How can I create a new array for summing array elements in place in Ruby?
[1,2,3,4,5].each_cons(2).map {|a, b| a + b }
gives me [3, 5, 7, 9] but expected result is [1,3,6,10,15].
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⇑ Welcome, PHP!Aleksei Matiushkin– Aleksei Matiushkin2016-11-22 09:12:05 +00:00Commented Nov 22, 2016 at 9:12
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Yes, just old school solution.. :)Murat Ustuntas– Murat Ustuntas2016-11-22 09:28:16 +00:00Commented Nov 22, 2016 at 9:28
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@MuratUstuntas what does "in place" mean?Stefan– Stefan2016-11-22 11:40:48 +00:00Commented Nov 22, 2016 at 11:40
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@Stefan, i means "in place" that you should update the original array rather than creating a new one.Murat Ustuntas– Murat Ustuntas2016-11-23 05:29:54 +00:00Commented Nov 23, 2016 at 5:29
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1@MuratUstuntas in that case, "How can i create a new array" is a bit misleading.Stefan– Stefan2016-11-23 09:33:30 +00:00Commented Nov 23, 2016 at 9:33
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5 Answers
More simple for understanding, I think:
temp_sum = 0
arr.map! {|e| temp_sum += e }
=> [1, 3, 6, 10, 15]
If you want to create a new array instead of existing one, just use map instead of map!
6 Comments
Aleksei Matiushkin
The first answer here so far answering “in place” part of the question. It has a drawback of producing N sums, but it could be fixed with
.with_object(temporary_sum).
Ilya
@mudasobwa, fixed:)
Aleksei Matiushkin
arr.each_with_object(sum: 0).map! {|e, acc| acc[:sum] += e }
Murat Ustuntas
@mudasobwa, this is shining.
Ilya
@SunnyMagadan, usually
in place means mutator. But I'll add your remark. |
Lots of ways to do this. One way is to create an Enumerator instance and use inject:
def adder array
enum = Enumerator.new do |y|
array.inject (0) do |sum, n|
y << sum + n
sum + n
end
end
enum.take array.size
end
adder [1,2,3,4,5] #=> [1, 3, 6, 10, 15]
2 Comments
Aleksei Matiushkin
(sum + n).tap &y.method(:<<)Murat Ustuntas
@mudasobwa, yes this is great idea.
[1, 2, 3, 4, 5].each_with_object([]){|e, a| a.push(a.last.to_i + e)}
# => [1, 3, 6, 10, 15]
5 Comments
Aleksei Matiushkin
to_i results in failing on, say, strings.
Eric Platon
#to_i is certainly there for the convention that nil.to_i returns 0. Aimed at numbers only.
sawa
@mudasobwa Not sure what you mean. Where does a string come from?
BDL
Although this might solve the problem, it is always good to add an explanation why/how this works.
Aleksei Matiushkin
@sawa Array might consist of any objects, having
#+ defined on, I don’t see any reason to reject anything save for Numerics. a.push(a.last ? a.last + e : e) would work for anything.Another variation:
> [1, 2, 3, 4, 5].reduce([]) {|acc, el| acc << el + (acc[-1] || 0); acc}
#=> [1, 3, 6, 10, 15]
Yuk.
1 Comment
Eric Duminil
If you use inject/reduce and have to return an object in a second statement inside the block, it means you could probably use each_with_object.
It's not very elegant, but it seems to work :
[1,2,3,4,5].each_with_object([]){|i,l| l<<(l.last||0)+i}
#=> [1,3,6,10,15]
3 Comments
Aleksei Matiushkin
@MuratUstuntas this is insane (besides that this is not a winner, re-examine your benchmarks.) The difference in half of nanosecond should not matter when choosing the proper solution.
Eric Duminil
I agree with @mudasobwa : your benchmark basically shows that there' s no big statistical variations between those implementations. So choose only depending on, say, code readability and familiarity to the used methods.
Murat Ustuntas
Yes you are right.. I just say a word. If i made a mistake, forgive me. :)