Data Compression Lec#3 2019

Irreversible Text Compression
Sometimes it is acceptable to “compress” text by simply throwing away some
information. This is called irreversible text compression or compaction. The
decompressed text will not be identical to the original, so such methods are not general
purpose; they can only be used in special cases.
For example: A run of consecutive blank spaces may be replaced by a single space.
This may be acceptable in literary texts and in most computer programs, but it should
not be used when the data is in tabular form.
In extreme cases all text characters except letters and spaces may be thrown away,
and the letters may be case flattened (converted to all lower- or all uppercase). This
will leave just 27 symbols, so a symbol can be encoded in 5 instead of the usual 8 bits.
The compression ratio is 5/8 = .625, not bad, but the loss may normally be too great.
Ad Hoc Text Compression
Here are some simple, intuitive ideas for cases where the compression must be
reversible (lossless).
If the text contains many spaces but they are not clustered, they may be removed and
their positions indicated by a bit-string that contains a 0 for each text character that is
not a space and a 1 for each space. Thus,