ResistanceCalculator

#include <stdio.h>
#include <math.h>

#define CALCULATE 1
#define HELP 2
#define EXIT 3

int bandColor[3];
float tolerance = 0.10;

int calculate()
{
    printf("These are the color choices for the bands\n");
    printf("\t0. black\n");
    printf("\t1. brown\n");
    printf("\t2. red\n");
    printf("\t3. orange\n");
    printf("\t4. yellow\n");
    printf("\t5. green\n");
    printf("\t6. blue\n");
    printf("\t7. violet\n");
    printf("\t8. grey\n");
    printf("\t9. white\n");
    printf("\t10. gold\n");
    printf("\t11. silver\n");

    printf("Pleas enter the color for band 1\n");
    scanf("%d", &bandColor[0]);
    while(0>bandColor[0] || 9< bandColor[0])
    {
        printf("Please enter a valid number\n");
        printf("Pleas enter the color for band 1\n");
        scanf("%d", &bandColor[0]);
    }

    printf("Pleas enter the color for band 2\n");
    scanf("%d", &bandColor[1]);
    while(0>bandColor[1] || 9< bandColor[1])
    {
        printf("Please enter a valid number\n");
        printf("Pleas enter the color for band 2\n");
        scanf("%d", &bandColor[1]);
    }

    printf("Pleas enter the color for band 3\n");
    scanf("%d", &bandColor[2]);
    while(0>bandColor[2] || 11< bandColor[2])
    {
        printf("Please enter a valid number\n");
        printf("Pleas enter the color for band 3\n");
        scanf("%d", &bandColor[2]);
    }

    printf("Pleas enter the color for band 4\n");
    scanf("%d", &bandColor[3]);
    while(10>bandColor[3] || 11< bandColor[3])
    {
        printf("Please enter a valid number\n");
        printf("Pleas enter the color for band 4\n");
        scanf("%d", &bandColor[3]);
    }

    printf("\nThe Value of the resistor is.\n");
    tolerance = bandColor[3] == 10 ? 0.05:0.10;
    metricScale();

    main();
}

char *getScale()
{
  switch(bandColor[2])
  {
    case 2:
    case 3:
    case 4:
      return " K-ohms";
    case 5:
    case 6:
    case 7:
      return " M-ohms";
    case 8:
    case 9:
      return " G-ohms";
    default:
      return " ohms";
  }
}

int metricScale()
{
    float power = bandColor[2] == 10 ? 0 :
                  bandColor[2] == 11 ? -1 : (bandColor[2]+1)%3; 
    float value = ((float)bandColor[0] + (float)bandColor[1]/10.0) * pow(10,power);
    tolerance = value * tolerance;

    printf("%.2f ~ %.2f %s\n", value, tolerance, getScale());
}

int help()
{
    printf("you will be prompted to enter the color of each band 1 - 4.\n");
    printf("The program will then give you the resistance in ohms of the resistor.\n\n");

    return main();
}

int main()
{
    int userInput;
    printf("Please choose an option.\n");
    printf("\t1. calculate the resistance value\n");
    printf("\t2. Help\n");
    printf("\t3. Exit\n");
    scanf("%d", &userInput);

    switch(userInput)
    {
        case CALCULATE:
            calculate();
            break;
        case HELP:
            help();
            break;
        case EXIT:
            printf("Have a Nice Day!\n");
            break;
        default:
            printf("Please Enter a Valid Number.\n");
            main();
            break;
    }
}