python - Finding a subset of values in another list - Stack Overflow

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Why does this always print false?

I expected a different outcome because the first two items in the 4th element in List1 are 61, 64 and those match 2 items in the 1st element of List2 which has 61, 62, 64, 70

List1 = [
    [60, 64, 67],
    [62, 67, 72],
    [62, 66, 69],
    [61, 64, 69],
    [64, 66, 71],
    [63, 66, 71],
]

List2 = [
    [61, 62, 64, 70],
    [60, 61, 62, 63],
    [64, 65, 67, 69],
    [65, 70, 66, 61],
]

for x in List1:             
    is_subset = all([(y in List2) for y in x[0:2]])
    print(is_subset)           

Why does this always print false?

I expected a different outcome because the first two items in the 4th element in List1 are 61, 64 and those match 2 items in the 1st element of List2 which has 61, 62, 64, 70

List1 = [
    [60, 64, 67],
    [62, 67, 72],
    [62, 66, 69],
    [61, 64, 69],
    [64, 66, 71],
    [63, 66, 71],
]

List2 = [
    [61, 62, 64, 70],
    [60, 61, 62, 63],
    [64, 65, 67, 69],
    [65, 70, 66, 61],
]

for x in List1:             
    is_subset = all([(y in List2) for y in x[0:2]])
    print(is_subset)           

• python
• list
• subset

Share Improve this question Follow edited Jan 31 at 16:07 Barmar 783k5656 gold badges547547 silver badges660660 bronze badges asked Jan 31 at 15:02 TimTim 111 silver badge66 bronze badges 4

• sorry edited the w bits out. so list 2 is just strings, then? How do I define the list (or other data structure) for this to work. I am basically trying to see if there is a match with 2 elements from sublists in list1 in any list 2 sublists – Tim Commented Jan 31 at 15:36
• How about using sets? Then you can use .issubset() to tell if one set is a subset of another. – Barmar Commented Jan 31 at 16:10
• 1 Aside: The argument to any() and all() can be a generator, you don't need to construct a list. This allows them to stop generating as soon as the answer is determined. – Barmar Commented Jan 31 at 16:12
• List 2 contains lists of integers, not strings. – Barmar Commented Jan 31 at 16:13

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2 Answers 2

Sorted by: Reset to default 1

You were comparing numbers with list items that are lists as well. This alternative code might satisfy your purpose.

List1 = [
    [60, 64, 67],
    [62, 67, 72],
    [62, 66, 69],
    [61, 64, 69],
    [64, 66, 71],
    [63, 66, 71],
]

List2 = [
    [61, 62, 64, 70],
    [60, 61, 62, 63],
    [64, 65, 67, 69],
    [65, 70, 66, 61],
]

for x in List1:
    for z in List2:
        is_subset =all([(y in z) for y in x[0:2]])
        if is_subset == True:
            break
    print(is_subset)    

Result:

False
False
False
True
False
False

Use sets and the issubset() method.

You should use any() to test if x[0:2] is a subset of any of the lists in List2, not all().

List1 = [
    [60, 64, 67],
    [62, 67, 72],
    [62, 66, 69],
    [61, 64, 69],
    [64, 66, 71],
    [63, 66, 71],
]

List2 = [
    [61, 62, 64, 70],
    [60, 61, 62, 63],
    [64, 65, 67, 69],
    [65, 70, 66, 61],
]

Sets2 = list(map(set, List2))

for x in List1:
    xset = set(x[0:2])
    is_subset = any(xset.issubset(y) for y in Sets2)
    print(is_subset)

Result:

False
False
False
True
False
False

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