/* 
 * Program to convert different kinds of "English" units to metric:
 *
 * Input:  Which conversion is wanted, number to convert (program
 *   prompts for these).  Repeats prompt until user inputs 'x' to exit.
 *
 * Output:  Result of conversion (e.g., inches to centimeters).
 */
#include <stdio.h>      /* has scanf(), printf() */

/* Constants for conversions. */
#define CM_PER_INCH 2.54
#define INCHES_PER_FOOT 12
#define FEET_PER_MILE 5280
#define CM_PER_M 100
#define KM_PER_M 1000
#define M_PER_FOOT \
	(INCHES_PER_FOOT * CM_PER_INCH / CM_PER_M)
#define KM_PER_MILE \
	(FEET_PER_MILE * INCHES_PER_FOOT * CM_PER_INCH / (CM_PER_M * KM_PER_M))

/* Function declarations.  (Comments with definitions, below main.) */
char getCommand(void);
double getNumber(void);
void convertInches(void);
void convertFeet(void);
void convertMiles(void);

/* Main program */
int main(void) {

	char cmd;

	do {
		cmd = getCommand();

		switch (cmd) {
			case 'i':
				convertInches();
				break;
			case 'f':
				convertFeet();
				break;
			case 'm':
				convertMiles();
				break;
			case 'x':
				break;
			default:
				printf("Not a valid choice\n");
		}
	} while (cmd != 'x');

	return 0;
}

/* Function definitions. */

/* Prompt user for menu selection and return result. */
char getCommand(void) {
	char cmd;
	printf("What would you like to do?\n");
	printf("  'i' to convert inches to centimeters\n");
	printf("  'f' to convert feet to meters\n");
	printf("  'm' to convert miles to kilometers\n");
	printf("  'x' to exit\n");
	printf("Your choice?\n");
	scanf(" %c", &cmd);     
	/* space before %c says "skip leading white space" */
	return cmd;
}

/* Get number from standard input.  Prints error message and prompts
 * again if non-numeric input is provided.  
 */
double getNumber(void) {
	double number;
	char c;
	/* keep trying to get input until we get something valid */
	while (scanf("%lf", &number) != 1) {
		/* not a number -- discard input until we get to end of line */
		do {
			scanf("%c", &c);
		} while (c != '\n');
		printf("Not a number.  Please try again:\n");
	}
	return number;
}

/* Do "inches to cm" conversion (prompt for input, print result). */
void convertInches(void) { 
	double inches;
	printf("how many inches?\n");
	inches = getNumber();
	printf("%lf inches is %lf cm\n", inches, CM_PER_INCH * inches);
}

/* Do "feet to meters" conversion (prompt for input, print result). */
void convertFeet() { 
	double feet;
	printf("how many feet?\n");
	feet = getNumber();
	printf("%lf feet is %lf m\n", feet, M_PER_FOOT * feet);
}

/* Do "miles to km" conversion (prompt for input, print result). */
void convertMiles() { 
	printf("miles\n");
	double miles;
	printf("how many miles?\n");
	miles = getNumber();
	printf("%lf miles is %lf cm\n", miles, KM_PER_MILE * miles);
}