Doubling all occurrences of a specific character in a string - c

Question

I want to write a C program, that returns a new string allocated on the heap. This string is obtained by doubling all occurrences of “c” in a string (“abcdc” becomes “abccdcc” after doubling “c”).
This is my code and I don't really see where the problem is to fix it!
size_t taille = stringLength(str);
size_t k=0;
size_t q=0;
while (*str!='\0')
{
if (*str == c)
{
k=k+1;
}
++str;
}
char *nouvelle=malloc(taille+1+k);
int i,j= 0;
while(*str !='\0')
{
if (str[i] != c)
{
j=i;
nouvelle[j]=str[i];
}
else
{
j=i;
++q;
nouvelle[j]=str[i];
j=i+q;
nouvelle[j++]=str[i];
}
++i;
}
nouvelle[taille+1+k]='\0';
return nouvelle;
}

Answer 1

There are two problems with your code.
The first one is that after this while loop
while (*str!='\0')
{
if (*str == c)
{
k=k+1;
}
++str;
}
the pointer str points to the end of the string that is to the terminating zero character '\0'.
The second one is that you are using the uninitialized variable i
int i,j= 0;
while(*str !='\0')
{
if (str[i] != c)
{
j=i;
//..
This declaration
int i,j= 0;
is not the same as
int i = 0,j= 0;
That is only the variable j is initialized by 0.
And the statement
j = i;
does not make sense.
Also it is unclear whether c denotes a variable or the character 'c'. If you mean the character 'c' then you need to write at least like
if (*str == 'c')
You could define the function for example the following way as shown in the demonstration program below.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char * duplicate_char( const char *s, char c )
{
size_t n = 0;
for (const char *p = s; *p; ++p)
{
if (*p == c) ++n;
}
char *nouvelle = malloc( strlen( s ) + n + 1 );
if (nouvelle)
{
if (n == 0)
{
strcpy( nouvelle, s );
}
else
{
char *p = nouvelle;
while (*s)
{
*p++ = *s++;
if (p[-1] == c) *p++ = c;
}
*p = '\0';
}
}
return nouvelle;
}
int main( void )
{
char *nouvelle = duplicate_char( "abcdc", 'c' );
if (nouvelle != NULL) puts( nouvelle );
free( nouvelle );
}
The program output is
abccdcc
If you want to use your own function stringLength instead of the standard C function strlen then it can look like
size_t stringLength( const char *s )
{
const char *p = s;
while ( *p ) ++p;
return p - s;
}