Perhaps the simplest way to get started with understanding programming for Hadoop is a simple word count functionality on a fairly large electronic book. The map program will read in every line of the text separated by a space or tab and return a key-value pair, which is by default assigned to a count of 1. The reduce program will read in all key-value pairs from the map program and sum up the number of similar words. Hadoop will produce an output file that contains a list of words in the book and the number of times the words have appeared.