# FAQ: Functional Programming - Combining all Three Higher-Order Functions

This community-built FAQ covers the “Combining all Three Higher-Order Functions” exercise from the lesson “Functional Programming”.

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## FAQs on the exercise Combining all Three Higher-Order Functions

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Hi there,
I’m having a difficult time to understand the order to apply in these higher functions.

first i thought the code was :

``````product = reduce(lambda x,y: x*y,  filter(lambda n: n < 10, map(lambda i: i+5, nums)))
``````

output 3024

the exercise was not right, so i went to the solution, that was:

``````product = reduce(lambda x,y: x*y, map(lambda i: i+5, filter(lambda n: n < 10, nums)))
``````

output 72648576

i understand the reduce always comes outside of all, but how can i know where to put the other two in the sequence?

thanks!

I have a similar question:

the order in all exercises was always reduce( map( filter()))
in exercise 8 it is reduce( filter( map())) with map() coming after filter() does the order of the higher-order functions matter, or not? I’ confused
just completed the exercise and the order in the two steps is contrary, first filter() than map(), than map() followed by filter(). is the first order particular when working with dictionaries?

There’s no reason function patterns have to start with reduce(), or go in any order for that matter. That’s just how these exercises are (maybe CC should add an exercise with another pattern).
You have to think about what the problem is asking for, not just fill it in as before. So for checkpoint 2: “find all numbers less than 10, add five to them, and find their total product”. This means filter (for less than 10), map (add 5 to each), then reduce (multiply all those together). Which means reduce(λ, map(λ, filter(λ, list))). Okay, turns out that’s a bad example since it’s the same order yet again (they really do need another exercise). But point is the order is case-by-case.

1 Like

Correction (can’t seem to edit without deleting entirely): There actually is a good reason reduce is the last operation in these. It returns one value instead of an iterable, and these three are iterable functions.