Reduce

Conversation started Nov 29, 2019 at 7:39.

Aladdin

Nov 29, 2019 07:39

imgur.com/a/FBtbNAc

Yes

I can't get how it works

John Rennie

Do you understand what the reduce() function does?

Aladdin

Kinda but I don't understand well

It preform some function on each element of Iterable

John Rennie

Suppose you call reduce(fn, [1,2,3,4])

Aladdin

And at the end gives 1 output

@JohnRennie ok

John Rennie

What it does is take the first two elements of the list, 1 and 2, then calculate a = fn(1,2). Then it replaces those first two elements with a.

Aladdin

Nov 29, 2019 07:43

John Rennie

So the list starts [1,2,3,4] and changes to [a,3,4] where a = fn(1,2). OK so far?

Aladdin

John Rennie

But then it repeats. So it takes the first two elements again, now a and 3, and replaces them by b = fn(a,3).

Aladdin

John Rennie

So the list changes from [a,3,4] to [b,4] where b = fn(a,3)

Aladdin

Nov 29, 2019 07:46

Hmm ok

John Rennie

And then it repeats again so the list changes from [b,4] to [c] where c = fn(b,4)

Aladdin

So on

So only 1 element remains at the end

John Rennie

And now it stops because the list has length one, and it returns the value of the one remaining element in the list.

@Aladdin yes

So suppose the function fn(x,y) just added x and y together.

Aladdin

Aha ok

John Rennie

So we start with [1,2,3,4] and after the first iteraction we get a list one element shrter [1+2, 3, 4].

The it iterates again to give [(1+2) + 3, 4]

And then again to give [((1+2)+3) + 4]

Aladdin

Nov 29, 2019 07:50

Got it

Makes, sense

John Rennie

Then it stops because the list now only has a single element, and it returns that value i.e. 1+2+3+4

Aladdin

How will u find maximum in a list using this

John Rennie

Well let's try. Suppose we do reduce(max, [1,2,3,4])

On the first iteration it takes the first two elements, 1 and 2, and does max(1,2) to get the result 2. So the list now looks like [2,3,4].

Then the next iteration does max(2,3) to give 3 so the list reduces to [3,4]

Aladdin

Next we get 4

John Rennie

And the last iteration gives [4] so it returns 4

@Aladdin yes

So this is a way of getting the maximum value in the list, though it's a rather strange way of getting the maximum.

Aladdin

Nov 29, 2019 07:55

@JohnRennie my teacher said something like we can do like reduce(fun, Iterable, 2)

So it will start with 2 and first element of Iterable

John Rennie

The reason you're being taught this is that it's a technique called functional programming. This is kind of trendy at the moment, though I have to confess I don't know much about it.

Aladdin

What's use of functional programming

John Rennie

@Aladdin In effect it prefixes the list with the last argument.

Aladdin

Didn't understand

John Rennie

So if you call reduce(fn, [1,2,3,4], 5) it's like calling reduce(fn, [5,1,2,3,4])

Conversation ended Nov 29, 2019 at 7:57.

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