Converting hex to dec problems

For some reason my algorithm to convert a hexadecimal number into a decimal number doens't work and I cna't figure it out. Any help would be appreciated.

Code:

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

//dest is a character array that holds my hexedecimal string
int todecimal(char *dest)
{
        int pos, sum, power_of_16;
                //pos is the current element in the dest array
       
        pos=strlen(dest);
        power_of_16=0;
       
        sum=0;
        do{
                --pos;
                if(dest[pos]<=9)
                        sum+=dest[pos]*pow(16,power_of_16);
                switch(dest[pos]){
                        case 'a': sum+=10*pow(16,power_of_16);
                                break;
                        case 'b':sum+=11*pow(16,power_of_16);
                                break;
                        case 'c':sum+=12*pow(16,power_of_16);
                                break;
                        case 'd':sum+=13*pow(16,power_of_16);
                                break;
                        case 'e':sum+=14*pow(16,power_of_16);
                                break;
                        case 'f':sum+=15*pow(16,power_of_16);
                                break;
                        default: /*do nothing*/
                                break;
                }
                power_of_16++;
        }while(pos!=0);
       
        return sum;
}

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>if(dest[pos]<=9)

The character value of a text digit may not be a decimal value. For example, an ASCII '4' is equal to 0x34, which is equal to 52. You may want to use the following.

Code:

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if ( dest[pos]-'0'<=9 )
  sum+=(dest[pos]-'0')*pow(16,power_of_16);

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May I ask what that does exactly? I'm new to C, but have been using C++ for a couple of years. Let me guess, it subtracts the ASCII value of zero from the ASCII value of dest[pos] and that should fix it?

Code:

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#include<stdio.h>

int main(void)
{
  int i;
  for(i = '0'; i <= '9'; ++i)
  {
      printf("i = '%c' = %d, i - '0' = %d - %d = %d\n", i, i, i, '0', i - '0');
  }
  return 0;
}

/* my output
i = '0' = 48, i - '0' = 48 - 48 = 0
i = '1' = 49, i - '0' = 49 - 48 = 1
i = '2' = 50, i - '0' = 50 - 48 = 2
i = '3' = 51, i - '0' = 51 - 48 = 3
i = '4' = 52, i - '0' = 52 - 48 = 4
i = '5' = 53, i - '0' = 53 - 48 = 5
i = '6' = 54, i - '0' = 54 - 48 = 6
i = '7' = 55, i - '0' = 55 - 48 = 7
i = '8' = 56, i - '0' = 56 - 48 = 8
i = '9' = 57, i - '0' = 57 - 48 = 9
*/

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Thank you very much! Now I'll move on to trying to convert signed decimals to 2's complement binary. I really appreciate the help!