# Joel Verhagen

a computer programming blog

# Three String Matching Algorithms in C++

During this post, I will show you my C++ implementation of three string algorithms:

1. a silly naive solution
2. the Rabin-Karp algorithm
3. the Knuth-Morris-Pratt algorithm

I've recently been fooling around with a ton of different programming projects (OpenCV, F#, C#, and of course my usual screen scraping antics...) but one topic that caught my interest is string matching algorithms. One of my recent classes in school was Algorithms I (more verbosely, the Design and Analysis of Algorithms I). Unfortunately, most of the class was spent on a brushing-over of universal hashing and more than one experimentation project concerning the choosing of pivots when implementing Quicksort. Big woop.

For the class, we were required to purchase the renowned Introduction to Algorithms... which we never opened throughout the entire class. In fact, my professor just taught off of Wikipedia for basically every lecture. Bummer. So I decided to look through it on my own time. Well there's an entire chapter dedicated to string matching! Cool! So I decided to throw together an implementation or two in C++.

Throughout this post, I used the following two terms frequently:

• needle: this is the string that you are searching for. So if you are searching for "Emma" in the string "Hermione Granger is Emma Watson", then "Emma" is the needle.
• haystack: this is the string that you are searching in. In the above example, "Hermione Granger is Emma Watson" would be the haystack.

To ease back into C++, I wrote a naive implementation which works perfectly well, but isn't very efficient (as there is absolutely no pre-processing step, which is where other algorithms get their speedup). It returns a vector of all of the indices where `needle` exists in `haystack`. Very intuitive. No fancy tricks.

The next one I implemented was the Rabin-Karp string searching algorithm. Basically the algorithm works by using a specific kind of hashing. For the pre-processing step, a hash of the needle is generated. Then a hash of the first `m` characters of the haystack is generated, where `m` is the length of the needle. If those two hashes match, then we know (or are relatively certain) that the haystack starts with an instance of the needle. We then shift forward one character in the haystack, calculate the new hash for the new position, then compare the hashes. The reason this is faster than the naive search is because the hashing algorithm used in Rabin-Karp is designed to be very cheap to change one character in the hashed value. So moving the bounds of the candidate string in the haystack forward one character is cheaper than rechecking the whole string, character-by-character.

The prime used for the hashing algorithm is the largest prime less than number values expressible in your hash data type (in my case, a 64-bit integer - 264) divided by your alphabet size (in my case, a `char` - 28). Wolfram Alpha is magic and tells me the prime number I can use.

The `offSetMatch` function just does a character-by-character check for a needle and haystack at a given offset in the haystack. Note that this function does no bounds checking!

The last searching algorithm is the Knuth-Morris-Pratt Algorithm. Like Rabin-Karp, it also does a pre-processing step on the needle before starting the search routine. It uses an array called the partial match table. I don't want to butcher the explanation, so I would recommend reading this article if you want to understand what's going on. Also, here's a neat little script that breaks down the steps a KMP execution. Great for seeing what's going on!

Here's my implementation.

Enjoy!