Length of 2d list in python

Question

I have a 2D list, for example mylist =[[1,2,3],[4,5,6],[7,8,9]].

Is there any way I can use len() function such that I can calculate the lengths of array indices? For example:

len(mylist[0:3])
len(mylist[1:3])
len(mylist[0:1])

Should give:

9
6
3

Answer 1

length = sum([len(arr) for arr in mylist])

sum([len(arr) for arr in mylist[0:3]]) = 9
sum([len(arr) for arr in mylist[1:3]]) = 6
sum([len(arr) for arr in mylist[2:3]]) = 3

Sum the length of each list in mylist to get the length of all elements.
This will only work correctly if the list is 2D. If some elements of mylist are not lists, who knows what will happen...

Additionally, you could bind this to a function:

len2 = lambda l: sum([len(x) for x in l])
len2(mylist[0:3]) = 9
len2(mylist[1:3]) = 6
len2(mylist[2:3]) = 3

Answer 2

You can use reduce to calculate the length of array indices like this, this can also handle the scenario when you pass in something like mylist[0:0]:

def myLen(myList):
    return reduce(lambda x, y:x+y, [len(x) for x in myList], 0)

myLen(mylist[0:3]) = 9
myLen(mylist[1:3]) = 6
myLen(mylist[0:1]) = 3
myLen(mylist[0:0]) = 0

Answer 3

You can flatten the list, then call len on it:

>>> mylist=[[1,2,3],[4,5,6],[7,8,9]]
>>> import collections
>>> def flatten(l):
...     for el in l:
...         if isinstance(el, collections.Iterable) and not isinstance(el, basestring):
...             for sub in flatten(el):
...                 yield sub
...         else:
...             yield el
...
>>> len(list(flatten(mylist)))
9
>>> len(list(flatten(mylist[1:3])))
6
>>> len(list(flatten(mylist[0:1])))
3