Look for start chars

acronym.h

#ifndef ACRONYM_H
#define ACRONYM_H

char *abbreviate(const char *phrase);

#endif

acronym.c

#include "acronym.h"
#include <ctype.h>
#include <stdlib.h>
#include <string.h>

char *abbreviate(const char *phrase)
{
   if (!phrase || !(*phrase))
      return NULL;

   char *acronyms = malloc(strlen(phrase) + 1);
   if (!acronyms)
      return NULL;
   char *acronyms_start = acronyms;

   const char start_trigger[] = " -";
   for (int start = 1; *phrase; phrase++) {
      if (start && isalpha(*phrase)) {
         *acronyms++ = toupper(*phrase);
         start = 0;
      } else if (strchr(start_trigger, *phrase))
         start = 1;
   }
   *acronyms = '\0';
   return acronyms_start;
}

This abbreviate function starts by checking for the input pointer being NULL or an empty string.

If the pointer is either NULL or en empty string, the function returns NULL.

The strlen function is used to define the size of the output string allocated by the malloc function. Note the strlen function does not include the terminating '\0' for the length, so 1 is added to the result of strlen for the malloc size. For example, if the input phrase is only one character, the result of strlen is 1. But trying to write the one character plus the terminating '\0' would result in overflow if the output string was allocated for only 1 byte.

If allocating the memory for the output string fails, malloc returns NULL, which would be returned by the function. Otherwise, a pointer is defined to keep the place at the beginning of the output string.

Based on the test input, a char array is defined that holds the characters that signal an upcoming beginning of a word. At the time of writing, those characters are a space or a hyphen.

The for loop initializes a start flag variable to 1. The for loop will iterate as long as the dereferenced phrase pointer is not a terminating null character ('\0'). The loop increments the phrase pointer after each iteration.

If the start flag is true (i.e., not zero), then the isalpha function is used to check if the current char is an alphabetic character. This check is made because the start flag could be true, but the current char could be another start trigger, or it could be an underscore. If the current char is alphabetic, then the isupper function is used to set the value at the current position of the acronyms pointer to the upppercased char. The acronyms pointer is then incremented, and the start flag is set to 0.

If either the start flag is not true, or the current char is not alphabetic, then the strchr function is used to check if the current char is one of the start trigger characters. If so, then the start flag is set to 1.

After the for loop is done, the terminating null character ('\0') is set at the current position of the acronyms pointer. The function then returns the pointer to the beginning of the acronyms string.