Find indexes of elements in list that is approximatly equal to element in another list

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I have 2 lists with floating point numbers:

list_1 = [1.03, 3.56, 4.91, 2.85, 5.55, 1.12, 1.73, 1.33, 2.56, 2.58, 3.53]
list_2 = [1.05, 1.55, 2.05, 2.55, 3, 3.55, 4]

The goal is to compare 2 lists and return the indexes of element from the first list (list_1) if they are approximatly equal based on the specific threshold.

def compare_lists(list1, list2, threshold=0.02):
    output_list = []
    for i in range(len(list1)):
        for j in range(len(list2)):
            print(abs(list1[i] - list2[j]))
            if abs(list1[i] - list2[j]) <= threshold:
                output_list.append(i)
    return output_list

This is only return [1, 8] that I think it is wrong. There are some indexes should be included in the output list such as 0 and the last one.

I think the problem is the subtraction of the floating points. I checked the difference between 1.03 and 1.05 and it returns 0.02000000000000018

How can I fix this problem? or I just could round them, but this is not a good choice because the list can contain varied decimal points. It doesn't scope just 2 points, as well as the threshold.

subtraction floating-point list python debugging
2 Answers

You might want to have more padding, due to round-off error:

list_1 = [1.03, 3.56, 4.91, 2.85, 5.55, 1.12, 1.73, 1.33, 2.56, 2.58, 3.53]
list_2 = [1.05, 1.55, 2.05, 2.55, 3, 3.55, 4]

output = [i for i, x in enumerate(list_1) for y in list_2 if abs(x - y) <= 0.02 + 1e-8]
print(output) # [0, 1, 8, 10]
user_114508200

Python has the decimal module https://docs.python.org/3/library/decimal.html "for fast correctly-rounded decimal floating point arithmetic".

from decimal import Decimal

list_1 = [1.03, 3.56, 4.91, 2.85, 5.55, 1.12, 1.73, 1.33, 2.56, 2.58, 3.53]
list_2 = [1.05, 1.55, 2.05, 2.55, 3, 3.55, 4]

def compare_lists(list1, list2, threshold=0.02):
    threshold = Decimal(str(threshold))
    list2 = [Decimal(str(y)) for y in list2]

    output_list = []
    for i, x in enumerate(list1):
        x = Decimal(str(x))
        if any(abs(x-y) <= threshold for y in list2):
            output_list.append(i)
    return output_list

print(compare_lists(list_1, list_2))  # prints [0, 1, 8, 10]
user_88440260
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