Questions tagged [list]
The list tag may refer to: a linked list (an ordered set of nodes, each referencing its successor), or a form of dynamic array. Not to be used for HTML lists, use [html-lists] instead.
142,720
questions
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28
answers
4.4m
views
How to access the index value in a 'for' loop?
How do I access the index while iterating over a sequence with a for loop?
xs = [8, 23, 45]
for x in xs:
print("item #{} = {}".format(index, x))
Desired output:
item #1 = 8
item #2 = ...
- 323k
5319
votes
32
answers
4.3m
views
How do I make a flat list out of a list of lists?
I have a list of lists like
[
[1, 2, 3],
[4, 5, 6],
[7],
[8, 9]
]
How can I flatten it to get [1, 2, 3, 4, 5, 6, 7, 8, 9]?
If your list of lists comes from a nested list ...
- 54k
4388
votes
46
answers
6.3m
views
How to find the index for a given item in a list?
Given a list ["foo", "bar", "baz"] and an item in the list "bar", how do I get its index 1?
- 48.3k
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votes
24
answers
2.2m
views
How do I clone a list so that it doesn't change unexpectedly after assignment?
While using new_list = my_list, any modifications to new_list changes my_list every time. Why is this, and how can I clone or copy the list to prevent it?
- 65.8k
3238
votes
31
answers
4.4m
views
How do I concatenate two lists in Python?
How do I concatenate two lists in Python?
Example:
listone = [1, 2, 3]
listtwo = [4, 5, 6]
Expected outcome:
>>> joinedlist
[1, 2, 3, 4, 5, 6]
- 65.7k
3226
votes
27
answers
4.9m
views
How do I check if a list is empty?
For example, if passed the following:
a = []
How do I check to see if a is empty?
- 190k
3115
votes
70
answers
1.7m
views
How do I split a list into equally-sized chunks?
How do I split a list of arbitrary length into equal sized chunks?
See also: How to iterate over a list in chunks.
To chunk strings, see Split string every nth character?.
- 32.9k
3111
votes
20
answers
3.3m
views
What is the difference between Python's list methods append and extend?
What's the difference between the list methods append() and extend()?
- 227k
2780
votes
22
answers
1.4m
views
How to sort a list of dictionaries by a value of the dictionary in Python?
How do I sort a list of dictionaries by a specific key's value? Given:
[{'name': 'Homer', 'age': 39}, {'name': 'Bart', 'age': 10}]
When sorted by name, it should become:
[{'name': 'Bart', 'age': 10}, ...
2750
votes
26
answers
3.2m
views
How do I get the last element of a list?
How do I get the last element of a list? Which way is preferred?
alist[-1]
alist[len(alist) - 1]
- 188k
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votes
18
answers
2.2m
views
How can I randomly select (choose) an item from a list (get a random element)?
How do I retrieve an item at random from the following list?
foo = ['a', 'b', 'c', 'd', 'e']
- 190k
2266
votes
18
answers
4.1m
views
How to remove an element from a list by index
How do I remove an element from a list by index?
I found list.remove(), but this slowly scans the list for an item by value.
- 323k
2266
votes
11
answers
3.8m
views
How do I get the number of elements in a list (length of a list) in Python?
How do I get the number of elements in the list items?
items = ["apple", "orange", "banana"]
# There are 3 items.
- 65.7k
2160
votes
30
answers
2.9m
views
How do I count the occurrences of a list item?
Given a single item, how do I count occurrences of it in a list, in Python?
A related but different problem is counting occurrences of each different element in a collection, getting a dictionary or ...
- 29.2k
2122
votes
26
answers
1.3m
views
How do I check if a variable is an array in JavaScript?
How do I check if a variable is an array in JavaScript?
if (variable.constructor == Array)
- 38.1k
2095
votes
10
answers
1.5m
views
Why is it string.join(list) instead of list.join(string)?
This has always confused me. It seems like this would be nicer:
["Hello", "world"].join("-")
Than this:
"-".join(["Hello", "world"])
Is ...
- 100k
1744
votes
22
answers
1.3m
views
Make a dictionary (dict) from separate lists of keys and values
I want to combine these:
keys = ['name', 'age', 'food']
values = ['Monty', 42, 'spam']
into a single dictionary:
{'name': 'Monty', 'age': 42, 'food': 'spam'}
How can I do this?
- 47.2k
1716
votes
29
answers
251k
views
Why not inherit from List<T>?
When planning out my programs, I often start with a chain of thought like so:
A football team is just a list of football players. Therefore, I should represent it with:
var football_team = new ...
- 26k
1681
votes
13
answers
2.0m
views
if/else in a list comprehension
How do I convert the following for-loop containing an if/else into a list comprehension?
results = []
for x in xs:
results.append(f(x) if x is not None else '')
It should yield '' if x is None, ...
- 91.7k
1636
votes
24
answers
1.8m
views
How to Sort a List<T> by a property in the object
I have a class called Order which has properties such as OrderId, OrderDate, Quantity, and Total. I have a list of this Order class:
List<Order> objListOrder = new List<Order>();
...
- 216k
1468
votes
58
answers
2.2m
views
Removing duplicates in lists
How can I check if a list has any duplicates and return a new list without duplicates?
- 20.5k
1357
votes
37
answers
1.9m
views
How do I reverse a list or loop over it backwards?
How do I iterate over a list in reverse in Python?
See also: How can I get a reversed copy of a list (avoid a separate statement when chaining a method after .reverse)?
- 14k
1350
votes
25
answers
2.2m
views
Get a list from Pandas DataFrame column headers
I want to get a list of the column headers from a Pandas DataFrame. The DataFrame will come from user input, so I won't know how many columns there will be or what they will be called.
For example, ...
- 24.7k
1298
votes
13
answers
2.0m
views
How do I return dictionary keys as a list in Python?
With Python 2.7, I can get dictionary keys, values, or items as a list:
>>> newdict = {1:0, 2:0, 3:0}
>>> newdict.keys()
[1, 2, 3]
With Python >= 3.3, I get:
>>> newdict....
user2015601
1297
votes
25
answers
2.0m
views
Converting array to list in Java
How do I convert an array to a list in Java?
I used the Arrays.asList() but the behavior (and signature) somehow changed from Java SE 1.4.2 (docs now in archive) to 8 and most snippets I found on the ...
- 25.5k
1261
votes
9
answers
1.2m
views
How do I iterate through two lists in parallel?
I have two iterables, and I want to go over them in pairs:
foo = [1, 2, 3]
bar = [4, 5, 6]
for (f, b) in iterate_together(foo, bar):
print("f:", f, " | b:", b)
That should ...
- 125k
1259
votes
12
answers
2.9m
views
Fastest way to check if a value exists in a list
What is the fastest way to check if a value exists in a very large list (with millions of values) and what its index is?
- 13.9k
1242
votes
33
answers
1.5m
views
Get difference between two lists with Unique Entries
I have two lists in Python:
temp1 = ['One', 'Two', 'Three', 'Four']
temp2 = ['One', 'Two']
Assuming the elements in each list are unique, I want to create a third list with items from the first list ...
- 63.2k
1231
votes
10
answers
964k
views
How do I sort a list of objects based on an attribute of the objects?
I have a list of Python objects that I want to sort by a specific attribute of each object:
[Tag(name="toe", count=10), Tag(name="leg", count=2), ...]
How do I sort the list by ....
- 36.9k
1206
votes
30
answers
2.8m
views
Get unique values from a list in python [duplicate]
I want to get the unique values from the following list:
['nowplaying', 'PBS', 'PBS', 'nowplaying', 'job', 'debate', 'thenandnow']
The output which I require is:
['nowplaying', 'PBS', 'job', '...
- 12.3k
1201
votes
27
answers
2.0m
views
Is there a simple way to delete a list element by value?
I want to remove a value from a list if it exists in the list (which it may not).
a = [1, 2, 3, 4]
b = a.index(6)
del a[b]
print(a)
The above gives the error:
ValueError: list.index(x): x not in ...
- 65.1k
1201
votes
15
answers
2.0m
views
Difference between del, remove, and pop on lists in Python
Is there any difference between these three methods to remove an element from a list in Python?
a = [1, 2, 3]
a.remove(2)
a # [1, 3]
a = [1, 2, 3]
del a[1]
a # [1, 3]
a = ...
- 12.9k
1186
votes
3
answers
3.8m
views
1174
votes
12
answers
2.2m
views
How to concatenate (join) items in a list to a single string
How do I concatenate a list of strings into a single string?
For example, given ['this', 'is', 'a', 'sentence'], how do I get "this-is-a-sentence"?
For handling a few strings in separate ...
- 118k
1152
votes
22
answers
464k
views
What's the difference between lists and tuples?
What's the difference between tuples/lists and what are their advantages/disadvantages?
- 40.8k
1125
votes
32
answers
678k
views
Randomize a List<T>
What is the best way to randomize the order of a generic list in C#? I've got a finite set of 75 numbers in a list I would like to assign a random order to, in order to draw them for a lottery type ...
- 14.1k
1109
votes
18
answers
839k
views
List comprehension vs. lambda + filter
I have a list that I want to filter by an attribute of the items.
Which of the following is preferred (readability, performance, other reasons)?
xs = [x for x in xs if x.attribute == value]
xs = ...
- 18.9k
1067
votes
33
answers
1.5m
views
How do I join two lists in Java?
Is there a simpler way than:
List<String> newList = new ArrayList<String>();
newList.addAll(listOne);
newList.addAll(listTwo);
Conditions:
Do not modify the original lists.
JDK only.
No ...
- 24.1k
1057
votes
13
answers
1.3m
views
How do I remove the first item from a list?
How do I remove the first item from a list?
[0, 1, 2, 3] → [1, 2, 3]
- 51.6k
1052
votes
31
answers
730k
views
How do I remove duplicates from a list, while preserving order?
How do I remove duplicates from a list, while preserving order? Using a set to remove duplicates destroys the original order.
Is there a built-in or a Pythonic idiom?
- 25.7k
978
votes
14
answers
1.1m
views
Remove empty strings from a list of strings
I want to remove all empty strings from a list of strings in python.
My idea looks like this:
while '' in str_list:
str_list.remove('')
Is there any more pythonic way to do this?
- 9,801
971
votes
26
answers
2.3m
views
Writing a list to a file with Python, with newlines
How do I write a list to a file? writelines() doesn't insert newline characters, so I need to do:
f.writelines([f"{line}\n" for line in lines])
- 10.3k
950
votes
19
answers
79k
views
List of lists changes reflected across sublists unexpectedly
I created a list of lists:
>>> xs = [[1] * 4] * 3
>>> print(xs)
[[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]
Then, I changed one of the innermost values:
>>> xs[0][0] = 5
>...
- 19.7k
934
votes
26
answers
1.1m
views
Shuffling a list of objects [duplicate]
How do I shuffle a list of objects? I tried random.shuffle:
import random
b = [object(), object()]
print(random.shuffle(b))
But it outputs:
None
- 11.3k
932
votes
14
answers
3.2m
views
Find a value in a list [duplicate]
I use the following to check if item is in my_list:
if item in my_list:
print("Desired item is in list")
Is "if item in my_list:" the most "pythonic" way of finding ...
- 39.4k
913
votes
25
answers
2.6m
views
How to make a new List in Java
We create a Set as:
Set myset = new HashSet()
How do we create a List in Java?
- 19k
911
votes
11
answers
951k
views
How to sort a list/tuple of lists/tuples by the element at a given index
I have some data either in a list of lists or a list of tuples, like this:
data = [[1,2,3], [4,5,6], [7,8,9]]
data = [(1,2,3), (4,5,6), (7,8,9)]
And I want to sort by the 2nd element in the subset. ...
- 37.7k
905
votes
30
answers
3.0m
views
How to define a two-dimensional array?
I want to define a two-dimensional array without an initialized length like this:
Matrix = [][]
But this gives an error:
IndexError: list index out of range
- 10.7k
901
votes
18
answers
2.3m
views
Create an empty list with certain size in Python
How do I create an empty list that can hold 10 elements?
After that, I want to assign values in that list. For example:
xs = list()
for i in range(0, 9):
xs[i] = i
However, that gives IndexError: ...
- 13.5k
892
votes
21
answers
723k
views
How to convert string representation of list to a list
I was wondering what the simplest way is to convert a string representation of a list like the following to a list:
x = '[ "A","B","C" , " D"]'
Even in cases ...
- 11.6k