#include <cstring>

static size_t appendSeparator(char *answer, size_t maxLength, size_t length, char separator)
{
	--maxLength; // reserve space for one more character
	while (length < maxLength && (answer[length]))
		++length;

	if ( (length > 0) && (answer[length] == 0) && (answer[length - 1] != separator) ) {
		answer[length++] = separator;
		answer[length] = 0;
	}

	return length;
}

int main()
{
	enum { ANSWER_SIZE = 32 };
	char answer[ANSWER_SIZE];
	size_t pos = 0;
	strncpy_s(answer, "Hello World", ANSWER_SIZE);
	pos = appendSeparator(answer, ANSWER_SIZE, pos, ',');

	return pos;
}