resistor.c

/*
 * resistor.c
 *
 * Convert a resistor color code into the resistor
 * value.  This program takes a three-color band list
 * and prints the value in Ohms.  It does not do anything
 * with the tolerance band.
 *
 * Example: red violet orange -> 27000-Ohms
 */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <locale.h>
#include <ctype.h>

#define VERSION "2018.01a"
#define VERSION_DATE "January 29, 2018"

/* lookup table */
struct resistor_st {
  char *color;
  int digit;
  double mult;
} resistor[] = {
  "black",  0,  1.0,
  "brown",  1,  10.0,
  "red",    2,  100.0,
  "orange", 3,  1000.0,
  "yellow", 4,  10000.0,
  "green",  5,  100000.0,
  "blue",   6,  1000000.0,
  "violet", 7,  10000000.0,
  "grey",   8,  0.01,
  "white",  9,  0.1,
  "gray",   8,  0.01, /* for those of us that spell grey gray */
  NULL,    -1, -1.0
};

/* function prototypes */
int color_to_digit(char *);
double color_to_mult(char *);
void error(char *);
int resist_out(double, FILE *);
int color_in(int, char **);
int resist_in(int, char **);
double find_mult(double);
int find_band1(double);
int find_band2(double);
int mult_to_digit(double);
void version(void);
void help(void);

int main(int argc, char **argv) {
  int result;

  setlocale(LC_ALL, "");
  
  if (argc==1) {
    result = color_in(argc, argv);
  }
  if (argc==2) {
    if (isdigit(argv[1][0])) {
      result = resist_in(argc, argv);
    } else {
      switch (tolower(argv[1][1])) {
      case 'h':
	help();
	return EXIT_SUCCESS;
	break;
      case 'v':
	version();
	return EXIT_SUCCESS;
	break;
      default:
	result = -1;
	break;
      }
    }
  }
  return result;
}

void version() {
  printf("RESISTOR version %s\n", VERSION);
  printf("\nMichael J. Chappell %s\n\n", VERSION_DATE);
  return;
}

void help() {
  printf("RESISTOR\n");
  printf("Usage: resistor [n[K|M]]\n");
  printf("\n");
  version();
  return;
}

int resist_in(int argc, char **argv) {
  double base, ohms;
  int digit1, digit2;
  double mult;
  char band1[255], band2[255], band3[255];
  char *p;
  
  base = strtod(argv[1], &p);

  if ((*p == 'M')||(*p == 'm'))
    ohms = base * 1000000.0;
  else if ((*p == 'K')||(*p == 'k'))
    ohms = base * 1000.0;
  else
    ohms = base;
  
  if (ohms <= 0.0) return EXIT_FAILURE;

  mult = find_mult(ohms);
  if (mult<0.0) return EXIT_FAILURE;

  digit1 = find_band1(ohms);
  if (digit1<0) return EXIT_FAILURE;

  digit2 = find_band2(ohms);
  if (digit2<0) return EXIT_FAILURE;
  
 /*
  printf("argv[1] = \"%s\".\n", argv[1]);
  printf("Ohms    = %'.2lf\n", ohms);

  printf("(band1,band2,mult) = (%d,%d,%'.2lf)\n", digit1, digit2, mult);
*/
  printf("The first three bands of an %s Ohm resistor are %s, %s, and %s.\n",
	 argv[1],
	 resistor[digit1].color, resistor[digit2].color,
	 resistor[mult_to_digit(mult)].color);
    
  return EXIT_SUCCESS;
}

double find_mult(double ohms) {
  if (ohms<         1.0) return        0.01;
  if (ohms<        10.0) return        0.10;
  if (ohms<       100.0) return        1.00;
  if (ohms<      1000.0) return       10.00;
  if (ohms<     10000.0) return      100.00;
  if (ohms<    100000.0) return     1000.00;
  if (ohms<   1000000.0) return    10000.00;
  if (ohms<  10000000.0) return   100000.00;
  if (ohms< 100000000.0) return  1000000.00;
  if (ohms<1000000000.0) return 10000000.00;
  return -1.0;
}

int find_band1(double ohms) {
  double x;
  double mult;

  mult = find_mult(ohms);
  x = ohms/mult;

  if (x>=90.0) return 9;
  if (x>=80.0) return 8;
  if (x>=70.0) return 7;
  if (x>=60.0) return 6;
  if (x>=50.0) return 5;
  if (x>=40.0) return 4;
  if (x>=30.0) return 3;
  if (x>=20.0) return 2;
  if (x>=10.0) return 1;
  if (x>=00.0) return 0;
  return -1;
}

int find_band2(double ohms) {
  double x;
  double mult;
  int band1;

  mult = find_mult(ohms);
  band1 = find_band1(ohms);
  x = ohms/mult -10*band1;

  if (x>=9.0) return 9;
  if (x>=8.0) return 8;
  if (x>=7.0) return 7;
  if (x>=6.0) return 6;
  if (x>=5.0) return 5;
  if (x>=4.0) return 4;
  if (x>=3.0) return 3;
  if (x>=2.0) return 2;
  if (x>=1.0) return 1;
  if (x>=0.0) return 0;
  return -1;
}

int mult_to_digit(double n) {
  int i;
  
  for (i=0; i<10; i++) {
    if (n == resistor[i].mult) return i;
  }
  return -1;
}

int color_in(int argc, char **argv) {
  double ohms;
  int digit1, digit2;
  double mult;
  char band1[255], band2[255], band3[255];

  /* get the color code */
  puts("Type colors (Ex: brown black red) + Return:");
  scanf("%s %s %s", band1, band2, band3);

  /* convert the code to a value */
  if  ((digit1 = color_to_digit(band1)) == -1)
    error(band1);
  if  ((digit2 = color_to_digit(band2)) == -1)
    error(band2);
  if  ((mult = color_to_mult(band3)) == -1.0)
    error(band1);

  ohms = (digit1*10 + digit2)*mult;
  printf(
	 "The value of a resistor with bands of %s, %s and %s\nis ",
	 band1, band2, band3);
  resist_out(ohms, stdout);
  printf(" Ohms.\n");
  
  return EXIT_SUCCESS;
}

int color_to_digit(char *color) {
  struct resistor_st *p;

  /* look for the code in the resistor table */
  p = resistor;
  while (p->color != NULL) {
    if ((strcmp(p->color, color))==0)
	break;
	++p;
  }

  return p->digit;
}

double color_to_mult(char *color) {
  struct resistor_st *p;

  /* look for the code in the resistor table */
  p = resistor;
  while (p->color != NULL) {
    if ((strcmp(p->color, color))==0)
	break;
	++p;
  }

  return p->mult;
}

void error(char *text) {
  fprintf(stderr, "bad color spec: %s\n", text);
  exit(EXIT_FAILURE);
}

int resist_out(double value, FILE *fp) {
  if (value>1000000.0) {
    fprintf(fp, "%'.2lfM", value/1000000.0);
    return 0;
  }
  if (value>1000.0) {
    fprintf(fp, "%'.2lfM", value/1000.0);
    return 0;
  }

  fprintf(fp, "%'.2lf", value);
  
  return 0;
}
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Wednesday, 12 August 2026 Michael J. Chappell Contact me at: mcsuper5@freeshell.org
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