Encoding and Decoding text in c

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

FILE *inputFile;
FILE *outputFile;

int encodeBinary[4] = {0x00, 0x01, 0x02, 0x03};
char encodeChars[4] = {':', '@', '\n', ' '};

void encode(const char * inFile, const char * outFile)
{

    inputFile = fopen(inFile, "r");
    outputFile = fopen(outFile, "w");
    char lineBuffer[BUFSIZ];

    if(inputFile == NULL)
    {
        perror("Error while opening file.\n");
        exit(EXIT_FAILURE);
    }

    while(fgets(lineBuffer, sizeof(lineBuffer), inputFile))
    {
        for(int i = 0; lineBuffer[i] != 0; i++)
        {
            if(lineBuffer[i] == encodeChars[0])
            {
                fprintf(outputFile, "%d", encodeBinary[0]);
            }
            else if(lineBuffer[i] == encodeChars[1])
            {
                fprintf(outputFile, "%d", encodeBinary[1]);
            }
            else if(lineBuffer[i] == encodeChars[2])
            {
                fprintf(outputFile, "%d", encodeBinary[2]);
            }
            else if(lineBuffer[i] == encodeChars[3])
            {
                fprintf(outputFile, "%d", encodeBinary[3]);
            }
        }
    }

    fclose(inputFile);
    fclose(outputFile);

}

void decode(const char * inFile, const char * outFile)
{

    inputFile = fopen(inFile, "r");
    outputFile = fopen(outFile, "w");
    char lineBuffer[BUFSIZ];

    if(inputFile == NULL)
    {
        perror("Error while opening file.\n");
        exit(EXIT_FAILURE);
    }

    while(fgets(lineBuffer, sizeof(lineBuffer), inputFile))
    {
        for(int i = 0; lineBuffer[i] != 0; i++)
        {
            if(lineBuffer[i] == '0')
            {
                fprintf(outputFile, "%c", encodeChars[0]);
            }
            else if(lineBuffer[i] == '1')
            {
                fprintf(outputFile, "%c", encodeChars[1]);
            }
            else if(lineBuffer[i] == '2')
            {
                fprintf(outputFile, "%c", encodeChars[2]);
            }
            else if(lineBuffer[i] == '3')
            {
                fprintf(outputFile, "%c", encodeChars[3]);
            }
        }
    }

    fclose(inputFile);
    fclose(outputFile);

}


void commands(const char * command, const char * inputFile, const char * outputFile)
{
    if(strcmp(command, "encode") == 0)
    {
        encode(inputFile, outputFile);
    }
    else if(strcmp(command, "decode") == 0)
    {
        decode(inputFile, outputFile);
    }
}

void testValues(int argc, const char * argv[])
{
    if(argc == 4)
    {
        commands(argv[1], argv[2], argv[3]);
    }
    else
        printf("USAGE: ./encode [input_file] [output_file]\n");
}

//MAIN
int main(int argc, const char * argv[])
{

    testValues(argc, argv);

    return 0;
}

Hi there. I have this piece of code. The code is supposed to get a text file in consisting of the characters : @ "newline" and "space". These characters are then supposed to be converted to binary, 0, 1, 10, 11. After that I also need a way to decode back to the original characters. What I can't seem to figure out is how to be able to read difference between the numbers, if there is 001, how can I know that we are talking about 0, 01, and not 00, 1. I read somewhere that you can use bitwise operations to do this? Any help appreciated!

So, I have change my code a bit. Now the problem is that when I store the values the file that is encoded is as large as the file that is to be encoded. How can I store the values in a file in such a way that it stores the values as hexadecimal (or binary) so that the encoded file is smaller than the original file?

Answer 1

{0, 1, 10, 11}; are not binary numbers, they are decimal numbers, which is the default number format in C source code. The other possible number bases are hex, written with a prefix 0x and octal, written with a prefix 0 . There is no way to write binary numbers in standard C code (probably because they are considered hard to read for humans).

So what you will have to do is to type the numbers in hex:

{0x00, 0x01, 0x02, 0x03}

The algorithm is pretty straight-forward:

• Read a character from the file.
• Search for a match of this character among encodeChars (which should be declared as const char [] ).
• If found, replace it with the corresponding index in "binary".
• Decoding is the other way around, just use the binary as lookup table instead.
• If performance is important, consider implementing this using binary search. It is an ideal example of where binary search should be used (sorted data, no duplicates).

EDIT

What I spoke of is the representation of numbers for the programmer , inside the programmer's own source code. Here you can only use decimal, hex and octal.

There is also the representation of numbers for the user , which I suppose is what you were looking for. This can be anything you fancy.

And finally there is the representation of numbers for the CPU . He only wants binary and nothing but binary.

Consider this: printf("%c", 0x41).

• The programmer sees hex 41.
• The user sees the letter A.
• The CPU sees something like "Store number 01000001 on the stack. Jump to subroutine."

To display some random byte as a binary number to the user , simply do something like:

#include <stdint.h>

uint8_t data = 0x41;

for(uint8_t i=0; i<8; i++)
{
  if( (data & (1<<i)) > 0)
  {
    printf("1");
  }
  else
  {
    printf("0");
  }

}