Ruby modify array items and return full array

You have several undefined variables in your example, which is very sloppy.

1. each_slice(1) is equivalent to each(), so it's not clear why you are using each_slice(1). In any case, both each() and map() step through the items in an Array one by one, but each() returns the original Array unchanged. On the other hand, you use map() when you want to create a new Array that contains changes to the items.

2. In the regex /\w{1,}/, there is a shortcut for the quantifier {1, }, and it's: +, so most people would write the regex as /\w+/, where + means 1 or more.

3. I want to modify certain items in the array based on regex, then return the full array with the modified items in it.

Here is an example:

results = [1, 2, 3].map do |num|
  if num == 2
    num + 4
  else
    num - 1
  end
end

p results

--output:--
[0, 6, 2]

Your current attempt with map() doesn't return anything if the conditional fails. Note how the example above returns something both when the condition fails AND when the condition succeeds. map() replaces an item with whatever is returned for that item.

Now look at this example:

results = [1, 2, 3].map do |num|
  if num == 2
    num + 4
  end
end

p results

--output:--
[nil, 6, nil]

If you don't return something for an item, then map() will use nil for that item. In the example, if the condition num == 2 is true then num+4 is returned--but if num == 2 is false, nothing is returned.

Edit:

words = %w[
  page.php?
  site=
  blah&
  id=
  1
]    #=> words = ["page.php?", "site=", "blah&", "id=", "1"]

suffix = 'hello'

results = words.map do |word|
  if word.end_with?('=')   
    "#{word}#{suffix}"
  else
    word
  end
end

p results

--output:--
["page.php?", "site=hello", "blah&", "id=hello", "1"]

Instead of parsing a URL with a regex, have you considered using the addressable gem?

require 'addressable/uri'

uri = Addressable::URI.parse('page.php?site=blah&id=1&bar')
uri.query_values = uri.query_values.map do |k, v| 
  [k, v.is_a?(String) ? v << 'foo' : v] 
end
puts uri.to_s # => page.php?site=blahfoo&id=1foo&bar

This won't handle very complex query parameters (it will just pass them through).

You can use respond_to? :sub! and v.sub! /$/, 'foo' instead of checking types if that makes you uneasy. (I wouldn't use :<< or :concat because those are valid methods for Arrays.)