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

/* 
 * Constants for conversions.
 * (Here I chose to compute everything from English-to-English,
 * metric-to-metric, and inches-to-centimeters factors, because
 * that seemed more likely to be as accurate as possible.  But
 * could have just looked up the needed conversion factors somewhere.
 */
#define CM_PER_INCH 2.54
#define INCHES_PER_FOOT 12
#define FEET_PER_MILE 5280
#define CM_PER_M 100
#define M_PER_KM 1000
/* m/ft = in/ft * cm/in * m/cm
        = (in/ft * cm/in) / (cm/m)
 */
#define M_PER_FOOT \
	(INCHES_PER_FOOT * CM_PER_INCH / CM_PER_M)
/* km/mi = ft/mi * in/ft * cm/in * km/cm
        = (ft/mi * in/ft * cm/in) / (cm/m * m/km)
 */
#define KM_PER_MILE \
	(FEET_PER_MILE * INCHES_PER_FOOT * CM_PER_INCH / (CM_PER_M * M_PER_KM))

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

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

	char cmd;
	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");
	}
	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);
	return cmd;
}

/* Get number from standard input.  Prints error message and exits
 * program if non-numeric input is provided.  (This might not be the
 * best thing to do, but the main point of this function is to 
 * illustrate using a function to avoid duplicating code.)
 */
float getNumber(void) {
	float number;
	if (scanf("%f", &number) == 1) {
		return number;
	}
	else {
		printf("Not a number\n");
		exit(1);
	}
}

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

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

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