License: CC0 (public domain) https://creativecommons.org/publicdomain/zero/1.0/

percenta.c

// Helper program translates from decimal numbers to C99 "%a" format.
// Arguments:
//     -r   : Translate %a to decimal instead
//     -d   : Use doubles
// Then enter numbers you want to convert one line at a time
//
// To compile just do: gcc percenta.c
#include <stdio.h>
#include <stdbool.h>
#include <string.h>
#include <float.h>

#ifndef FLT_DECIMAL_DIG
// Oops we're in C++
#include <limits>
#define FLT_DECIMAL_DIG (std::numeric_limits<float>::max_digits10)
#define DBL_DECIMAL_DIG (std::numeric_limits<double>::max_digits10)
#endif

bool check(int result) {
	if (result == 0) {
		printf("Unrecognized\n");
		while (1) {
			int ch = getchar();
			if (ch == '\n' || ch == '\r' || ch == EOF)
				return false;
		} 
	}
	return true;
}

int main(int argc, const char **argv) {
	bool reverse = false, doubled = false;
	int arg = 1;

	while (arg < argc) {

		if (strcmp(argv[arg], "-r") == 0)
			reverse = true;
		else if (strcmp(argv[arg], "-d") == 0)
			doubled = true;
		else {
			printf("Usage: %s [-r] [-d])\n", argv[0]);
		}
		arg++;
	}

	if (doubled) {
		if (reverse) {
			while (1) {
				double f = 0;
				int found = scanf("%la", &f);
				if (found == EOF)
					break;
				if (check(found))
					printf("%.*lg\n", DBL_DECIMAL_DIG, f);
			}
		} else { 
			while (1) {
				double f = 0;
				int found = scanf("%lf", &f);
				if (found == EOF)
					break;
				if (check(found))
					printf("%la\n", f);
			}
		}
	} else { 
		if (reverse) {
			while (1) {
				float f = 0;
				int found = scanf("%a", &f);
				if (found == EOF)
					break;
				if (check(found))
					printf("%.*g\n", FLT_DECIMAL_DIG, f);
			}
		} else { 
			while (1) {
				float f = 0;
				int found = scanf("%f", &f);
				if (found == EOF)
					break;
				if (check(found))
					printf("%a\n", f);
			}
		}
	}
	return 0;
}