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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?
Answer1: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)
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 <strong>set</strong> though, because it has a much faster containment check.
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]]
<em>(This should be a comment, but I can't leave them yet.)</em>
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
Answer4:This code works fine for me:
>>> a = [[1, 2], [3, 4], [13, 11]]
>>>
>>> for i in range(10):
... for j in range(10):
... if [i, j] in a:
... print [i, j]
...
[1, 2]
[3, 4]
>>>
I'm not sure what is wrong with your code. For sure you have missing ']' in first line.
Answer5: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
