Ruby array map and join in one loop

Question

Array exemple

[
 [
   "Francis",
   "Chartrand",
   "[email protected]"
 ],
 [
   "Francis",
   "Chartrand",
   "[email protected]"
 ],...
]

Result wanted

"[email protected], [email protected], ..."

My solution (two loop)

array.map{|a| a[2]}.join(", ")

Is it possible to do this with one loop?

@SergioTulentsev Ahah not for a homework, just for refactoring and personal informations — Francis Chartrand
– Francis Chartrand, Commented Oct 1, 2013 at 14:26
Seems more like refuctoring to me. map/join is common idiom, don't stray from it. — Sergio Tulentsev
– Sergio Tulentsev, Commented Oct 1, 2013 at 14:27
Well. you could consider join to be a loop, when you look at the runtime (it is implemented using a O(n) loop) — tessi
– tessi, Commented Oct 1, 2013 at 14:27
I believe that map/join is better, and faster than one loop cyle with a condition check. — Малъ Скрылевъ
– Малъ Скрылевъ, Commented Oct 1, 2013 at 14:28

tessi · Accepted Answer · 2013-10-01 15:27:29Z

8

Using Enumerable#inject we can do the task in one loop:

a = [
  ["Francis", "Chartrand", "[email protected]"],
  ["Francis", "Chartrand", "[email protected]"]
]
a.inject(nil) {|str, arr| str ? (str << ', ' << arr[2]) : arr[2].dup}
#=> "[email protected], [email protected]"

However, this is an academic thing only, because map/join is faster and more readable anyways. See this benchmark:

             user   system    total       real
map/join 1.440000 0.000000 1.440000 ( 1.441858)
inject   2.220000 0.000000 2.220000 ( 2.234554)

edited Oct 1, 2013 at 15:27

answered Oct 1, 2013 at 14:23

tessi

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5 Comments

Francis Chartrand Over a year ago

Nice answer, but map and join still faster gist.github.com/chartrandf/6779594

sawa Over a year ago

@FrancisChartrand Of course map and join is not only faster, but is more straightforward. And that is what everyone is telling you. But since you insist on not using map and join, and since you insist on doing it in one loop, and since you do not mention the speed in the question, this is what you get.

tessi Over a year ago

FrancisChartrand, sawa you both are right. map/join is faster (I have created a second benchmark where my answer is optimized for performance (so its a little more fair)). I better state that in my answer :)

toro2k Over a year ago

@tessi Pay attention that the optimization you have done alters the original array. After executing it a[0] is ["Francis", "Chartrand", "[email protected], [email protected]"].

tessi Over a year ago

@toro2k good catch. Fixed it (which makes the code look a little clumsy). But we all agree to use map/join anyways, so clumsiness shouldn't really matter :)

Cary Swoveland · Accepted Answer · 2013-10-02 03:48:23Z

1

Here's one approach, but it may not be especially fast.

s = ''          
array.flatten.each_slice(3) {|e| s += e.last + ', '} 
s.chop.chop

Here's another:

array.transpose[2].join(', ')

I assume you wanted a single string of email addresses for the entire array.

answered Oct 2, 2013 at 3:48

Cary Swoveland

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