Given a list ["foo", "bar", "baz"]
and an item in the list "bar"
, how do I get its index 1
?
How to find the index for a given item in a list?
6.3m views Asked by Eugene M AtThere are 46 answers
![Alejadro Xalabarder](/img/user-default.png)
A function "indexof" similar to another languages that returs -1 if the element is not found can be written like
def indexof (obj, elem, offset=0):
if elem in obj[offset:]:
return offset + obj[offset:].index(elem)
return -1
obj = ["foo", "bar", "baz", "foo"]
print (indexof(obj, "not here"))
print (indexof(obj, "baz"))
print (indexof(obj, "foo", 1))
which returns
-1
2
3
or an optimized version for big lists
def indexof (obj, elem, offset=0):
if offset == 0:
# no need of a sublist
if elem in obj:
return obj.index(elem)
return -1
sublist = obj[offset:]
if elem in sublist:
return offset + sublist.index(elem)
return -1
![Siddharth Satpathy](/img/user-default.png)
Let’s give the name lst
to the list that you have. One can convert the list lst
to a numpy array
. And, then use numpy.where to get the index of the chosen item in the list. Following is the way in which you will implement it.
import numpy as np
lst = ["foo", "bar", "baz"] #lst: : 'list' data type
print np.where( np.array(lst) == 'bar')[0][0]
>>> 1
![pylang](/img/user-default.png)
As indicated by @TerryA, many answers discuss how to find one index.
more_itertools
is a third-party library with tools to locate multiple indices within an iterable.
Given
import more_itertools as mit
iterable = ["foo", "bar", "baz", "ham", "foo", "bar", "baz"]
Code
Find indices of multiple observations:
list(mit.locate(iterable, lambda x: x == "bar"))
# [1, 5]
Test multiple items:
list(mit.locate(iterable, lambda x: x in {"bar", "ham"}))
# [1, 3, 5]
See also more options with more_itertools.locate
. Install via > pip install more_itertools
.
![saolof](/img/user-default.png)
Don't. If you absolutely need to, use the .index(item...)
method on list. However, it takes linear time, and if you find yourself reaching for it, you are probably misusing lists to do something you should not do with them.
Most likely, you care either about 1) a two-way mapping between integers and items, or 2) about finding an item in a sorted list of items.
For the first one, use a pair of dictionaries. If you want a library that does this for you, use the bidict
library.
For the second one, use methods that can properly make use of the fact that the list is sorted. Use the built-in bisect
module in python.
If you find yourself wanting to insert items in a sorted list, you should also not use a sorted list. Either weaken the sorted requirement to a heap using the builtin heapq
module, or use the sortedcontainers
library.
It is bad practice to use a data structure that is not designed for what you want to do. Using one that matches the task you give it will both telegraph to the reader that you want to do that specific thing, and also make your solution a lot faster/more scalable in practice.
![Sanu](/img/user-default.png)
A simple solution in python:
li1=["foo","bar","baz"]
for i in range(len(li1)):
if li1[i]=="bar":
print(i)
The data type of the list elements is irrelevant. Just replace the "bar" with the element you are looking for. We can also write a function for this as:
def indexfinder(element,lis):
for i in range(len(lis)):
if lis[i]==element:
return i
![Vlad Bezden](/img/user-default.png)
If you are going to find an index once then using "index" method is fine. However, if you are going to search your data more than once then I recommend using bisect module. Keep in mind that using bisect module data must be sorted. So you sort data once and then you can use bisect. Using bisect module on my machine is about 20 times faster than using index method.
Here is an example of code using Python 3.8 and above syntax:
import bisect
from timeit import timeit
def bisect_search(container, value):
return (
index
if (index := bisect.bisect_left(container, value)) < len(container)
and container[index] == value else -1
)
data = list(range(1000))
# value to search
value = 666
# times to test
ttt = 1000
t1 = timeit(lambda: data.index(value), number=ttt)
t2 = timeit(lambda: bisect_search(data, value), number=ttt)
print(f"{t1=:.4f}, {t2=:.4f}, diffs {t1/t2=:.2f}")
Output:
t1=0.0400, t2=0.0020, diffs t1/t2=19.60
![MD SHAYON](/img/user-default.png)
I find this two solution is better and I tried it by myself
>>> expences = [2200, 2350, 2600, 2130, 2190]
>>> 2000 in expences
False
>>> expences.index(2200)
0
>>> expences.index(2350)
1
>>> index = expences.index(2350)
>>> expences[index]
2350
>>> try:
... print(expences.index(2100))
... except ValueError as e:
... print(e)
...
2100 is not in list
>>>
![MrWonderful](/img/user-default.png)
And now, for something completely different...
... like confirming the existence of the item before getting the index. The nice thing about this approach is the function always returns a list of indices -- even if it is an empty list. It works with strings as well.
def indices(l, val):
"""Always returns a list containing the indices of val in the_list"""
retval = []
last = 0
while val in l[last:]:
i = l[last:].index(val)
retval.append(last + i)
last += i + 1
return retval
l = ['bar','foo','bar','baz','bar','bar']
q = 'bar'
print indices(l,q)
print indices(l,'bat')
print indices('abcdaababb','a')
When pasted into an interactive python window:
Python 2.7.6 (v2.7.6:3a1db0d2747e, Nov 10 2013, 00:42:54)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def indices(the_list, val):
... """Always returns a list containing the indices of val in the_list"""
... retval = []
... last = 0
... while val in the_list[last:]:
... i = the_list[last:].index(val)
... retval.append(last + i)
... last += i + 1
... return retval
...
>>> l = ['bar','foo','bar','baz','bar','bar']
>>> q = 'bar'
>>> print indices(l,q)
[0, 2, 4, 5]
>>> print indices(l,'bat')
[]
>>> print indices('abcdaababb','a')
[0, 4, 5, 7]
>>>
Update
After another year of heads-down python development, I'm a bit embarrassed by my original answer, so to set the record straight, one can certainly use the above code; however, the much more idiomatic way to get the same behavior would be to use list comprehension, along with the enumerate() function.
Something like this:
def indices(l, val):
"""Always returns a list containing the indices of val in the_list"""
return [index for index, value in enumerate(l) if value == val]
l = ['bar','foo','bar','baz','bar','bar']
q = 'bar'
print indices(l,q)
print indices(l,'bat')
print indices('abcdaababb','a')
Which, when pasted into an interactive python window yields:
Python 2.7.14 |Anaconda, Inc.| (default, Dec 7 2017, 11:07:58)
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def indices(l, val):
... """Always returns a list containing the indices of val in the_list"""
... return [index for index, value in enumerate(l) if value == val]
...
>>> l = ['bar','foo','bar','baz','bar','bar']
>>> q = 'bar'
>>> print indices(l,q)
[0, 2, 4, 5]
>>> print indices(l,'bat')
[]
>>> print indices('abcdaababb','a')
[0, 4, 5, 7]
>>>
And now, after reviewing this question and all the answers, I realize that this is exactly what FMc suggested in his earlier answer. At the time I originally answered this question, I didn't even see that answer, because I didn't understand it. I hope that my somewhat more verbose example will aid understanding.
If the single line of code above still doesn't make sense to you, I highly recommend you Google 'python list comprehension' and take a few minutes to familiarize yourself. It's just one of the many powerful features that make it a joy to use Python to develop code.
![sahasrara62](/img/user-default.png)
using dictionary , where process the list first and then add the index to it
from collections import defaultdict
index_dict = defaultdict(list)
word_list = ['foo','bar','baz','bar','any', 'foo', 'much']
for word_index in range(len(word_list)) :
index_dict[word_list[word_index]].append(word_index)
word_index_to_find = 'foo'
print(index_dict[word_index_to_find])
# output : [0, 5]
![HongboZhu](/img/user-default.png)
index()
returns the first index of value!
| index(...)
| L.index(value, [start, [stop]]) -> integer -- return first index of value
def all_indices(value, qlist):
indices = []
idx = -1
while True:
try:
idx = qlist.index(value, idx+1)
indices.append(idx)
except ValueError:
break
return indices
all_indices("foo", ["foo","bar","baz","foo"])
![user459872](/img/user-default.png)
Pythonic way would to use enumerate
but you can also use indexOf
from operator
module. Please note that this will raise ValueError
if b
not in a
.
>>> from operator import indexOf
>>>
>>>
>>> help(indexOf)
Help on built-in function indexOf in module _operator:
indexOf(a, b, /)
Return the first index of b in a.
>>>
>>>
>>> indexOf(("foo", "bar", "baz"), "bar") # with tuple
1
>>> indexOf(["foo", "bar", "baz"], "bar") # with list
1
![Blackjack](/img/user-default.png)
It just uses the python function array.index()
and with a simple Try / Except it returns the position of the record if it is found in the list and return -1 if it is not found in the list (like on JavaScript with the function indexOf()
).
fruits = ['apple', 'banana', 'cherry']
try:
pos = fruits.index("mango")
except:
pos = -1
In this case "mango" is not present in the list fruits
so the pos
variable is -1, if I had searched for "cherry" the pos
variable would be 2.
![rbrisuda](/img/user-default.png)
If you want all indexes, then you can use NumPy:
import numpy as np
array = [1, 2, 1, 3, 4, 5, 1]
item = 1
np_array = np.array(array)
item_index = np.where(np_array==item)
print item_index
# Out: (array([0, 2, 6], dtype=int64),)
It is clear, readable solution.
![bvanlew](/img/user-default.png)
A variant on the answer from FMc and user7177 will give a dict that can return all indices for any entry:
>>> a = ['foo','bar','baz','bar','any', 'foo', 'much']
>>> l = dict(zip(set(a), map(lambda y: [i for i,z in enumerate(a) if z is y ], set(a))))
>>> l['foo']
[0, 5]
>>> l ['much']
[6]
>>> l
{'baz': [2], 'foo': [0, 5], 'bar': [1, 3], 'any': [4], 'much': [6]}
>>>
You could also use this as a one liner to get all indices for a single entry. There are no guarantees for efficiency, though I did use set(a) to reduce the number of times the lambda is called.
![FatihAkici](/img/user-default.png)
If performance is of concern:
It is mentioned in numerous answers that the built-in method of list.index(item)
method is an O(n) algorithm. It is fine if you need to perform this once. But if you need to access the indices of elements a number of times, it makes more sense to first create a dictionary (O(n)) of item-index pairs, and then access the index at O(1) every time you need it.
If you are sure that the items in your list are never repeated, you can easily:
myList = ["foo", "bar", "baz"]
# Create the dictionary
myDict = dict((e,i) for i,e in enumerate(myList))
# Lookup
myDict["bar"] # Returns 1
# myDict.get("blah") if you don't want an error to be raised if element not found.
If you may have duplicate elements, and need to return all of their indices:
from collections import defaultdict as dd
myList = ["foo", "bar", "bar", "baz", "foo"]
# Create the dictionary
myDict = dd(list)
for i,e in enumerate(myList):
myDict[e].append(i)
# Lookup
myDict["foo"] # Returns [0, 4]
![tanzil](/img/user-default.png)
A problem will arise if the element is not in the list. This function handles the issue:
# if element is found it returns index of element else returns None
def find_element_in_list(element, list_element):
try:
index_element = list_element.index(element)
return index_element
except ValueError:
return None
![ggorlen](/img/user-default.png)
Amazingly, next
's fallback value second parameter mentioned in this comment in a duplicate thread hasn't been shown here yet.
The basic .index()
works well when you can compare whole objects, but it's common to need to search a list of objects or dicts for a particular item by a certain property, in which case a generator with a condition is the natural choice:
>>> users = [{"id": 2, "name": "foo"}, {"id": 3, "name": "bar"}]
>>> target_id = 2
>>> found_user = next(x for x in users if x["id"] == target_id)
>>> found_user
{'id': 2, 'name': 'foo'}
This stops at the first matching element and is reasonably succinct.
However, if no matching element is found, a StopIteration
error is raised, which is a little awkward to deal with. Luckily, next
offers a second parameter next(gen, default)
fallback to provide a more natural, except
-free control flow:
>>> found_user = next((x for x in users if x["id"] == target_id), None)
>>> if not found_user:
... print("user not found")
...
user not found
This is a bit more verbose, but still fairly readable.
If an index is desired:
>>> found_idx = next((i for i, x in enumerate(users) if x["id"] == 1), None)
>>> found_idx
None
>>> next((i for i, x in enumerate(users) if x["id"] == 3), None)
1
As this comment points out, it may be best not to return the typical -1 for a missing index, since that's a valid index in Python. Raising is appropriate if None
seems odd to return.
These are a bit verbose, but feel free to bury the code in a helper function if you're using it repeatedly, providing an arbitrary predicate.
>>> def find(it, pred):
... return next((x for x in it if pred(x)), None)
...
>>> find(users, lambda user: user["id"] == 2)
{'id': 2, 'name': 'foo'}
>>> print(find(users, lambda user: user["id"] == 42))
None
>>> find("foobAr", str.isupper) # works on any iterable!
'A'
![Graham Giller](/img/user-default.png)
All of the proposed functions here reproduce inherent language behavior but obscure what's going on.
[i for i in range(len(mylist)) if mylist[i]==myterm] # get the indices
[each for each in mylist if each==myterm] # get the items
mylist.index(myterm) if myterm in mylist else None # get the first index and fail quietly
Why write a function with exception handling if the language provides the methods to do what you want itself?
![TerryA](/img/user-default.png)
The majority of answers explain how to find a single index, but their methods do not return multiple indexes if the item is in the list multiple times. Use enumerate()
:
for i, j in enumerate(['foo', 'bar', 'baz']):
if j == 'bar':
print(i)
The index()
function only returns the first occurrence, while enumerate()
returns all occurrences.
As a list comprehension:
[i for i, j in enumerate(['foo', 'bar', 'baz']) if j == 'bar']
Here's also another small solution with itertools.count()
(which is pretty much the same approach as enumerate):
from itertools import izip as zip, count # izip for maximum efficiency
[i for i, j in zip(count(), ['foo', 'bar', 'baz']) if j == 'bar']
This is more efficient for larger lists than using enumerate()
:
$ python -m timeit -s "from itertools import izip as zip, count" "[i for i, j in zip(count(), ['foo', 'bar', 'baz']*500) if j == 'bar']"
10000 loops, best of 3: 174 usec per loop
$ python -m timeit "[i for i, j in enumerate(['foo', 'bar', 'baz']*500) if j == 'bar']"
10000 loops, best of 3: 196 usec per loop
![Coder123](/img/user-default.png)
name ="bar"
list = [["foo", 1], ["bar", 2], ["baz", 3]]
new_list=[]
for item in list:
new_list.append(item[0])
print(new_list)
try:
location= new_list.index(name)
except:
location=-1
print (location)
This accounts for if the string is not in the list too, if it isn't in the list then location = -1
![My Car](/img/user-default.png)
Try the following code:
["foo", "bar", "baz"].index("bar")
Refer to: https://www.programiz.com/python-programming/methods/list/index
![Arnaldo P. Figueira Figueira](/img/user-default.png)
All indexes with the zip
function:
get_indexes = lambda x, xs: [i for (y, i) in zip(xs, range(len(xs))) if x == y]
print get_indexes(2, [1, 2, 3, 4, 5, 6, 3, 2, 3, 2])
print get_indexes('f', 'xsfhhttytffsafweef')
![PythonProgrammi](/img/user-default.png)
Getting all the occurrences and the position of one or more (identical) items in a list
With enumerate(alist) you can store the first element (n) that is the index of the list when the element x is equal to what you look for.
>>> alist = ['foo', 'spam', 'egg', 'foo']
>>> foo_indexes = [n for n,x in enumerate(alist) if x=='foo']
>>> foo_indexes
[0, 3]
>>>
Let's make our function findindex
This function takes the item and the list as arguments and return the position of the item in the list, like we saw before.
def indexlist(item2find, list_or_string):
"Returns all indexes of an item in a list or a string"
return [n for n,item in enumerate(list_or_string) if item==item2find]
print(indexlist("1", "010101010"))
Output
[1, 3, 5, 7]
Simple
for n, i in enumerate([1, 2, 3, 4, 1]):
if i == 1:
print(n)
Output:
0
4
![jihed gasmi](/img/user-default.png)
Since Python lists are zero-based, we can use the zip built-in function as follows:
>>> [i for i,j in zip(range(len(haystack)), haystack) if j == 'needle' ]
where "haystack" is the list in question and "needle" is the item to look for.
(Note: Here we are iterating using i to get the indexes, but if we need rather to focus on the items we can switch to j.)
![Russia Must Remove Putin](/img/user-default.png)
Finding the index of an item given a list containing it in Python
For a list
["foo", "bar", "baz"]
and an item in the list"bar"
, what's the cleanest way to get its index (1) in Python?
Well, sure, there's the index method, which returns the index of the first occurrence:
>>> l = ["foo", "bar", "baz"]
>>> l.index('bar')
1
There are a couple of issues with this method:
- if the value isn't in the list, you'll get a
ValueError
- if more than one of the value is in the list, you only get the index for the first one
No values
If the value could be missing, you need to catch the ValueError
.
You can do so with a reusable definition like this:
def index(a_list, value):
try:
return a_list.index(value)
except ValueError:
return None
And use it like this:
>>> print(index(l, 'quux'))
None
>>> print(index(l, 'bar'))
1
And the downside of this is that you will probably have a check for if the returned value is
or is not
None:
result = index(a_list, value)
if result is not None:
do_something(result)
More than one value in the list
If you could have more occurrences, you'll not get complete information with list.index
:
>>> l.append('bar')
>>> l
['foo', 'bar', 'baz', 'bar']
>>> l.index('bar') # nothing at index 3?
1
You might enumerate into a list comprehension the indexes:
>>> [index for index, v in enumerate(l) if v == 'bar']
[1, 3]
>>> [index for index, v in enumerate(l) if v == 'boink']
[]
If you have no occurrences, you can check for that with boolean check of the result, or just do nothing if you loop over the results:
indexes = [index for index, v in enumerate(l) if v == 'boink']
for index in indexes:
do_something(index)
Better data munging with pandas
If you have pandas, you can easily get this information with a Series object:
>>> import pandas as pd
>>> series = pd.Series(l)
>>> series
0 foo
1 bar
2 baz
3 bar
dtype: object
A comparison check will return a series of booleans:
>>> series == 'bar'
0 False
1 True
2 False
3 True
dtype: bool
Pass that series of booleans to the series via subscript notation, and you get just the matching members:
>>> series[series == 'bar']
1 bar
3 bar
dtype: object
If you want just the indexes, the index attribute returns a series of integers:
>>> series[series == 'bar'].index
Int64Index([1, 3], dtype='int64')
And if you want them in a list or tuple, just pass them to the constructor:
>>> list(series[series == 'bar'].index)
[1, 3]
Yes, you could use a list comprehension with enumerate too, but that's just not as elegant, in my opinion - you're doing tests for equality in Python, instead of letting builtin code written in C handle it:
>>> [i for i, value in enumerate(l) if value == 'bar']
[1, 3]
Is this an XY problem?
The XY problem is asking about your attempted solution rather than your actual problem.
Why do you think you need the index given an element in a list?
If you already know the value, why do you care where it is in a list?
If the value isn't there, catching the ValueError
is rather verbose - and I prefer to avoid that.
I'm usually iterating over the list anyways, so I'll usually keep a pointer to any interesting information, getting the index with enumerate.
If you're munging data, you should probably be using pandas - which has far more elegant tools than the pure Python workarounds I've shown.
I do not recall needing list.index
, myself. However, I have looked through the Python standard library, and I see some excellent uses for it.
There are many, many uses for it in idlelib
, for GUI and text parsing.
The keyword
module uses it to find comment markers in the module to automatically regenerate the list of keywords in it via metaprogramming.
In Lib/mailbox.py it seems to be using it like an ordered mapping:
key_list[key_list.index(old)] = new
and
del key_list[key_list.index(key)]
In Lib/http/cookiejar.py, seems to be used to get the next month:
mon = MONTHS_LOWER.index(mon.lower())+1
In Lib/tarfile.py similar to distutils to get a slice up to an item:
members = members[:members.index(tarinfo)]
In Lib/pickletools.py:
numtopop = before.index(markobject)
What these usages seem to have in common is that they seem to operate on lists of constrained sizes (important because of O(n) lookup time for list.index
), and they're mostly used in parsing (and UI in the case of Idle).
While there are use-cases for it, they are fairly uncommon. If you find yourself looking for this answer, ask yourself if what you're doing is the most direct usage of the tools provided by the language for your use-case.
![Caveman](/img/user-default.png)
For one comparable
# Throws ValueError if nothing is found
some_list = ['foo', 'bar', 'baz'].index('baz')
# some_list == 2
Custom predicate
some_list = [item1, item2, item3]
# Throws StopIteration if nothing is found
# *unless* you provide a second parameter to `next`
index_of_value_you_like = next(
i for i, item in enumerate(some_list)
if item.matches_your_criteria())
Finding index of all items by predicate
index_of_staff_members = [
i for i, user in enumerate(users)
if user.is_staff()]
![Badri Paudel](/img/user-default.png)
There is a chance that that value may not be present so to avoid this ValueError, we can check if that actually exists in the list .
list = ["foo", "bar", "baz"]
item_to_find = "foo"
if item_to_find in list:
index = list.index(item_to_find)
print("Index of the item is " + str(index))
else:
print("That word does not exist")
![mirapole](/img/user-default.png)
For those coming from another language like me, maybe with a simple loop it's easier to understand and use it:
mylist = [
"foo", "bar",
"baz", "bar"
]
for index, item in enumerate(mylist):
if item == "bar":
print(index, item)
I am thankful for So what exactly does enumerate do?. That helped me to understand.
See the documentation for the built-in
.index()
method of the list:Caveats
Linear time-complexity in list length
An
index
call checks every element of the list in order, until it finds a match. If the list is long, and if there is no guarantee that the value will be near the beginning, this can slow down the code.This problem can only be completely avoided by using a different data structure. However, if the element is known to be within a certain part of the list, the
start
andend
parameters can be used to narrow the search.For example:
The second call is orders of magnitude faster, because it only has to search through 10 elements, rather than all 1 million.
Only the index of the first match is returned
A call to
index
searches through the list in order until it finds a match, and stops there. If there could be more than one occurrence of the value, and all indices are needed,index
cannot solve the problem:Instead, use a list comprehension or generator expression to do the search, with
enumerate
to get indices:The list comprehension and generator expression techniques still work if there is only one match, and are more generalizable.
Raises an exception if there is no match
As noted in the documentation above, using
.index
will raise an exception if the searched-for value is not in the list:If this is a concern, either explicitly check first using
item in my_list
, or handle the exception withtry
/except
as appropriate.The explicit check is simple and readable, but it must iterate the list a second time. See What is the EAFP principle in Python? for more guidance on this choice.