How to check whether a tuple exists in a Python list?

Question

I am new to Python, and I am trying to check whether a pair [a,b] exists in a list l=[[a,b],[c,d],[d,e]]. I searched many questions, but couldn't find precise solution. Please can someone tell me the right and shortest way of doing it?

when i run :

a=[['1','2'],['1','3']]
for i in range(3):
    for j in range(3):
        if [i,j] in a:
            print a

OUTPUT IS BLANK

how to achieve this then?

Your code works for me, output is `[[1,2], [1,3]]` as expected. — bereal, Mar 11 '12 at 11:44
Ah, after you edited the code it's quite obvious, `[1,2]` != `['1','2']`. — bereal, Mar 11 '12 at 11:45

score 23 · Answer 1 · answered Mar 11 '12 at 11:31

23

Here is an example:

>>> [3, 4] in [[2, 1], [3, 4]]
True

If you need to do this a lot of times consider using a set though, because it has a much faster containment check.

answered Mar 11 '12 at 11:31

orlp

106,415
33
201
300

i am using nested loops for `i in range(3):` and `for j in range(3)` and then trying for `[i,j] in a`....but its not identifying – sum2000 Mar 11 '12 at 11:36

score 9 · Accepted Answer · answered Mar 11 '12 at 11:50

The code does not work because '1' != 1 and, consequently, ['1','2'] != [1,2] If you want it to work, try:

a=[['1','2'],['1','3']]
for i in range(3):
    for j in range(3):
        if [str(i), str(j)] in a: # Note str
            print a

(But using in or sets as already mentioned is better)

score -2 · Answer 3 · answered Mar 11 '12 at 11:45

In my interpreter (IPython 0.10, Python 2.7.2+) your code gives the correct output:

In [4]: a=[[1,2],[1,3]]

In [5]: for i in range(3):
   ...:         for j in range(3):
   ...:             if [i,j] in a:
   ...:                 print a
   ...: 
[[1, 2], [1, 3]]

(This should be a comment, but I can't leave them yet.)

EDIT:

Turns out you had strings in the a list. Then you need to convert your ints to str as well:

a=[['1','2'],['1','3']]
for i in range(3):
    for j in range(3):
        if [str(i), str(j)] in a:
            print a

score -2 · Answer 4 · answered Mar 11 '12 at 12:02

Don't forget that [a, b] is not [b, a] in python so you might want to order the 2 values in your tuples if you want to consider [A, B] and [B, A] is the same:

You might also want to use set(your_list) if your list is big and it has redundancy.

In your code example you are compaing integers and strings :

['1', '2'] # this is a 2-list of strings '1' and '2'
[1, 2] # this is a 2-list of integers 1 and 2

How to check whether a tuple exists in a Python list?

4 Answers4

Linked