How to stop overwriting previous strings pushed into an array?

Question

I'm trying to push all the derivation of the sentence.

For example: The < animal > in the < object >

I want derivations to hold the expansions, so derivation[0] would have "The cat in the < object >", while derivation[1] would have "The cat in the hat".

However, all the derivations before getting overwritten by the final derivation.

How do I prevent this from happening?

while is_non_terminal?(sentence)
 $key = sentence.match(/(\<[a-zA-Z0-9_\-*]+\>)/)
 sentence.sub!(/(\<[a-zA-Z0-9_\-*]+\>)/){grammar[$key.to_s].sample.join(' ')}
 derivations.push(sentence)
 puts derivations.to_s
 #puts sentence
end

simple fix would be to change `derivations.push(sentence)` to `derivations.push(sentence.dup)` - this will push a copy of `sentence` at its present state to your `derivations` collection. — dinjas, Apr 12 '18 at 19:13
Remember `$` is a **global** variable in Ruby and should not be used casually like this. — tadman, Apr 12 '18 at 19:58

score 2 · Answer 1 · answered Apr 12 '18 at 18:08

2

You only have one string in your code, and your continually modifying it and pushing new references to it into an array. Each entry in your array is simply a reference to the same string.

You should use sub instead of sub! to return a modified copy of your string that you can push into the array, so that each iteration of the loop produces a new string instead of modifying the same string.

answered Apr 12 '18 at 18:08

user229044

232,980
40
330
338

when I do sub instead of sub! it messes everything up – msc Apr 12 '18 at 18:12
1

@msc: "it messes everything up" - unfortunately, this tells us nothing. – Sergio Tulentsev Apr 12 '18 at 18:13
it tries to derive my sentence but goes on an infinite loop. With sub! it derives it just fine – msc Apr 12 '18 at 18:16
is there another way to accomplish this – msc Apr 12 '18 at 18:19
@msc: Have you considered investigating what _exactly_ causes infinite loop? You _know_ it's a bug/flaw in your code, right? – Sergio Tulentsev Apr 12 '18 at 18:21
I fixed it. I just did String.new within the parameters – msc Apr 12 '18 at 18:27
@SergioTulentsev quick question...how would I print something like " – msc Apr 12 '18 at 18:45
@msc: it does, check page source. – Sergio Tulentsev Apr 12 '18 at 18:45
@SergioTulentsev You're right! it says
["The
– msc Apr 12 '18 at 18:50
@msc: yes there is. https://stackoverflow.com/questions/7381974/which-characters-need-to-be-escaped-on-html – Sergio Tulentsev Apr 12 '18 at 18:57

score 0 · Answer 2 · answered Apr 12 '18 at 20:05

A more Ruby way of tackling this is:

def subst(template, vars)
  template = template.dup
  derivations = [ ]

  while (template.sub!(/\<([a-zA-Z0-9_\-*]+)\>/) { vars[$1].sample })
    derivations << template.dup
  end

  derivations
end

Where that can work when called like:

subst("The <animal> in the <object>", 'animal' => [ 'cat' ], 'object' => [ 'hat' ]);
# => ["The cat in the <object>", "The cat in the hat"]

Note that this can go into an infinite loop if one of your substitutions contains text like <animal> which contains text like <animal> which contains text like <animal>...

How to stop overwriting previous strings pushed into an array?

2 Answers2