Convert a hexadecimal string to an integer efficiently in C?

Question

In C, what is the most efficient way to convert a string of hex digits into a binary unsigned int or unsigned long?

For example, if I have

Answer 1

Try this to Convert from Decimal to Hex

    #include
    #include

    int main(void)
    {
      int count=0,digit,n,i=0;
      int hex[5];
      clrscr();
      printf("enter a number   ");
      scanf("%d",&n);

      if(n<10)
      {
          printf("%d",n);
      }

      switch(n)
      {
          case 10:
              printf("A");
            break;
          case 11:
              printf("B");
            break;
          case 12:
              printf("B");
            break;
          case 13:
              printf("C");
            break;
          case 14:
              printf("D");
            break;
          case 15:
              printf("E");
            break;
          case 16:
              printf("F");
            break;
          default:;
       }

       while(n>16)
       {
          digit=n%16;
          hex[i]=digit;
          i++;
          count++;
          n=n/16;
       }

       hex[i]=n;

       for(i=count;i>=0;i--)
       {
          switch(hex[i])
          {
             case 10:
                 printf("A");
               break;
             case 11:
                 printf("B");
               break;
             case 12:
                 printf("C");
               break;
             case  13:
                 printf("D");
               break;
             case 14:
                 printf("E");
               break;
             case 15:
                 printf("F");
               break;
             default:
                 printf("%d",hex[i]);
          }
    }

    getch();

    return 0;
}