The purpose of source coding is to reduce the number of bits required to convey the information provided by the information source. Central to source coding is the use of sequences. By this, we mean that codewords are not simply associated to a single outcome, but to a sequence of outcomes. To see why this is useful, let us return to the problem of the rain forecast. Suppose we group the outcomes in threes, according to their probability, and assign binary codewords to these grouped outcomes.
Table shows such a code, and the probability of each codeword occurring.
This example shows how using sequences permits us to decrease the average number of bits per symbol. Moreover, without difficulty, we have found a code that has an average bit usage less than the source entropy. However, there is a difficulty with the code in Table . Before a codeword can be decoded, it must be parsed. Parsing describes the activity of breaking the message string into its component codewords.