如何在Python中的两个长列表中减去特定切片的相应元素？

Question

suppose I have following two lists:

x = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]

y = [None,None,None,None,None,10,20,30,40,50,60,70,80,90,100]

I need to subtract the corresponding elements of each list (x[i] - y[i] ) and I want to start subtracting from 6th element of x which is 6, so that the corresponding element in y is not null(None). the following code is what I have tried and got error:

result = []

for i in x[5:]:
  result.append(x[i] - y[i])

Index Error: list index out of range

Answer 1

You should do it like this:

for val1, val2 in zip(x[5:], y[5:]):
    result.append(val1 - val2)

OR

for val1, val2 in list(zip(x, y))[5:]:
    result.append(val1 - val2)

You could also just skip over the None values, like this:

for val1, val2 in zip(x, y):
    if val2 is not None:  # can also check if val1 is not None if needed
        result.append(val1 - val2)

The reason you're getting the IndexError is that the i in your loop is getting assigned the values (not indeces!) of the x list, and you're trying to index the lists with those values. So for example on the last go of the loop i = 15 when the index of that element is only 14 .

Answer 2

You get an error, because you're trying to access y[15] (since 15 is the value of x[14] ).

But y only has 15 elements, and since list items start at index 0, you get list index out of range .

A more flexible approach - as long as your both lists have the same length, is to use zip and skip the None values:

x = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]

y = [None,None,None,None,None,10,20,30,40,50,60,70,80,90,100]

result = []

for val_x, val_y in zip(x, y):
    if(val_y is None):
        continue
    result.append(val_x - val_y)

print(result)

Output:

[-4, -13, -22, -31, -40, -49, -58, -67, -76, -85]

Or as a list comprehension:

result = [ (val_x - val_y) for val_x, val_y in zip(x, y) if (val_y is not None) ]

Answer 3

You can use enumerate

Ex:

x = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
y = [None,None,None,None,None,10,20,30,40,50,60,70,80,90,100]

result = []
for i, v in enumerate(x[5:], 5):
    result.append(v - y[i])
print(result)

Output:

[-4, -13, -22, -31, -40, -49, -58, -67, -76, -85]

Answer 4

As @meowgoesthedog said in his comment, array indices start from 0.

Also, as your 2 list are of same size, you can zip them, to make the code look nicer, so here's a solution with zipped lists :

result = []

for i, j in list(zip(x, y))[5:]:
  result.append(i - j)

And here's a solution without :

result = []

for i in range(x[5:] - 1):
  result.append(x[i] - x[j])

Answer 5

Try this, it will take care of "None"s in both the lists. We need not track them manually. If we don't need first 5 then we can just add a slice on subtract.

subtrct=[None if val is None else (val - (0 if y[index] is None else y[index])) for index, val in enumerate(x)]

Answer 6

You can use the functions starmap() and dropwhile() :

from itertools import starmap, dropwhile

x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
y = [None, None, None, None, None, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100]

# substract corresponding elements
m = starmap(lambda x, y: x - y if y else None, zip(x, y))

# filter None
list(dropwhile(lambda x: not x, m))
# [-4, -13, -22, -31, -40, -49, -58, -67, -76, -85]