Sorting char arrays by swapping pointers, C++

Question

I am trying to sort an array of char pointers (char * _string) by swapping pointers.

I have this method, and what I want to do is use the values I get from _string and sort them by not manipulating _string, but the empty helper array (char * _output) which I also hand over to the method.

Can anyone help me and tell me what I am doing wrong?

void sortAsc(char* _string, char* _output) 
{

    int length = strlen(_string);

        // output and string now point to the same area in the memory
    _output = _string; 

    for( int i = 0; i < length; i++) {
          for( int j = 0; j < length; j++) {
                if( *(_output) > (_output[j] ) ) {

                    // save the pointer
                    char* tmp = _output;

                    // now output points to the smaller value   
                    _output = _output+j; 

                    // move up the pointer to the smaller value
                    _output + j; 

                    // now the pointer of the smaller value points to the higher value
                    _output = tmp; 

                    // move down to where we were + 1
                    _output - j + 1; 

            }
        }
    }

    //_output[length]='\0';

    //delete chars;
 }

In my main-Method, I do something like this:

char * string = {"bcdae"};
char * output = new char[5];
sortAsc(string, output);

After that code, I want the output array to contain the sorted values.

Answer 1

This sorts the string into an already allocated buffer, and if the buffer isn't large enough tells you how big it has to be:

std::size_t sortAsc(char const* string, char* dest, std::size_t dest_length) {
  std::size_t str_length = strlen(string);
  char const* str_end = string + str_length;
  if (dest_length < str_length+1)
    return str_length+1;
  std::copy( string, str_end, output );
  output[str_length] = '\0';
  std::sort( output, output+strlen(output) );
  return str_length+1;
}

This does the poor "allocate a new string" pattern, using the above implementation:

char* allocate_and_sortAsc(char const* string) {
  std::size_t str_length = strlen(string);
  char* retval = new char[str_length+1];
  std::size_t count = sortAsc( string, retval, str_length+1);
  ASSERT( count <= str_length );
  return retval;
}

And don't use variable names that start with an _ , it is a bad practice because it wanders really near compiler reserved names. _Capital is reserved everywhere, and _lower in global scope, and foo__bar everywhere.

Answer 2

Let's do the selection sort for a 10 size int array using pointer notation, you can simply change it to an array list.

      *---*---*---*---*---* ........
a[] = | 1 | 2 | 4 | 0 | 3 | ........
      *---*---*---*---*---* ........
        ^--------We start here looking for the smaller numbers and sort the array.


for( i = 0; i < 10; i++ ){
    k = i;
    bypass = *( a + i );
    for( j = i + 1; j < 10; j++ ){

        /* To get Increasing order. */
        if( bypass > *( a + j ) ){
           bypass = *( a + j );
           k = j;
        }
    }
    if ( k != i ){
         *( a + k ) = *( a + i );
         *( a + i ) = bypass;
    }
}