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An efficient LZW implementation: Basic decoding. Return to the main page.

An efficient LZW implementation: Basic decoding

Decoding LZW-encoded data is simpler and easier to be done efficiently because dictionary searches are not needed. Thus the decoder does not need the extra index values in the string elements as the encoder did for the fast search. Recreating a string from an index The encoding step wrote only index values to the encoded result. When decoding, one of the crucial steps is, naturally, to recreate the original string from the dictionary using such an index value. The index value points to a string in the dictionary. Thus it becomes clear that to reconstruct a string from an index we need to traverse the dictionary strings backwards, following each successive prefix index until this prefix index is the empty index. We can build the reversed string into a temporary array and then output the byte values by traversing the array backwards.

The basic decoding algorithm Again, we assume that all 256 possible byte values are used. Decoding with dynamic bit sizes. Input/output with files. C++ provides the following classes to perform output and input of characters to/from files: ofstream: Stream class to write on filesifstream: Stream class to read from filesfstream: Stream class to both read and write from/to files.

Input/output with files

These classes are derived directly or indirectly from the classes istream and ostream. We have already used objects whose types were these classes: cin is an object of class istream and cout is an object of class ostream. Therefore, we have already been using classes that are related to our file streams. And in fact, we can use our file streams the same way we are already used to use cin and cout, with the only difference that we have to associate these streams with physical files.

Let's see an example: This code creates a file called example.txt and inserts a sentence into it in the same way we are used to do with cout, but using the file stream myfile instead. But let's go step by step: Open a file Closing a file Text files Checking state flags bad() fail() eof() CodingUnit Programming Tutorials. This C++ programming language tutorial will be in two parts.

CodingUnit Programming Tutorials

The first (this one) will cover the theory behind IO and in the second tutorial we will look at some examples. Input-Output Input – Output is a process of transfer of data from one computer device to another or from one part of the computer to another. There are three categories of Input-Output: Standard IOMemory IONetwork IO Standard IO is used frequently for which C++ provides cin, cout, cerr and clog streams. We can use the ifstream, ofstream and fstream classes to perform file IO.

File IO means transfer of data from secondary memory (hard disk) to main memory or vice-versa. Note: The arrows indicate the flow of data. Text and binary files The C++ language supports two types of files: Text filesBinary files The basic difference between text files and binary files is that in text files various character translations are performed such as “\r+\f” is converted into “\n”, whereas in binary files no such translations are performed. Windows XP - Using command redirection operators.

Huffman compression class in C++ Introduction I've written, some many years ago, a dynamic Huffman algorithm to compress and decompress data.

Huffman compression class in C++

It is mainly targeted to data with some symbols occurring more often than the rest (e.g., having some data file consisting of three different symbols, and their total number of occurrences in that file is s1(1000), s2(200), s3(30), so the total length of the file is 1000+200+30=1230 bytes; it is encoded, assigning one bit to s1 and two bits to s2, s3, so the encoded length is 1*1000+2*(200+30)=1460 bits=182 bytes).

In the best case, a file consisting of just one symbol will be encoded with a compression ratio of 1:8. Huffman coding is used in image compression; however, in JPEG2000, an arithmetic codec is employed. Background The article intends to provide the code only, and is not a Huffman tutorial.