CODEWITHSUNDEEP
pythonlistlambda
javareigns
javareigns

Reputation: 61

Python reduce on len() function

I have

>>> reduce( (lambda x,y: x + y), [1,2,3])

6

And

>>> reduce( (lambda x,y: x + y), ['cat','dog','hat'])

'catdoghat'

Then, I'd like to get the sum of all string elements in the list:

>>> reduce( (lambda x,y: len(x) + len(y)), ['cat','dog','hat'])

It produces an error.

What am I doing wrong?

Upvotes: 2

Views: 1380

Answers (2)

juanpa.arrivillaga
juanpa.arrivillaga

Reputation: 95948

Think about your reduce operation:

(len((len('cat') + len('dog')))+ len('hat'))

You will end up calling len on an int. You probably want:

sum(map(len, ['cat','dog','hat']))

And stop re-inventing the wheel and using reduce for sum.

Note also, using sum (which is equivalent to reduce(lambda x,y: x+y, ...) is going to have terrible performance for strings, i.e. O(n^2). Instead, use ''.join(sequence_of_strings)

Indeed, the interpreter will yell at you if you even try!

>>> sum(['a','b','c'], '')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sum() can't sum strings [use ''.join(seq) instead]

Upvotes: 4

Nir Alfasi
Nir Alfasi

Reputation: 53525

reduce is a process in which you iteratively go over all the items and apply an operation, let's try to do it with your example:

reduce( (lambda x,y: len(x) + len(y)), ['cat','dog','hat'])

so first x is 'cat' and y is 'dog' and the output is 6, now we'll have x as 6 and y as 'hat' and we'll fail on len(6)

What you want to do instead is sum the lengths of the strings, you can use map for that because unlike reduce map applies a function on all the items (which is exactly what you want - to apply len to each and every item) and sum will take care of the addition:

sum(map(len, ['cat','dog','hat']))

Upvotes: 3

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