Inserting a string into a string within 2 loops

Question

I am looping through a list of words, and am inserting each letter of the alphabet at the beginning of each word, with this code:

def add_charac_front
  ("a".."z").each do |letter|
    @array.each do |list_word|
      list_word.insert(0, letter)
      puts list_word
    end #ends @array loop
  end #ends alphabet loop
end #ends method

but .insert is changing @array so that when I loop through @array for the letter "b", the first list_word in @array is not "Hello" but "aHello".

I need the exact same behavior, but for @array to be the same array for each letter loop I run. It is working correctly when I do this code:

def add_charac_front
  ("a".."z").each do |letter|
    @array.each do |list_word|
      puts "#{letter}#{list_word}"
    end #ends @array loop
  end #ends alphabet loop
end #ends method

But I eventually want to insert letters in different parts of the list_word, not just the front.

I guess the other way I could do this is to .split("") the list_word, then .insert(0, letter), then .join. But it seems way more cumbersome.

How do I do this?

Then where do you store the position you want to insert the letter to ? — Benoît
– Benoît, Commented Sep 14, 2012 at 5:42
Here list_word.insert(0, letter) the 0 is indicating the 0 index (in other words 'position'), whereas I could put 3 to insert the letter into the 3 index. — Fralcon
– Fralcon, Commented Sep 14, 2012 at 5:54
These requirements are difficult because it doesn't make sense for the object to be printing itself. IOW, you should probably be returning the new array and letting the caller print it out (or do whatever else with it that it wants). Which means that the correct answer is probably going to be different than the answer that just solves the problem. Unfortunately, it's really hard to tell what is the right thing to do without more context. — Joshua Cheek
– Joshua Cheek, Commented Sep 14, 2012 at 7:17

Taryn East · Accepted Answer · 2012-09-14 05:44:29Z

2

Ruby passes by reference, not by value. This means that the list_word is actually pointing to inside the original words inside @array.

If you want to just play with the list_word without it affecting the original array, you need to make a throw-away copy of it eg:

def add_charac_front
  ("a".."z").each do |letter|
    @array.each do |orig_word|
      list_word = orig_word.dup # duplicates the list_word
      list_word.insert(0, letter)
      puts list_word
    end #ends @array loop
  end #ends alphabet loop
end #ends method

answered Sep 14, 2012 at 5:44

Taryn East

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4 Comments

Fralcon Over a year ago

Thanks, this is probably what I will need to do. I was surprised that .insert changes the actual array. From my understanding, because there is no bang after .insert, it would be non-destructive. Any insight?

Joshua Cheek Over a year ago

The insert doesn't change the array, it changes the word. The array does not change. As a metaphor, if I'm holding a note and I write on it, my hand didn't change, I'm not holding a different note, it's the same note but the note itself changed. This solution duplicates the note and writes on the duplicate.

Jörg W Mittag Over a year ago

Ruby passes by value. Always. No ifs. No buts. No exceptions. But don't take my word for it, ask her yourself:

def is_ruby_pass_by_value?(foo) foo = 'No, Ruby is pass-by-reference.' end; bar = 'Yes, of course, Ruby *is* pass-by-value!'; is_ruby_pass_by_value?(bar); p bar # 'Yes, of course, Ruby *is* pass-by-value!'

Taryn East Over a year ago

Hmm - yes, in the case of parameters to methods. in this case, however - the things it passes are pointing at the actual objects inside of the array. My guess is I have a nomenclature mistake here. That "pass by reference" refers only to method-parameters? in which case - what is the correct description for what I have just explained here?

sunkencity · Accepted Answer · 2012-09-14 07:45:07Z

You could also easily create a new string with double quotes and use #{varname} this creates a new string.

irb> @array = %w(foo bar baz)
=> ["foo", "bar", "baz"]
irb> ("a".."e").inject([]) { |m,c| 
                m.concat(@array.map { |item| "#{c}-#{item}" });
                m }
=> ["a-foo", "a-bar", "a-baz", "b-foo", "b-bar", "b-baz", "c-foo", "c-bar", "c-baz", "d-foo", "d-bar", "d-baz", "e-foo", "e-bar", "e-baz"]

It's less error prone to create a new array with map, and then when you need to fold another array into it use inject. If you feel uncomfortable using inject, it's possible to achieve the same result using map and flatten:

irb> ("a".."e").map { |c| @array.map { |item| "#{c}-#{item}" } }
=> [["a-foo", "a-bar", "a-baz"], ["b-foo", "b-bar", "b-baz"], ["c-foo", "c-bar", "c-baz"],     ["d-foo", "d-bar", "d-baz"], ["e-foo", "e-bar", "e-baz"]]
irb> ("a".."e").map { |c| @array.map { |item| "#{c}-#{item}" } }.flatten
=> ["a-foo", "a-bar", "a-baz", "b-foo", "b-bar", "b-baz", "c-foo", "c-bar", "c-baz", "d-foo", "d-bar", "d-baz", "e-foo", "e-bar", "e-baz"]

Using chained operations on Enumerables makes it easy to inspect the state at each point in execution. And then when you need to print it out you can just append .each { |x| puts x }

If you're doing this a lot, why not install the gem cartesian, which makes it dead simple to get the cartesian product of two arrays, which is what you need:

irb> require 'cartesian'
=> true
irb> Cartesian::product(("a".."c"), %w(foo bar baz))
=> [["a", "foo"], ["a", "bar"], ["a", "baz"], ["b", "foo"], ["b", "bar"], ["b", "baz"], ["c",   "foo"], ["c", "bar"], ["c", "baz"]]
irb> Cartesian::product(("a".."c"), %w(foo bar baz)).map { |x| "#{x.first}-#{x.last}" }
=> ["a-foo", "a-bar", "a-baz", "b-foo", "b-bar", "b-baz", "c-foo", "c-bar", "c-baz"]

Martin Velez · Accepted Answer · 2012-09-14 05:42:38Z

0

If you feel that you need to use insert, then you can create a new copy of the string.

def add_charac_front
  ("a".."z").each do |letter|
     @array.each do |list_word|
       t = String.new(list_word).insert(0,letter)
       puts t
     end #ends @array loop
  end #ends alphabet loop
end #ends method

edited Sep 14, 2012 at 5:42

answered Sep 14, 2012 at 5:31

Martin Velez

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2 Comments

Fralcon Over a year ago

The main problem is I need to be able to insert letter into random positions of the list_word string. With .insert I can do that. But the problem is .insert changes the original array.

Martin Velez Over a year ago

I prefer to use new because it is a common way to indicate that you are creating a new object. dup is less common.

Monica Wilkinson · Accepted Answer · 2012-09-14 05:44:30Z

0

Not sure I follow the requirements but I would recommend just manipulating the arrays themselves

# example.rb

@alpha = %w{a b c d e f g h i j k l m n o p q r s t u v w x y z}

def add_charac_front(array)
  return @alpha + array
end

Or the code above is actually inserting the letters in front of each other so its the equivalent of reversing the alpha array and inserting it in front

@reverse_alpha = alpha.reverse

def add_charac_front(array)
  return @reverse_alpha + array
end

Hope this helps. You can also use ranges like this to concatenate sub arrays

a2 = %w{b c}
a3 = @alpha[0..4] + a2 + @alpha[5..13]

answered Sep 14, 2012 at 5:44

Monica Wilkinson

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