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Ruby: How does `grep` method work?
Context
Problem
Let's say I want to find all even numbers in an array then double them up.
Example:
# Input:
a = [1, 2, 3, 4, 5]
# Output:
[4, 8]
I came up with 2 solutions:
Solution 1:
a.select{|x| x.even?}.map{|x| x * 2}
# => [4, 8]
Solution 2:
is_even = ->(x){x.even?}
a.grep(is_even){|x| x * 2}
# => [4, 8]
Question
In Solution 1, there are 2 iterations:
selectwill iterate througha, pick out even numbers, then return array[2, 4]mapwill iterate through[2, 4]to double them up
I have a question about the grep method in Solution 2 :
How many iterations does ruby walk through inside method grep?
Does it iterate through a, check if an element is even, if it is, yield it right away to the supplied code block? In this case, there's only one iteration.
Or does it walk through 2 iterations like in Solution 1.?
I hope I made my question clear enough. Thank you.
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1 CÂU TRẢ LỜI
+1
This is source code of Enumerable#grep
static VALUE
enum_grep(VALUE obj, VALUE pat)
{
VALUE ary = rb_ary_new();
struct MEMO *memo = MEMO_NEW(pat, ary, Qtrue);
rb_block_call(obj, id_each, 0, 0, rb_block_given_p() ? grep_iter_i : grep_i, (VALUE)memo);
return ary;
}
Returns an array of every element in enum for which Pattern === element. If the optional block is supplied, each matching element is passed to it, and the block's result is stored in the output array.
So I think it's just only one iteration.
And as you said, solution 1 uses the output array of select as input of map.
FYI https://ruby-doc.org/core-2.5.0/Enumerable.html#method-i-grep https://ruby-doc.org/core-2.5.0/Enumerable.html#method-i-select https://ruby-doc.org/core-2.5.0/Enumerable.html#method-i-map
Thank you for your answer.
Honestly, I've read the source code of Enumerable#grep but don't understand a word.
So, is it safe to say Solution 2 is more efficient than Solution 1 because it takes less iteration to process?
@toan not sure. you should check the benchmark for that 
require 'benchmark'
big_array = (1..10**7).to_a
is_even = ->(x){x.even?}
puts Benchmark.measure { big_array.select{|x| x.even?}.map{|x| x * 2} }
# => 0.980000 0.040000 1.020000 ( 1.015895)
puts Benchmark.measure { big_array.grep(is_even){|x| x * 2} }
# => 1.480000 0.020000 1.500000 ( 1.521293)
Solution 1 is faster as the benchmark result
