In this section we will briefly introduce file compression algorithms. These are concerned with ways to reduce the space a file occupies on the file store. Methods are required that can (fairly) rapidly compress a file, such that the compressed file occupies less space, yet can easily be expanded to recreate the original file.
As a simple example of how we might compress a file, suppose we replace every occurence of the word 'the' with the symbol '#' (and deal with actual occurences of '#' in some way!). The resulting file would take up less space, but it would be easy to recreate the original. In general we can try and replace common sequences with shorter 'codes' for those sequences.