I got to solve the problem I was trying to solve, but the thing is that I am not sure why it worked, I just started adding methods.
So if anyone could explain why worked:
def replace(string1, letter_a, letter_b)
replacements = {letter_a => letter_b}
#this is the part I am not sure why is working:
initial_string.split('').map{|i| replacements[i] || i}.join
end
Firstly I recommend to use built-in methods String#gsub or String#tr
string.gsub(%r{#{replaceable_letter}}, replacing_letter)
"abcdef".gsub(/a/, "b") # => "bbcdef"
string.tr(replaceable_letter, replacing_letter)
"abcdef".tr("a", "b") # => "bbcdef"
Instead of initial_string.split('').map you can use initial_string.each_char.map
Explanation of your code:
replacements = {letter_a => letter_b}
is hash where replaceable letter is key and replacing letter is value
For example { "a" => "b" }
Than you split your string to chars array
After that map over this array
For every char you check the hash, for example:
replacements["a"] # => "b"
replacements["c"] # => nil
If hash has such key, you take replacing letter, if not take origin letter. Compare and read about || operator:
nil || "f" # => "f"
"b" || "a" # => "b"
And finally join new array
