Data compression: Huffman coding and run-length encoding

Overview

Why data is compressed, lossy and lossless compression, building and reading Huffman codes, bits saved against 7-bit ASCII, and run-length encoding with worked pairs.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is data compression?

   Model answer

   Data compression reduces the number of bits needed to store a piece of data.

2. 2

   Why is data often compressed?

   Model answer

   A compressed file takes up less storage space. A compressed file also transfers faster over a network and uses less bandwidth.

3. 3

   What is lossless compression?

   Model answer

   Lossless compression makes a file smaller without losing any data. The original can be rebuilt exactly, as with Huffman coding and RLE.

4. 4

   What is lossy compression?

   Model answer

   Lossy compression permanently removes some data to make a file much smaller. Lossy compression suits photos and music, where small losses are hard to notice.

5. 5

   Describe how Huffman coding compresses a piece of text.

   Model answer

   Huffman coding counts how often each character appears. Frequent characters get short codes from a tree and rare characters get longer codes.

6. 6

   How is a character's Huffman code read from a Huffman tree?

   Model answer

   Follow the branches from the root down to the character. A left branch adds a 0 and a right branch adds a 1.

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