Thread: Compressing problem

Hi

I'm currently writing a program that can take a file of ascii symbols and compress each symbol into 2 bits, and then decompress it again and print out the file. My decompress method works 100% but my compress method is writing out what seems to be spaces inbetween some characters, so that I get an obscured version of the original ascii image, but there is no doubt about it that the image is 75% correct IE you see that its semi correct. The problem probably lies in something I've written in the compress algorithm, so I'm gonna post both the decompress and compress ones and hopefully you can help me out. Thanks in advance

Decompress:

Code:

void decompress(char a)
{

	char bits[4];
	int k = 0;

	bits[0] = (a >> 6) &3;
	bits[1] = (a >> 4) &3;
	bits[2] = (a >> 2) &3;
	bits[3] = a & 3;
	
	for(; k < 4; k++){
	  if(bits[k] == 0) printf(" ");
	  else if(bits[k] == 1) printf(":");
	  else if(bits[k] == 2) printf("@");
	  else if(bits[k] == 3) printf("\n");
	}// end for
}// end decompress

compress:

Code:

else if(strcmp("e", argc[1]) == 0){
      c = fgetc(f);
      f2 = fopen(argc[3], "w");
     
	if(f2 == NULL) printf("Can't open file %s\n", output);
	else{
	  while(c != EOF){
		    
	    int l = 0;
	    int com = 0;
	    for(; l < 4; l++){	      
	      if(c == ' ') com += ((0 << (l*2)));
	      else if(c == ':') com += ((1 << (l*2)));
	      else if(c == '@') com += ((2 << (l*2)));
	      else if(c == '\n')  com += ((3 << (l*2)));
	      c = fgetc(f);
	    }// end for
	   
	    fprintf(f2, "%c",com);
	  }// end while
	  fclose(f);
	  fclose(f2);
	}// end else
      }// end else if

The decompressor does indeed look fine, except that if the input contained a number of symbols that isn't a multiple of 4, then it will generate too many symbols on output.

As far as the compressor is concerned, you should be or'ing, not adding. On a side note, you should check for EOF each time fgetc is called. If it is an EOF then you should report an error if 'l' is less than 3 (or else factor in some way to communicate to the decoder the number of 'end' bytes to generate). Also check for unexpected symbols, just for good measure. And just to be sure, 'c' is declared as an 'int' (not 'char'), correct?

One small point: is this a direct copy-paste? It's strange to see the command-line arguments in an argc vector.

Thats a direct copy paste yes. Thats how I've been told to do command line arguments for now at least.

The c was a char, I then changed it to an int after seeing your post and it didnt change anything. As for or'ing, I tried doing that and it only obscured the picture more, but then again im very new to these operations.

Originally Posted by Stenland

Thats a direct copy paste yes. Thats how I've been told to do command line arguments for now at least.

The c was a char, I then changed it to an int after seeing your post and it didnt change anything. As for or'ing, I tried doing that and it only obscured the picture more, but then again im very new to these operations.

Post the entire code, if possible.

Code:

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

//const char EOL = '\n';

/* decompress(char a)
   Denne metoden leser inn en char(int) i form av en byte.
   Deretter puttes de 4 forskjellige verdiene i byten(siden vi jobber med 2 bit per tegn, og de skiller vi ut
   ved å flytte hele byten n tegn til høyre(6, 4, 2, 0) og så dytte de inn i en array.Hvis vi f.eks har en innlesing:
   11011000 så vil vi etter å ha flyttet bitene og sagt at vi skal ha en maks verdi(mask) på 3 dvs 2 bit står vi igjen da med:
   {3, 1, 2, 0} i arrayet vårt. Så kjøres en for løkke 4 ganger slik at vi kan gå gjennom alle parene i en byte. Derettes testes verdiene
   mot sitt tegn, og vi får en utskrift. I eksemplet over vil utskriften bli: \n : @ mellomrom.

   
 */

void decompress(char a)
{

	char bits[4];
	int k = 0;

	bits[0] = (a >> 6) &3;
	bits[1] = (a >> 4) &3;
	bits[2] = (a >> 2) &3;
	bits[3] = a & 3;
	
	for(; k < 4; k++){
	  if(bits[k] == 0) printf(" ");
	  else if(bits[k] == 1) printf(":");
	  else if(bits[k] == 2) printf("@");
	  else if(bits[k] == 3) printf("\n");
	}// end for
}// end decompress



int o2(int argv, char *argc[])
{
  FILE *f;
  FILE *f2;
  char c;
  int cc;
  unsigned char byte;
  
  char input[400];
  char output[400];
  
  strcpy(input, argc[2]);
  //strcpy(output, argc[3]);

  f = fopen(input, "r");
  
  if(f == NULL) printf("Can't open file %s\n", input);
  else{
    
    if(strcmp("p", argc[1]) == 0){
      c = fgetc(f);
      while(c != EOF){
	  printf("%c", c);
	  c = fgetc(f);
      }// end while
      fclose(f);
    }// end if

    else if(strcmp("e", argc[1]) == 0){
      
      cc = fgetc(f);
      f2 = fopen(argc[3], "w");
     
	if(f2 == NULL) printf("Can't open file %s\n", output);
	else{
	  while(cc != EOF){
		    
	    int l = 0;
	    unsigned char com = 0;
	    for(; l < 4; l++){	      
	      if(cc == ' ') com += ((0 << (l*2)));
	      else if(cc == ':') com += ((1 << (l*2)));
	      else if(cc == '@') com += ((2 << (l*2)));
	      else if(cc == '\n')  com += ((3 << (l*2)));
	      else com += ((0 << (l*2)));

	      /*   if(cc == ' ') com += (com | cc);
	      else if(cc == ':') com += (com | cc);
	      else if(cc == '@') com += (com | cc);
	      else if(cc == '\n')  com += (com | cc);
	        else com
	      */
	    cc = fgetc(f);
	    }// end for
	   
	    fprintf(f2, "%c",com);
	  }// end while
	  fclose(f);
	  fclose(f2);
	}// end else
      }// end else if
    
    else if(strcmp("d", argc[1]) == 0){
      int d;
      int k = 0;
      d = fgetc(f);
     
      while(d != EOF){
	decompress((char) d);
	d = fgetc(f);
	
      }// end 
      printf("\n");
    }// end if
    
    else printf("Please enter a valid option(p,e or d)\n");
	

  }// end else


}// end main

Comments in Norwegian, but shouldn't matter much. Also adding that in the above outcommented code with oring, was my first attempt at it ever and I tried other solutions that netted the same result

Thanks for the help Sebastiani, always helpful