Finding average length of items in a list. Python

Question

my_list = [['abcdefg','awdawdawd'],['awdwer'],[],'rtgerfwed','wederfrtg']]

so i have a list and i want to find the average lengths for each sublist within the list

i tried this but it didn't seem to work

lengths = []
lengths = my_list

for k in lengths:
    k = len(k)

print lengths

# i get this again -> [['abcdefg','awdawdawd'],['awdwer'],[],'rtgerfwed','wederfrtg']]

i want the output to be

lengths = [8,6,0,9]

Answer 1

A list of the lengths of each element in a list:

lengths = [len(i) for i in my_list]

For the average:

def averageLen(lst):
    lengths = [len(i) for i in lst]
    return 0 if len(lengths) == 0 else (float(sum(lengths)) / len(lengths)) 

If it's a list of lists:

lengths = [averageLen(i) for i in my_list]

Answer 2

First, iterating over a list in python only copies the value of each item into a temporary variable. So in fact, in your example, k is assigned a value of each item in the list, but changing it doesn't change the original list. Second, it seems that you tried to map each item in the list to its length, and not to the average length of its items. Mapping each item to its length could be easily done with the builtin function map: map(len,lengths). However, to accomplish what you request, we're gonna need to improve a little bit. Let's define a function to calculate the average length of a lists items:

def average_len(l):
  return sum(map(len, l))/float(len(l))

Now we can use map again to get the average of each sublist: map(average_len, lengths).