Question

Is it possible to use variables in both fields of the gsub method ?
I’m trying to get this piece of code work :

$I = 0
def random_image
  $I.to_s
  random = rand(1).to_s
  logo = File.read('logo-standart.txt')
  logo_aleatoire = logo.gsub(/#{$I}/, random)
  File.open('logo-standart.txt', "w") {|file| File.puts logo_aleatoire}
  $I.to_i
  $I += 1
end

Thanks in advance !

Answer 1

filecontents = File.read('logo-standart.txt')
filecontents.gsub!(/\d+/){rand(100)}
File.open("logo-standart.txt","w"){|f| f << filecontents }

The magic line is the second line.

• The gsub! function modifies the string in-place, unlike the gsub function, which would return a new string and leave the first string unmodified.
• The single parameter that I passed to gsub! is the pattern to match. Here, the goal is to match any string of one or more digits — this is the number that you’re going to replace. There’s no need to loop through all of the possible numbers running gsub on each one. You can even match numbers as high as a googol (or higher) without your program taking longer and longer to run.
• The block that gsub! takes is evaluated each time the pattern matches to programmatically generate a replacement number. So each time, you get a different random number. This is different from the more usual form of gsub! that takes two parameters — there the parameter is evaluated once before any pattern matching occurs, and all matches are replaced by the same string.

Note that the way this is structured, you get a new random number for each match. So if the number 307 appears twice, it turns into two different random numbers.

If you wanted to map 307 to the same random number each time, you could do the following:

filecontents = File.read('logo-standart.txt')
randomnumbers = Hash.new{|h,k| h[k]=rand(100)}
filecontents.gsub!(/\d+/){|match| randomnumbers[match]}
File.open("logo-standart.txt","w"){|f| f << filecontents }

Here, randomnumbers is a hash that lets you look up the numbers and find what random number they correspond to. The block passed when constructing the hash tells the hash what to do when it finds a number that it hasn’t seen before — in this case, generate a new random number, and remember what that random number the mapping. So gsub!‘s block just asks the hash to map numbers for it, and randomnumbers takes care of generating a new random number when you encounter a new number from the original file.