[Shantanu_Dutta]

/* Program to convert hexAdeCimal into binary form........ Q_13......... */

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


     int *input( int *l_pos )
      {
    *l_pos = 0 ;
    int i = 0 , j;
    int t_bit , *arr ;
     printf("\n\n\tEnter the bits in hexadecimal form( 0 to 9 and A to F in CAPS ) in ") ;
     printf("\n\tsequence and to quit entering , press 'RETURN' key ........ ");

     printf("\n\n\tStart entering bits .............  \n\n\t");

      while( ( t_bit = getche() ) != 13 )
       {
          *( arr + i ) = t_bit ;
          *l_pos = i ;
          ++i ;
          printf("  ");
       }

      return arr ;
      }

     int *dec_calc( int bit )
      {
    int j , *arr ;
    bit = bit - 48 ;

     for( j = 0 ; j < 4 ; j++ )
      {
        *( arr + j ) = 0 ;
      }
     j-- ;

     while( bit != 0 )
      {
        *( arr + j ) = bit % 2 ;
        j-- ;
        bit /= 2 ;
      }

    return arr ;
      }

     int *alfa_calc( int bit )
      {
    int j = 0 , *arr ;

    switch( bit )
     {
       case 65 : bit = 10 ; break ;
       case 66 : bit = 11 ; break ;
       case 67 : bit = 12 ; break ;
       case 68 : bit = 13 ; break ;
       case 69 : bit = 14 ; break ;
       case 70 : bit = 15 ; break ;
     }

     for( j = 0 ; j < 4 ; j++ )
      {
        *( arr + j ) = 0 ;
      }
     j-- ;

     while( bit != 0 )
      {
        *( arr + j ) = bit % 2 ;
        j-- ;
        bit /= 2 ;
      }

    return arr ;
      }


     void main()
      {
    clrscr();
    int *arr , *f_arr ;
    int l_pos , f_lpos = 0 , pos = 0 , i = 0 , j , *bin_diz , chk = 0 ;

     arr = input( &l_pos ) ;

    while( i <= l_pos )
     {

        for( j = 48 ; j <= 57 ; j++ )
         {
           if( *( arr + i ) == j )
         {
           bin_diz = dec_calc( *( arr + i ) ) ;

           for( pos = 0 ; pos < 4 ; pos++ )
            {
              *( f_arr + f_lpos ) = *( bin_diz + pos ) ;
              ++f_lpos ;
            }
           pos = 0 ;
           chk = 1 ;
           break ;
         }
         }

      if( !chk )
       {
        for( j = 65 ; j <= 70 ; j++ )
         {
           if( *( arr + i ) == j )
         {
           bin_diz = alfa_calc( *( arr + i ) ) ;
           while( pos < 4 )
            {
              *( f_arr + f_lpos ) = *( bin_diz + pos ) ;
              ++f_lpos , ++pos ;
            }
            pos = 0 ;
           chk = 1 ;
           break ;
         }
         }


        if( !chk )
         {
          for( j = 71 ; j <= 90 ; j++ )
           {
        if( *( arr + i ) == j )
         {
           printf("\n\n\tHexadecimal bits entered are not valid ! ");
           chk = 1 ;
           break ;
         }
           }

         if( !chk )
          {
           for( j = 97 ; j <= 122 ; j++ )
        {
         if( *( arr + i ) == j )
          {
           printf("\n\n\tHexadecimal bits entered are not valid ! ");
           chk = 1 ;
           break ;
          }
        }

           if( !chk )
        {
           printf("\n\n\tHexadecimal bits entered are not valid ! ");
           chk = 1 ;
           break ;
        }

          }
         }
       }

      chk = 0 ;
      i++ ;

     }

    printf("\n\n\n\tAfter conversion , number in binary form is : ' " );

     for( i = 0 ; i < f_lpos  ; i++ )
      {
       printf("%d" , *( f_arr + i ) );
      }
     printf(" '");

    getch();

      }

[xpi0t0s]

Well, we can't read your mind. What does it do wrong? Does it build? If not what are the errors? If it builds, what does it do when you run it? What line of code goes wrong? If it displayed the wrong output, what output did you get, and what did you expect? It looks like it might take some input from the user, so what input did you give it?