#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <assert.h>

bool parse_digit(int* a_value, char digit) {
    if(digit >= '0' && digit <= '9') {
        *a_value = digit - '0';   // for '0', we return 0; for '5' we return 5.
        return true;
    }
    return false;  // indicates that the digit was not a numeric digit.
}

int main(int argc, char* argv[]) {
    int value;
    bool is_valid = parse_digit(&value, '8');
    assert(value == 8);
    assert(is_valid);

    is_valid = parse_digit(&value, '_');
    assert(!is_valid);

    return EXIT_SUCCESS;
}
/* vim: set tabstop=4 shiftwidth=4 fileencoding=utf-8 noexpandtab: */