What is happening here?
reduce(lambda x,y: x+y, [x for x in range(1,1000) if x % 3 == 0 or x % 5 == 0])
I understand how x is iterating through all of the numbers from 1 to 999 and taking out those that are divisible by 3 or 5, but the 'lambda x,y: x+y' part is stumping me.
This is bad Python for
sum(x for x in range(1,1000) if x % 3 == 0 or x % 5 == 0)
It simply sums all numbers in the range 1..999 divisible by 3 or 5.
reduce() applies the given function to the first two items of the iterable, then to the result and the next item of the iterable, and so on. In this example, the function
lambda x, y: x + y
simply adds its operands.
saying
f = lambda x, y : x + y
is almost the same as saying
def f(x, y):
return x + y
in other words lambda returns a function that given the parameters to the left of the : sign will return the value of the expression on the right of it.
In respect to a function is however quite limited, for example allows only one expression and no statements are allowed. This is not a serious problem however because in Python you can define a full function even in the middle of another function and pass that instead.
The usage you shown is however quite bad bacause a lambda there is pointless... Python would allow to compute that sum directly instead of using reduce.
Also, by the way, for the result of that computation there is an easy closed-form solution that doesn't require any iteration at all... (hint: the sum of all numbers from 1 to n is n*(n+1)/2 and the sum of all multiples of 5 from 5 to n is 5*(sum of all numbers from 1 to n/5) ... therefore ...)
A lambda designates an anonymous function. The syntax lambda x,y: x+y can be stated in English as "declare a nameless function taking two parameters named x and y. Perform the operation x+y. The return value of this nameless function will by the result of this operation"
reduce applies some function sequentially to the first two elements of a supplied list, then to the result of that function and the third element, and so on. Therefore, the lambda in the supplied code is used by reduce to add together the elements of the supplied list, which will contain all of the multiples of 3 and 5 less than 1000.
