Python different results from same code

Question

A very odd and peculiar thing happened. I was trying to answer the question at Compare 1 column of 2D array and remove duplicates Python and I made the following answer (which I did not post since some of the existing answers to that question are much compact and efficient) but this is the code I made:

array = [['abc',2,3,],
        ['abc',2,3],
        ['bb',5,5],
        ['bb',4,6],
        ['sa',3,5],
        ['tt',2,1]]

temp = []
temp2 = []

for item in array:
    temp.append(item[0])

temp2 = list(set(temp))

x = 0
for item in temp2:
    x = 0
    for i in temp:
        if item == i:
            x+=1
        if x >= 2:
            while i in temp:
                temp.remove(i)

for u in array:
    for item in array:
        if item[0] not in temp:
            array.remove(item)

print(array)

The code should work, doing what the asker at the given link requested. But I get two pairs of results:

[['sa', 3, 5], ['tt', 2, 1]]

And

[['bb', 4, 6], ['tt', 2, 1]]

Why does the same code on the same operating system on the same compiler on the same everything produce two different answers when run? Note: the results do not alternate. It is random between the two possible outputs I listed above.

You're iterating over temp and array while removing values out of them. Is this what you want? — ForceBru
– ForceBru, Commented Jan 22, 2017 at 14:16
Ah! Thank you @ForceBru. If you post as an answer I can mark as correct. — Octo
– Octo, Commented Jan 22, 2017 at 14:20

Mike Müller · Accepted Answer · 2017-01-22 14:42:21Z

4

In Python sets don't have any specific order, i.e. the implementation is free to choose any order and hence it could be different for each program run.

You convert to a set here:

temp2 = list(set(temp))

Ordering the result should give you consistent (but maybe not right) results :

temp2 = sorted(set(temp))

My results for array.

Sorted:

temp2 = sorted(set(temp))

array looks like this:

[['bb', 4, 6], ['tt', 2, 1]]

Reversed:

temp2 = sorted(set(temp), reverse=True)

array looks like this:

[['sa', 3, 5], ['tt', 2, 1]]

edited Jan 22, 2017 at 14:42

answered Jan 22, 2017 at 14:34

Mike Müller

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