How to average list elements in Python

Question

I have a list L. I want to average elements of each list within this list. But I am getting an error. I present the expected output.

L=[[], [1.3342713788981633], [1.3342713788981633], [0.40896852187708455,1,3]]
L_avg=[(i+j)/len[i,j] for [i,j] in L]
print(L_avg)

The error is

in <listcomp>
    L_avg=[(i+j)/len[i,j] for [i,j] in L]

ValueError: not enough values to unpack (expected 2, got 0)

The expected output is

L_avg=[[], [1.3342713788981633], [1.3342713788981633], [(0.40896852187708455+1+3)/3]]

francand · Accepted Answer · 2022-08-29 08:06:26Z

1

How about

L_avg=[sum(sublist)/len(sublist) if sublist else [] for sublist in L]

answered Aug 29, 2022 at 8:06

francand

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Comments

ychiucco · Accepted Answer · 2022-08-29 09:31:38Z

0

You're getting an error because of this:

for [i,j] in L

In this way you are trying to assign elements of L to [i,j] but expressions such as [i,j]=[] or [i,j]=[1.3342713788981633] just make no sense.

Here's what you should do instead

L=[[], [1.3342713788981633], [1.3342713788981633], [0.40896852187708455,1,3]]

L_avg = [
    [sum(x)/len(x)] if x else [] for x in L
]

edited Aug 29, 2022 at 9:31

answered Aug 29, 2022 at 8:20

ychiucco

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Comments

Fra93 · Accepted Answer · 2022-08-29 08:03:58Z

0

You can try this:

L=[[], [1.3342713788981633], [1.3342713788981633], [0.40896852187708455,1,3]]

L_avg = [ sum(l)/len(l) if len(l) > 0 else [] for l in L ]

print(L_avg)

Explanation: this iterates over all the sublists in L and calculates the average as the sum of the elements over the number of elements. The if-else is necessary to handle the case when the sublist is empty, which would give a division by 0.

answered Aug 29, 2022 at 8:03

Fra93

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SSlinky · Accepted Answer · 2022-08-29 08:06:18Z

0

If you think about how to do this in a for loop it'll make it easier to understand a comprehension.

for sl in L:
    print(sum(sl)/len(sl) if sl > 0 else [])

So the comprehension is:

[sum(sl)/len(sl) if sl > 0 else [] for sl in L]

answered Aug 29, 2022 at 8:06

SSlinky

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Comments

Massifox · Accepted Answer · 2022-08-29 08:07:57Z

0

Try this code:

L_avg = [sum(l)/len(l) if len(l)>0 else [] for l in L]

it would probably make more sense to return None instead of [] when the list on computing the average is empty:

L_avg = [sum(l)/len(l) if len(l)>0 else None for l in L]

answered Aug 29, 2022 at 8:07

Massifox

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gvn · Accepted Answer · 2022-08-29 08:10:58Z

0

You only be able to do this averaging if all your list members have values inside, in your case the first is empty, hence the error. It expected 2 parameters and got 0.

You can filter out the ones with len zero:

L_avg = [sum(item)/len(item) if len(item)>0 else []  for item in L]

edited Aug 29, 2022 at 8:10

answered Aug 29, 2022 at 8:04

gvn

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