A bitstring is just a sequence of bits, e$.$g$.10010110.$ Normally, printable characters in a file are encoded

as bytes, or bitstrings of length $8.$ This is called ASCII code. For example, the ASCII code for the character

$$a$$ is the bitstring $01100001,$ which is $97$ in binary.

However, we are not forced to use a fixed$-$length encoding. If we choose a di

erent encoding in which the codes can have di

erent lengths, then we can assign shorter strings to the more frequent characters, thereby saving space.

To illustrate, let$$s do this for bitstrings of length $2.$ Suppose we have a file that is exactly $2000$ bits long,

so it can be viewed as a sequence of $10002-$bit bitstrings. Suppose we scan the file and find that the four

$2-$bit bitstrings $00,01,10,11$ occur in the file with the frequencies as shown in Fig. $1.$