Problem with input and output to and from .txt file - c

Question

My homework is to calculate the resistance in Ohms of a resistor give a text file with information about the resistor. Using this text file, I calculate the resistance and print it to another text file. When I run this code I get a debug error "Run-Time check failure #3-T" and no output to the file. Can you tell me where I am going wrong?
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include<math.h>
int col_to_num(char color, int choice) {
int num;
switch (color) {
case 'B':if (choice == 1) num = 0;
else if (choice == 2) num = 1;
else if (choice == 3)num = 6;
case 'b':if (choice == 1) num = 0;
else if (choice == 2) num = 1;
else if (choice == 3)num = 6;
case 'G':if (choice == 1) num = 5;
else if (choice == 2) num = 8;
else if (choice == 3)num = -1;
case 'g':if (choice == 1) num = 5;
else if (choice == 2) num = 8;
else if (choice == 3)num = -1;
case 'R':num = 2;
case 'r':num = 2;
case 'O':num = 3;
case 'o':num = 3;
case 'Y':num = 4;
case 'y':num = 4;
case 'V':num = 7;
case 'v':num = 7;
case 'W':num = 9;
case 'w':num = 9;
case 'S':num = 10;
case 's':num = 10;
}
return num;
}
int main() {
double resistance, thirdband;
char color1, color2, color3;
int val1, val2, val3;
FILE *inp, *outp;
inp = fopen("resistorcolor.txt", "r");
outp = fopen("resistorvalue.txt", "w");
if (inp == NULL) {
printf("The input file does not exist\n");
}
else {
while (fscanf(inp, "%c %d %c %d %c %d%*c", &color1, &val1, &color2, &val2, &color3, &val3) != EOF) {
if (col_to_num(color3, val3) == -1)thirdband = .01;
if (col_to_num(color3, val3) == 10)thirdband = .01;
if (col_to_num(color3, val3) == 0)thirdband = 1;
if (col_to_num(color3, val3) == 1)thirdband = 10;
if (col_to_num(color3, val3) == 2)thirdband = 100;
if (col_to_num(color3, val3) == 3)thirdband = 1000;
if (col_to_num(color3, val3) == 4)thirdband = 10000;
if (col_to_num(color3, val3) == 5)thirdband = 100000;
if (col_to_num(color3, val3) == 6)thirdband = 1000000;
resistance = ((col_to_num(color1, val1) * 10) + col_to_num(color2, val2))*thirdband;
fprintf(outp, "%.2lf Ohm", resistance);
thirdband = 0;
}
fclose(outp);
}
return 0;
}

Answer 1

Run-Time check failure #3 mean something was used without being initialized. There is more information which you are not seeing. The error message should be "Run-Time check failure #3: The variable 'XXX' is being used without being initialized", where XXX is a variable in your code.
In any case you have a lot of uninitialized variables, those should be initialized to some default value:
double resistance = 0.0, thirdband = 0.0;
char color1 = 0, color2 = 0, color3 = 0;
int val1 = 0, val2 = 0, val3 = 0;
FILE *inp = nullptr, *outp = nullptr;
and for the function:
int col_to_num(char color, int choice) {
int num = -1;
Most likely it is int num from the col_to_num function, which will never hit a case if you didn't pass in one of those values. You need to break to exit the switch statement, usually after your condition is hit, however you can group cases together by not breaking allowing you to easily do the same action for different cases. Think of it as falling through starting at the case that is matched. You also need a default case to handle if no cases are matched like this:
int col_to_num(char color, int choice) {
int num = -1;
switch (color) {
case 'B':
case 'b':
if (choice == 1) num = 0;
else if (choice == 2) num = 1;
else if (choice == 3)num = 6;
break;
case 'G':
case 'g':
if (choice == 1) num = 5;
else if (choice == 2) num = 8;
else if (choice == 3)num = -1;
break;
case 'R':
case 'r':
num = 2;
break;
case 'O':
case 'o':
num = 3;
break;
case 'Y':
case 'y':
num = 4;
break;
case 'V':
case 'v':
num = 7;
break;
case 'W':
case 'w':
num = 9;
break;
case 'S':
case 's':
num = 10;
break;
default:
printf("Invalid input: %c\n", color);
}
return num;
}

Answer 2

perhaps you got a wrong understanding of switch case. You need to include break statement after every case if not added, all the cases will run whatever the input may be. And second thing is, you have to initialize the variables before using them. Hope that helps. Cheers !! Feel free to ask any doubts.