Defining a hexadecimal number as in Problem 67, write the function int hexToDec( const int hexNum[]) to convert a four-digit hexadecimal number to a nonnegative decimal integer. Do not output the decimal integer in the function. Test your function with interactive input.

#include <stdio.h>

int hexToDec(const int hexNum[]){

   int result = 0, prod = 1;

   for(int i = 3;i>=0;i-=1){

       result += (prod*hexNum[i]);

       prod = prod * 16;

   }

   return result;

}

int main()

{

   int hexNum[4], i;

   char s[5], ch;

   printf("Enter hexa decimal value: ");

   scanf("%s",s);

   for(i = 0;i<4;i++){

       ch = s[i];

       if(ch>='0' && ch<='9'){

         hexNum[i] = ch-'0';

      }

      else if((ch>='a' && ch<='f') || (ch>='A' && ch<='F')){

         if(ch=='A' || ch=='a'){

            hexNum[i] = 10;

         }

         else if(ch=='B' || ch=='b'){

            hexNum[i] = 11;

         }

         else if(ch=='C' || ch=='c'){

            hexNum[i] = 12;

         }

         else if(ch=='D' || ch=='d'){

            hexNum[i] = 13;

         }

         else if(ch=='E' || ch=='e'){

            hexNum[i] = 14;

         }

         else{

            hexNum[i] = 15;

         }

      }

   }

   printf("%d",hexToDec(hexNum));

   return 0;

}

Explanation:

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that without the operating systems you would have to control and synchronize computer hardware.