How to average list elements in Python-CodePudding

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]]

CodePudding user response：

How about

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

CodePudding user response：

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
]

CodePudding user response：

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.

CodePudding user response：

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]

CodePudding user response：

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]

CodePudding user response：

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]