writing bits into a c++ file

Question

I'm working on Huffman coding and I have built the chars frequency table with a

std::map<char,int> frequencyTable;

Then I have built the Huffman tree and then i have built the codes table in this way:

std::map<char,std::vector<bool> > codes;

Now I would read the input file, character by character ,and encode them through the codes table but i don't know how write bits into a binary output file. Any advice?

UPDATE: Now i'm trying with these functions:

void Encoder::makeFile()
{
char c,ch;
unsigned char ch2;
while(inFile.get(c))
{
    ch=c;
    //send the Huffman string to output file bit by bit
    for(unsigned int i=0;i < codes[ch].size();++i)
    {
        if(codes[ch].at(i)==false){
            ch2=0;
        }else{
            ch2=1;
        }
        encode(ch2, outFile);
    }
}
ch2=2; // send EOF
encode(ch2, outFile);

inFile.close();
outFile.close();
}

and this:

void Encoder::encode(unsigned char i, std::ofstream & outFile)
{
int bit_pos=0; //0 to 7 (left to right) on the byte block
unsigned char c; //byte block to write

if(i<2) //if not EOF
{
    if(i==1)
        c |= (i<<(7-bit_pos)); //add a 1 to the byte
    else //i==0
        c=c & static_cast<unsigned char>(255-(1<<(7-bit_pos))); //add a 0
    ++bit_pos;
    bit_pos%=8;
    if(bit_pos==0)
    {
        outFile.put(c);
        c='\0';
    }
}
else
{
    outFile.put(c);
}
}

but ,I don't know why ,it doesn't work, the loop is never executed and the encode function is never used, why?

As there are only 256 possible char values, consider creating an array of 256 instead of a map with char as the key. This may use additional memory if only a few possible chars are used, but will be much faster than map look up. — Neil Kirk, Feb 13 '15 at 15:12
This is a project for school and my main goal is to make the program work, because, unfortunately, I have little time left — Alfredo Liardo, Feb 13 '15 at 15:30

score 5 · Answer 1 · edited May 23 '17 at 11:47

5

You can't write a single bit directly to a file. The I/O unit of reading/writing is a byte (8-bits). So you need to pack your bools into chunks of 8 bits and then write the bytes. See Writing files in bit form to a file in C or How to write single bits to a file in C for example.

edited May 23 '17 at 11:47

Community

1
1

answered Feb 13 '15 at 13:41

Jean-Baptiste Yunès

34,548
4
48
69

score 0 · Answer 2 · answered Feb 13 '15 at 13:04

0

The C++ Standard streams support an access of the smallest unit the underlying CPU supports. That's a byte.

There are implementation of a bit stream class in C++ like the Stanford Bitstream Class.

Another approach could use the std::bitset class.

answered Feb 13 '15 at 13:04

harper

13,345
8
56
105

Wouldn't it be easier with a `std::vector` instead? – didierc Feb 13 '15 at 13:08
A C++ `bool` is not a bit. – Jean-Baptiste Yunès Feb 13 '15 at 13:36
@Jean-BaptisteYunès that's interesting. how's it implemented or any reference? – Abhinav Gauniyal Feb 13 '15 at 13:58
Need to have a look into the standard, but it is commonly a `int`. – Jean-Baptiste Yunès Feb 13 '15 at 16:21
3.9.1/7 Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types. A synonym for integral type is integer type. – Jean-Baptiste Yunès Feb 13 '15 at 16:23
std::bitset - The class emulates an array of bool elements – Risinek May 04 '19 at 10:09

writing bits into a c++ file

2 Answers2