Two String Exercise via Programming Praxis

Here's another one (or two rather) from Programming Praxis. Let's take a look at the second one first as it's trivial in ruby. If we're given a string, how do we replace multiple spaces with single spaces. F/or example the string "a b c" would become "a b c". For both of these, we're going to monkey patch the string class. Here's the code ...

class String

    def remove_consecutive_spaces

        self.gsub(/ +/, " ")

    end

end

All we do is do a global substitution of one or more spaces with a single space. This would be quite a bit trickier in C say which is why it ends up in interview questions.

The second (or first) problem is to remove duplicate characters from a string. For example, "aaaabbbb" becomes "ab" and "abcbd" becomes "abcd". This one is a bit trickier but shows another good example of how useful inject() can be. Here's the code ...

class String

    def remove_duplicate_characters

        self.split(//).inject([]) { |a, c| a << c if !a.include?(c); a }.join

    end

end

Going through it from left to right, we have first the split(//) which will turn the array into a string of characters. With that string of characters we do an inject([]) which a) initializes a new empty array (sometimes called the "memo") and then runs through the character array adding a character "c" to the array "a" if it's not already there include?. The ; a returns the current array back to the inject. Finally, we recreate the string by doing a join on the character string array.

Let me know if you have questions or comments.