Thread: String Processing

Each implementation of the standard functions may be and often are different. The Standard only specifies how it works for the user, not how it is implemented, but here is a possible source:

Code:

char *strncat ( char *s1, const char *s2, size_t n )
{
  char *s;
  for ( s = s1; *s != '\0'; s++ );
  for ( ; n > 0 && *s2 != '\0'; n-- )
    *s++ = *s2++;
  *s = '\0';
  return s1;
}

>if the function call was strncat("wow", "awesome", 3) the output would be "wowawe"
This is incorrect, "wow" is a string literal and may be in read-only memory, so you can't modify it or access memory that you do not own. A proper call to strncat would be:

char a[7] = "wow";
strncat ( a, "awesome", 3 );

-Prelude

Code:

char *strrchr ( const char *s, int c )
{
  const char ch = c;
  const char *sc;
  for ( sc = NULL; ; s++ ) {
    if ( *s == ch )
      sc = s;
    if ( *s == '\0' )
      return (char *)sc;
  }
}

Once again your call to the function is incorrect, it should be:

test = strrchr ( "this is a test", 't' );
// Work with test.

-Prelude

Okay this is making me mad. I'm making my own strncat() function called StrComb(). It does the same thing, but like with StrComp(), I like doing it my own way. However, this just isn't working. Here's my code. Could someone please help me to find where I strayed off? It copies the first string to a new one just fine, but then it has issues appending to it and adding the second:

Code:

char * StrComb(char * str1, char * str2, int size)
{
	int i;
	int pos;
	char * result = new char;

	// first enter in the 1st string
	for (i = 0; i < GetStrSize(str1); i++)
		result[i] = str1[i];

	pos = i + 1;

	for(i = 0; i < size; i++)
		result[pos] = str2[i];
	// return the result
	return result;
}

Thanks!

Brendan

First off, you allocated a single char and then try to copy an array of chars to it! Memory corruption!

And this does nothing since pos isn't incremented:

for(i = 0; i < size; i++)
result[pos] = str2[i];


So if you really want to allocate new memory, declare it like:

char * result = new char[GetLength(str1) + GetLength(str2) + 1];

Next, keep in mind that you will have to deallocate the memory on th calling side every time you use this function:

char *s = StrComb("The Cat ", "in the Hat", 20);
//...use it...
delete[] s;

Also, if returning new memory, why use the third parameter at all?

The standard strncat copies the second string to the first, exceeding no more than string one's total size, then returns a pointer to the first element of string one. Keeping that in mind we have:

Code:

char * StrComb(char * str1, char * str2, int size)
{
  int i = 0, k = 0, length = GetStringLengh( str2);

  while( str[i] != 0 ) i++; //...reach null terminator.

  while( i < size && k < length)
   {
     str1[i++] = str2[k++];
   } 

 str1[i] = 0; //...null term.

 return str1; //...return a pointer to str1. 
}