C - function that compresses characters - c

Question

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
char * compress(char *input, int size){
char *inputa;
char compressedString[100];
inputa = (char*)malloc(sizeof(size));
snprintf (inputa, size, "%s", input);
int i = 0;
int x;
int counter;
while(i < size){
counter = 1;
x = i;
while (inputa[x] == inputa[x + 1] && (x+1) < size){
x++;
counter++;
}
if (i != x){
i = x;
}else{
i++;
}
}
return inputa;
}
main(){
char ez[] = "blaablaaa";
printf("%s \n", compress(ez, sizeof(ez)));
printf("%s", ez);
return 0;
}
So, I am trying to make this function that compresses consecutive characters (eg. "blaablaaa" to "bla2bla3"). My thought process is to put the inputa[x] on the compressed array and next to it the counter, but I can't seem to make it to work.

Answer 1

Lets take a look at these two lines:
inputa = (char*)malloc(sizeof(size));
snprintf (inputa, size, "%s", input);
size has type int, so sizeof(size) is the size of an integer, which is probably 4.
You used malloc to allocate 4 bytes.
Then you use snprintf to try to copy all of your input (blaablaaa, 10-bytes long) into a buffer that is only 4 bytes long.
10 bytes won't fit into a 4 byte buffer.
I'm not sure what you're trying to do there, but it is not correct.

Answer 2

1) Your allocated buffer was too short:
inputa = (char*)malloc(sizeof(size));
It allocates only 4 bytes.
You needed
inputa = (char*)malloc(sizeof(char)*size + 1 ));
2) You forgot to release the allocated memory.
3) The algorithm itself needed the improvements. Comments in the code:
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
/* reverse: reverse string s in place */
void reverse(char s[])
{
int i, j;
char c;
for (i = 0, j = strlen(s)-1; i<j; i++, j--) {
c = s[i];
s[i] = s[j];
s[j] = c;
}
}
/* itoa is not a standard function */
/* itoa: convert n to characters in s */
void itoa1(int n, char s[])
{
int i, sign;
if ((sign = n) < 0) /* record sign */
n = -n; /* make n positive */
i = 0;
do { /* generate digits in reverse order */
s[i++] = n % 10 + '0'; /* get next digit */
} while ((n /= 10) > 0); /* delete it */
if (sign < 0)
s[i++] = '-';
s[i] = '\0';
reverse(s);
}
char * compress(char *input, int size){
int i = 0;
int r; // number of repetitions
char add[2]; // current character buffer
char rep[32]; // repetitions buffer
char c; // current character
char *compr = (char* )malloc(sizeof(char)*size + 1); // memory for the compressed string
compr[0] = 0; // terminate the buffer
add[1] = 0; // terminate the buffer
while(i < size){
c = add[0] = input[i]; // get a character
strcat(compr,add); // add to compr
r = 1; // default number of repetitions is one
while(1) // count and add to the string
{
if(c == input[i+1] )
{ // find how many characters follows c
r++; // number of repetition
i++; // moving along the input buffer
}
else
{
// check the r for number of repetitions
if( r > 1)
{
// there were repetitions:
// char * itoa ( int value, char * str, int base );
itoa1(r,rep); // get the number
strcat(compr,rep); // add repetition number to the compressed string
}
i++;// advance to the next character
break;
} // else
}// while
} //while
return compr;
}
int main(void){
char sg7[] = "BLaaaBBLLaaaaXXXaaY";
char ez[] = "blaablaaa";
char *ptr;
printf("%s \n", ptr = compress(sg7, strlen(sg7) ) );
printf("%s \n", sg7);
free(ptr);
printf("\n");
printf("%s \n", ptr = compress(ez, strlen(ez)));
printf("%s \n", ez);
free(ptr);
return 0;
}
Output:
BLa3B2L2a4X3a2Y
BLaaaBBLLaaaaXXXaaY
bla2bla3
blaablaaa
I hope it helps.