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?

Proceeding

Your code works for me, output is [[1,2], [1,3]] as expected. – Marvelofperu

Ah, after you edited the code it's quite obvious, [1,2] != ['1','2']. – Marvelofperu

sorry, i wanted this, i did'nt write properly – Proceeding

@Marvelofperu can you tell me how to do it now..? – Proceeding

Answer 1

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)

Marvelofperu

Answer 2

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.

Verve

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 – Proceeding

Answer 3

a=[['1','2'],['1','3']]
[[x,y] for x, y in a if [x,y]  == ['1','3']]

As per my understanding of you question, you were trying to find a specific list within another list.

for example ['1', '3'] within a. using list comprehension you can solve this.

Tybie

Answer needs supporting information Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. – Engdahl

Answer 4

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

Misconduct

Answer 5

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

Burlburlap