Find the index of an element in a list

Question

I am having an issue to find the index of each similar values within a list.

Example:

b = ["a", "b", "a", "a", "b", "b"]
for x in b:
    if "a" in x:
        print(b.index(x))

my current results:

0
0
0

expected results:

0
2
3

(In my actual case we have two to 100 of similar values within a list so .index is not working well.)

Not sure what you are trying to do. Are you only interested in finding the indices of `"a"`? — Selcuk, Aug 27 '19 at 01:27
Yes, I wanted to learn new methods and just in case I came across such issue in the future. — Anonymous, Aug 27 '19 at 01:27
You may want to look up the built-in [enumerate](https://docs.python.org/3/library/functions.html#enumerate) function. — Justin Ezequiel, Aug 27 '19 at 01:32

score 3 · Accepted Answer · answered Aug 27 '19 at 01:29

3

Use enumerate:

[n for n, i in enumerate(b) if i == 'a']

Output:

[0, 2, 3]

answered Aug 27 '19 at 01:29

Chris

27,139
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23
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score 3 · Answer 2 · answered Aug 27 '19 at 01:30

3

This is a problem of algorithm. the method of list.index(val) returns the index of the first value to appear. If you want to find all indices of similar values, then you need to change your algorithm.

A common brute force approach for this is like:

b = ["a", "b", "a", "a", "b", "b"]
for i in range(len(b)):
    if b[i] == 'a':
        print(i)

Which will probably output, as you have said:

0
2
3

answered Aug 27 '19 at 01:30

Seraph Wedd

814
5
14

thank you this is what I want. – Anonymous Aug 27 '19 at 01:31
I will mark as answer in 6mins cool down – Anonymous Aug 27 '19 at 01:33
The method above is, as said, brute force. There are codes much better than that efficiency/time/memory wise. – Seraph Wedd Aug 27 '19 at 01:35

score 1 · Answer 3 · answered Aug 27 '19 at 01:29

1

Fairly straightforward:

def indices(arr, val):
    return [n for n, x in enumerate(arr) if x == val]

b = ["a", "b", "a", "a", "b", "b"]

print(indices(b, "a"))

answered Aug 27 '19 at 01:29

Grismar

20,449
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score 0 · Answer 4 · answered Aug 27 '19 at 01:30

0

Correct Code:

characters = ["a", "b", "a", "a", "b", "b"]
for i in range(len(characters)):
    if characters[i] == "a":
        print(i)

answered Aug 27 '19 at 01:30

Mike

1,231
10
17

Find the index of an element in a list

4 Answers4