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python - Length of a list with the reduce() function

I need some help to count the numbers of elements in a list by using the reduce function.

def lenReduce(L):
  return reduce(lambda x: x + 1, L)

With this one I get the following error message:

TypeError: <lambda>() takes 1 positional argument but 2 were given

Greetings from Berlin. ;-)

Solution

  • The function argument to reduce takes two arguments: the return value of the previous call, and an item from the list.

    def counter(count, item):
    return count + 1

    In this case, you don't really care what the value of item is; simply passing it to counter means you want to return the current value of the counter plus 1.

    def lenReduce(L):
    return reduce(counter, L)

    or, using a lambda expression, 

    def lenReduce(L):
    return reduce(lambda count, item: count + 1, L)

    Even though your function ignores the second argument, reduce still expects to be able to pass it to the function, so it must be defined to take two arguments.