from Q to Z

#include <stdlib.h>     // qsort, toupper
#include <string.h>     // strlen
#include <stdbool.h>    // true
#include <stdio.h>      // puts, fflush, fgets

#define BUF_SIZE 80

int tovalid(int c)
{
	c = toupper(c);
	if (c < 'Q' || c > 'Z') {
		c = 0;
	}
	return c;
}

void filter(char* dst, char* src)
{
	int c;
	while (c = (unsigned char)*src++) {
		if (c = tovalid(c)) {
			*dst++ = c;
		}
	}
	*dst = '\0';
}

int charcmp(const void* a, const void* b)
{
	int code_a = *(unsigned char*)a;
	int code_b = *(unsigned char*)b;
	return code_a - code_b;
}

int main(void)
{
	char input[BUF_SIZE];
	char output[BUF_SIZE];
	bool Isbull = true;
	while (Isbull) {
		puts("Enter string:");
		fflush(stdin);
		fgets(input, sizeof(input), stdin);
		if (input[0] == '\n') { break; }
		filter(output, input);
		puts(output);
		qsort(output, strlen(output), sizeof(char), charcmp);
		puts(output);
		Isbull = false;
	}
	getchar();
	return 0;
}