Szathmáry László honlapja @ DEIK | Nim2 / filter and map operations

Let's see the filter and map operations. In Python, we use list comprehensions.

(1) filter, map
def main(): li = [1, 8, 2, 5, 9, 0, 7] small = [x for x in li if x < 5] print(small) # [1, 2, 0] double = [2 * x for x in li] print(double) # [2, 16, 4, 10, 18, 0, 14] both = [2 * x for x in li if x < 5] print(both) # [2, 4, 0]	import std/sequtils # map, filter import std/sugar # => operator proc main() = let li = @[1, 8, 2, 5, 9, 0, 7] let small = li.filter(x => x < 5) echo small # @[1, 2, 0] let double = li.map(x => 2 * x) echo double # @[2, 16, 4, 10, 18, 0, 14] let both = li.filter(x => x < 5).map(x => 2 * x) echo both # @[2, 4, 0]

Again, when we write li.map(…), that's actually UFCS magic.

Notice that the result of filter() and map() is a new sequence. Every time you call these functions, a new sequence is created. When you work with huge sequences and you chain together several filter and map calls, it might be memory-heavy. See the zero_functional library if you need lazy evaluation.

"=>" is a macro, actually. It's syntactic sugar for anonymous procedures.

Can we simplify it?

Sure :) Nim has two useful templates: mapIt and filterIt. We'll see templates later. At the moment, it's enough to see how to use it:

(2) filterIt, mapIt
# same Python code	import std/sequtils # toSeq(iter), map, filter # std/sugar is not needed proc main() = let li = @[1, 8, 2, 5, 9, 0, 7] let small = li.filterIt(it < 5) echo small # @[1, 2, 0] let double = li.mapIt(2 * it) echo double # @[2, 16, 4, 10, 18, 0, 14] let both = li.filterIt(it < 5).mapIt(2 * it) echo both # @[2, 4, 0]

As you can see, it means the current element.

Using the collect macro

Nim has a macro too, called collect that can do something similar. What is a macro? We'll see later :)

(3) collect
# same Python code	# std/sequtils is not needed import std/sugar # collect proc main() = let li = @[1, 8, 2, 5, 9, 0, 7] let small = collect: for x in li: if x < 5: x echo small # @[1, 2, 0] let double = collect: for x in li: 2 * x echo double # @[2, 16, 4, 10, 18, 0, 14] let both = collect: for x in li: if x < 5: 2 * x echo both # @[2, 4, 0]

Here, when you use both the filter and map operations (last example), then no intermediate sequence is built. Only the resulting sequence is constructed.