TheAlgorithms-C/conversions/to_decimal.c

/*
 * convert from any base to decimal
 */

#include <stdio.h>
#include <ctype.h>

int main(void) {
	int base, i, j;
	char number[100];
	unsigned long decimal = 0;
	
	printf("Enter the base: ");
	scanf("%d", &base);
	printf("Enter the number: ");
	scanf("%s", &number[0]);
	
	for (i = 0; number[i] != '\0'; i++) {
		if (isdigit(number[i]))
			number[i] -= '0';
		else if (isupper(number[i]))
			number[i] -= 'A' - 10;
		else if (islower(number[i]))
			number[i] -= 'a' - 10;
		else
			number[i] = base + 1;
		
		if (number[i] >= base){
			printf("invalid number\n");
			return 0;
		}
	}
	
	for (j = 0; j < i; j++) {
		decimal *= base;
		decimal += number[j];
	}
	
	printf("%lu\n", decimal);
}