如何读取二进制文件来计算霍夫曼树的频率？

Question

I have to calculate frequency of Huffman tree from a "binary file" as sole argument. I have a doubt that binary files are the files which contains "0" and "1" only.

Whereas frequency is the repetition of the number of alphabets (eg, abbacdd here freq of a=2, b=2 ,c=1, d=2). And my structure must be like this:

struct Node
{
unsigned char symbol;   /* the symbol or alphabets */
int freq;               /* related frequency */
struct Node *left,*right; /* Left and right leafs */
};

But i not at all understand how can i get the symbol and from ".bin" file (which consists of only "0" and "1") ?

When i try to see the contents of a file i get:

hp@ubuntu:~/Desktop/Internship_Xav/Huf_pointer$ xxd -b out.bin 
0000000: 00000000 00000000 00000000 00000000 00000000 00000000  ......
0000006: 00000000 00000000 00000000 00000000 00000000 00000000  ......
000000c: 00000000 00000000 00000000 00000000 00000000 00000000  ......
0000012: 00000000 00000000 00000000 00000000 00000000 00000000  ......
0000018: 00000000 00000000 00000000 00000000 00000000 00000000  ......
000001e: 00000000 00000000 00000000 00000000 00000000 00000000  ......
0000024: 00000000 00000000 00000000 00000000 00000000 00000000  ......
000002a: 00000000 00000000 00000000 00000000 00000000 00000000  ......
0000030: 00000000 00000000 00000000 00000000 00000000 00000000  ......
.........//Here also there is similar kind of data    ................
00008ca: 00010011 00010011 00010011 00010011 00010011 00010011  ......
00008d0: 00010011 00010011 00010011 00010011 00010011 00010011  ......
00008d6: 00010011 00010011 00010011 00010011 00010011 00010011  ..... 

So , I not at all understand where are the frequencies and where are the symbols. How to store the symbols and how to calculate frequencies. Actually after having frequencies and symbols i will create HUffman tree using it.

Answer 1

First, you need to create some sort of frequency table.
You could use a std::map .
You would do something like this:

#include <algorithm>
#include <fstream>
#include <map>
#include <string>

std::map <unsigned char, int> CreateFrequencyTable (const std::string &strFile)
{
    std::map <unsigned char, int> char_freqs ; // character frequencies

    std::ifstream file (strFile) ;

    int next = 0 ;
    while ((next = file.get ()) != EOF) {
        unsigned char uc = static_cast <unsigned char> (next) ;

        std::map <unsigned char, int>::iterator iter ;
        iter = char_freqs.find (uc) ;

        // This character is in our map.
        if (iter != char_freqs.end ()) {
            iter->second += 1 ;
        }

        // This character is not in our map yet.
        else {
            char_freqs [uc] = 1 ;
        }
    }

    return char_freqs ;
}

Then you could use this function like this:

std::map <unsigned char, int> char_freqs = CreateFrequencyTable ("file") ;

You can obtain the element with the highest frequency like this:

std::map <unsigned char, int>::iterator iter = std::max_element (
    char_freqs.begin (), 
    char_freqs.end (), 
    std::map <unsigned char, int>::value_comp
) ;

Then you would need to build your Huffman tree.
Remember that the characters are all leaf nodes, so you need a way to differentiate the leaf nodes from the non-leaf nodes.

Update

If reading a single character from the file is too slow, you could always load all of the contents into a vector like this:

// Make sure to #include <iterator>
std::ifstream file ("test.txt") ;
std::istream_iterator <unsigned char> begin = file ;
std::vector<unsigned char> vecBuffer (begin, std::istream_iterator <unsigned char> ()) ;

You would still need to create a frequency table.

Answer 2

A symbol in a huffman tree could be anything,
but as you have to use an unsigned char per symbol
you should probably take a byte?
So no, not only 0 or 1, but eight time 0 or 1 together.

Like 00010011 somewhere in your output of xxd
xxd -b will just give you eight 0/1 per byte.
You could write a number between 0 and 255 as well,
or two times one character of 0123456789abcdef
There are lots of possibilies how to show a byte on the screen,
but that does not matter at all.

If you know how to read the content of a file in C/C++,
just read unsigned char until the file ends
and count which value is how often in there. That´s all.

As you´re probably writing decimal numbers in your program code,
there are 256 different values (0,1,2...255).
So you will need 256 integers (in an array, or your Node struct...)
to count how often each value appears.