average or mean of float values present in list in python

Question

I have a list lt[][] which contains float values.Now when I try to find the average or mean of these float values I get an error as either float object has no attribute mean or float object is not iterable. The code that I am using is:

for i in range(100):  
    acc_pos = marks[i][5] # list of float values
    pos_acc.append((sum(acc_pos))/(len(acc_pos))) # when used then 2nd error comes
    neg_acc.append(acc_pos.mean()) # when used then 1st error comes

NOTE: I am not using both the method together but either of them.The error comes according to the line I used

UPDATE: marks is a list of list-something like [78.3,[1,0,,1...],...]. So by writing marks[i][5], I am trying to access 0 index element for each row.

@JoelCornett I thought that `mean()` is valid as it did not showed error while I typed in and also the compile error said the float object does not has attribute mean. I am new to python so I am not aware of it — user2916886, Apr 20 '14 at 08:45
Well, then the object in question is not actually a list. Are you using `numpy`? `numpy` arrays have a `mean()` method (but they are not lists). — Joel Cornett, Apr 20 '14 at 09:01

score 3 · Answer 1 · answered Apr 20 '14 at 08:33

3

I thnik the problem is at the second line.

acc_pos = marks[i][0]

That line does not put a list of floats in acc_pos, it puts only one float at pos[i][0] in matrix.

Replace it with

acc_pos = marks[i]

answered Apr 20 '14 at 08:33

Valdrinium

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`marks` is a list of list.So to get the list I need, shouldn't I need to use marks[i][0]? – user2916886 Apr 20 '14 at 08:35
@user2916886 No, of course not. – juanchopanza Apr 20 '14 at 08:36
No. That is just one element. – Valdrinium Apr 20 '14 at 08:36
I made an update to code above. If I want to access marks[i][5], then how would `marks[i]` give me the fifth element of each row? – user2916886 Apr 20 '14 at 08:43
marks[i][5] would give you the 5th element of every row. – Valdrinium Apr 20 '14 at 08:45
if marks is a matrix, the the syntax for accessing a single element is marks[row][colon] – Valdrinium Apr 20 '14 at 08:46
Then it should not give the 2nd error(sum/length) right? – user2916886 Apr 20 '14 at 08:47
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You really didn't get it. marks[i][5] is the 5th element of the ith row. You must first make an empty list and for every row of the matrix, you add to the list the 5th element. Then you apply sum and length for THAT list. – Valdrinium Apr 20 '14 at 08:51
that is what I have mentioned that `marks` is a `list of list`, a 2D array in short.And that is why I am accessing by using `marks[i][5]` or `marks[i][0]`.But it throws compile error – user2916886 Apr 20 '14 at 08:53
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@Scooby Either that, or simply sum up the elements yourself. Building a list just for calling sum() on it is wasteful. – Matthias Urlichs Apr 20 '14 at 08:54
@Matthias Urlichs, you are right. I was just trying not to change too much the original code. – Valdrinium Apr 20 '14 at 08:57

score 0 · Answer 2 · edited May 23 '17 at 12:28

The way to calculate the mean over a list of floats is solved in this stackoverflow question.

If you were looking for the overall mean of the list of lists, the below will work even if the lists are of different lengths:

numerator = reduce(lambda x, y: x + sum(y), marks, 0.0)
denominator = reduce(lambda x, y: x + len(y), marks, 0.0)
result = numerator / denominator
5.0  # result

As a note for debugging (don't have the rep to comment on a question apparently) -- Python is an interpreted language and will not give compiler errors on your program. Python's interactive console makes debugging easy. You could have at least diagnosed your error message above by doing the following in the console:

# (0) define the 'marks' test variable
marks = [[1., 2., 3.], [4., 5., 6.], [7., 8., 9.]]

# (1) do the fist iteration of the loop by hand
i = 0
acc_pos = marks[i][0]
acc_pos.mean()  # Error occurs

# (2) print out the variable
acc_pos

# At this point you realize acc_pos is a float not a list

Helpful resources: tutorial for Python 2, and for Python 3. They are faster than the tutorial sites like Udemy or Code Academy if you already know another language.

average or mean of float values present in list in python

2 Answers2