Using #inject to join strings from an array

Question

I'm going through an online lesson, which usually has a very simple one line solution. A problem states that, given the following array:

["emperor", "joshua", "abraham", "norton"]

I must use #inject to get a single string of all names joined together with a string, each name initial capped, like this:

"Emperor Joshua Abraham Norton"

While this could easily be done with #map and #join, this particular exercise requires the use of #inject only. I came up with something like this:

["emperor", "joshua", "abraham", "norton"].inject("") do |memo, word|
   memo << word.capitalize << " "
end

which would give me:

"Emperor Joshua Abraham Norton "

where the whitespace at the end of the string doesn't pass as the correct solution.

How do I achieve this without the whitespace at the end?
Is this even the right way to use #inject, passing an empty string?
Am I making correct use of the << to combine strings?

Thanks for all the answers! I know it seems silly to use #inject in this case, but it's just one of those exercises meant to test your understanding of the concepts and not necessarily your ability to find the shortest and most efficient solution. — Darek Rossman, Mar 14 '12 at 13:50

Ivaylo Strandjev · Accepted Answer · 2012-03-14T14:35:38.410

10

Try this:

a.map{|t| t.capitalize}.join(" ")

I don't think you can escape from the extra space with inject. Also you need to do

memo = memo + word.capitalize + " "

EDIT: as the statement has changed to force you not to use join and map, here is a bit ugly solution with inject:

a.inject("") do |memo, world|
  memo << " " unless memo.empty?
  memo << word.capitalize
end

edited Mar 14 '12 at 14:35

answered Mar 14 '12 at 13:32

Ivaylo Strandjev

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Thanks! The exercise says I must provide a solution using #inject and that I must not use #map and #join. I updated my original question to say that. – Darek Rossman Mar 14 '12 at 13:37
That did it! Another poster used unless memo.empty? which also worked. Thanks! – Darek Rossman Mar 14 '12 at 13:42
I have improved my answer a little bit thanks to user1252434 so that I use unless and empty to make the code a bit more ruby like. Please take another look – Ivaylo Strandjev Mar 14 '12 at 13:43
1

Better use `<<` than `+=` for performance. The former is `O(n)` while the later is `O(n^2)` – Marc-André Lafortune Mar 14 '12 at 14:26
@Marc-AndréLafortune I did not know that. Thank you for the insight. I will edit my answer accordingly – Ivaylo Strandjev Mar 14 '12 at 14:35
@izomorphius You can think of: `long_string + "x"` implies copying all of `long_string` to a new string. `long_string << "x"` doesn't need to do that, just append one character. – Marc-André Lafortune Mar 14 '12 at 14:41
This post is very valuable I wish I could add it to my favorites list – Donato Oct 01 '14 at 17:18

megas · Answer 2 · 2012-03-15T19:58:22.000

2

a = ["emperor", "joshua", "abraham", "norton"]
a.drop(1).inject(a.first.capitalize){|res,m| res << ' '+m.capitalize }

edited Mar 15 '12 at 19:58

answered Mar 14 '12 at 13:26

megas

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Thanks for the response! I edited my original question to clarify that this exercise requires the use of #inject. It specifically says not to use #map and #join. – Darek Rossman Mar 14 '12 at 13:35
Oh that's interesting...thanks! Can you explain the use of << after res and the + before m.capitalize? Is there a difference between the two in this case? – Darek Rossman Mar 14 '12 at 13:54
1

' '+m.capitalize will return " Joshua" and it adds to initial value "Emperor". The result of operation will be "Emperor Joshua" which will be passed to the next iteration. – megas Mar 14 '12 at 13:59
Close, but the first element is added twice. The code results in "Emperor Emperor Joshua Abraham Norton". Should work if you call `inject` on `a[1..-1]`. – user1252434 Mar 15 '12 at 08:21

score 1 · Answer 3 · answered Mar 14 '12 at 13:35

There are better ways then #inject, see the other answers. But if you insist you could just String#rstrip the trailing space character.

Or turn the block around and check whether memo is empty before adding the character.

memo << " " unless memo.empty?
memo << name.capitalize

I'm not sure about the << operator. I would use +, but that is probably just be a personal preference.

score 1 · Answer 4 · edited Sep 20 '20 at 11:34

1

Checking to add " " every turn is more expensive then to chop! last output. Your choise about << is true you can look string concatenation.

%w(emperor joshua abraham norton).inject("") do |m,w|
  m << w.capitalize << " "
end.chop!
"Emperor Joshua Abraham Norton"

edited Sep 20 '20 at 11:34

Sebastián Palma

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answered Mar 14 '12 at 14:22

Selman Ulug

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Ah very nice, thanks! I've got a lot of methods to learn yet. – Darek Rossman Mar 14 '12 at 14:58

score 0 · Answer 5 · answered Aug 12 '15 at 10:25

0

puts x.inject { |memo, val| "#{memo} #{val.upcase}" }

answered Aug 12 '15 at 10:25

Karol Rossa

1

score 0 · Answer 6 · edited May 10 '16 at 08:44

0

Don't use #inject , there are better ways to solve that:

["emperor", "joshua", "abraham", "norton"].map(&:capitalize).join(' ')

Your first objective should always be to solve the problem at hand. You could use #inject, but it's just not the ideal, or even clear, solution here. You have better things to do with your time than try to figure out how to trick #inject into giving you the right results.

edited May 10 '16 at 08:44

Community

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answered Mar 14 '12 at 13:25

wersimmon

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I would, however, suggest asking this question on the [Code Golf SE](http://codegolf.stackexchange.com/), just to see what people come up with. – wersimmon Mar 14 '12 at 13:30
The problem states that while it could (and should) easily be done with #map and #join, it wants you to use #inject. The exercise is meant to give you an understanding of what these methods do and how they work. – Darek Rossman Mar 14 '12 at 13:31
So test your understanding with a problem that makes sense: like "add all the numbers from 1 to 100, but subtract numbers divisible by 5". – wersimmon Mar 14 '12 at 13:43

Using #inject to join strings from an array

6 Answers6