the list here ...\nlst = [x for l in lst for x in l] print(lst)\n# [2, 2, 4, 1, 2, 3, 4, 1, 2, 3]<\/pre>\n
Explanation<\/strong>: In the nested list comprehension statement [x for l in lst for x in l]<\/code>, you first iterate over all lists in the list of lists (for l in lst<\/code>). Then, you iterate over all elements in the current list (for x in l<\/code>). This element, you just place in the outer list, unchanged, by using it in the “expression” part of the list comprehension statement [x<\/strong> for l in lst for x in l]<\/code>. <\/p>\n
Try It Yourself<\/strong>: You can execute this code snippet yourself in our interactive Python shell. Just click “Run” and test the output of this code. <\/p>\n