i, n, foo: integer;
...
result := 0;
for i := 1 to length( s ) do
  begin
  // validate digit is a binary digit
  if not (s[i] in ['0','1'])
    then complain_that_its_not_a_binary_number;
  // convert that digit into a number
  val( s[i], n, foo );
  // power up and add
  result := result * 2;
  result := result + n;
  end;

function bintohex(binary:string):string;
var
counter,upperbound,lowerbound:integer;
tempbin,hex:string;
begin
  counter:=length(binary);
  upperbound:=0;
  lowerbound:=0;
  tempbin:='';
  hex:='';
      while(Upperbound<counter) do
        begin
           while(lowerbound<=(upperbound+4)) do
             begin
                 tempbin:=tempbin+binary[lowerbound];
                 lowerbound:=lowerbound+1;
             end;
 
            if(bintodec(tempbin)=0) then
                       begin
                         hex:=hex+'0';
                       end;
                       if(bintodec(tempbin)=1) then
                       begin
                         hex:=hex+'1';
                       end;if(bintodec(tempbin)=2) then
                       begin
                         hex:=hex+'2';
                       end;
                       if(bintodec(tempbin)=3) then
                       begin
                         hex:=hex+'3';
                       end;
                       if(bintodec(tempbin)=4) then
                       begin
                         hex:=hex+'4';
                       end;
                       if(bintodec(tempbin)=5) then
                       begin
                         hex:=hex+'5';
                       end;
                       if(bintodec(tempbin)=6) then
                       begin
                         hex:=hex+'6';
                       end;
                       if(bintodec(tempbin)=7) then
                       begin
                         hex:=hex+'7';
                       end;
                       if(bintodec(tempbin)=8) then
                       begin
                         hex:=hex+'8';
                       end;
                       if(bintodec(tempbin)=9) then
                       begin
                         hex:=hex+'9';
                       end;
                      if(bintodec(tempbin)=10) then
                       begin
                         hex:=hex+'A';
                       end;
                       if(bintodec(tempbin)=11) then
                       begin
                         hex:=hex+'B';
                       end;
                       if(bintodec(tempbin)=12) then
                       begin
                         hex:=hex+'C';
                       end;
                       if(bintodec(tempbin)=13) then
                       begin
                         hex:=hex+'D';
                       end;
                       if(bintodec(tempbin)=14) then
                       begin
                         hex:=hex+'E';
                       end;
                       if(bintodec(tempbin)=15) then
                       begin
                         hex:=hex+'F';
                       end;
             Upperbound:=upperbound+4;
             tempbin:='';
        end;
         bintohex:=hex;
end;

Program NumberConversion;
 
Uses wincrt;
 
Const
   max = 30; {this is used for declaring ARRAY}
 
Type
   hexadecimal = ARRAY[1..max] OF integer;
 
Var
   decimal: longint;
   i: integer;
   hexa: hexadecimal;
 
Procedure Menu; {Procedure to display the Menu.}
Begin
     writeln(' Number conversion:'); {This line displayed only {Number conversion) and same for other line.}
     writeln('   1. Binary to decimal');
     Writeln('   2. Octal to decimal');
     Writeln('   3. Binary to hexadecimal');
     Writeln('   4. Exit');
     Write(' Enter your choice: ')
End; {Stop the Menu procedure.}
 
 
 
Procedure PressAnyKey; {Procedure to show the Menu by pressing any key when a conversion is finished.}
Begin
     Writeln; {leave a line.}
     Write('Press Any Key for Main Menu...');
     readkey; {Read any key on the keyboard.}
     clrscr; {Clear the screen.}
     Menu; {Display the Menu.}
End; {Stop the PressAnyKey procedure.}
 
 
function BinToDec(binary:string):integer; {Procedure to convert binary to decimal.}
Var  {this line Binary is declared as string.}
   Len:integer; {Len means Length declared as integer.}
   base:integer; {base is the base of the binary and declared as integer.}
   Result:longint; {Result is the final result and declared as integer.}
Begin
     Write('Enter a binary number: ');
     Readln(binary);
     result:=0; {Result initialised as 0.}
     base:=1; {base initialised as 1.}
     for Len:=length(binary) downto 1 do {loop from  string length down to 1 (binary reads from right to left).}
         begin                                                                                                                
               If not (binary[Len] in ['1','0']) then {validation, if the string does not contains 1 , 0 or both.}
                  begin           
 
                        writeln('This is not a binary number!');
                        writeln; {leave a line.}
                        Halt; {Stop the Loop.}
 
                  end
               else
                   if (binary[len]='1') then {Verify if see 1 in string}
                      result:= result+ base; {convert Binary to Decimal (The result).}
                      base:=base*2; {increment base by 2.}            
         end;
    BinToDec:=Result;
End;
 
Procedure OctToDec;
Var
   Octal:string;
   Len:integer;
   counter:integer;
   base:integer;
   result:longint;
   i,code:integer;
Begin
     Write('Enter an octal number: ');
     readln(Octal);
     result:=0;
     base:=1;
     for Len:=length(octal) downto 1 do {loop from  string length down to 1 (Octal reads from right to left).}
         Begin
              If not (octal[Len] in ['0'..'7']) then {validation, if the string does not contain only from 0 to 7.}
                 Begin
                      Writeln('This is not an Octal number!');
                      Writeln;
                      Halt; {stop the loop.}
 
                 end
              Else
                  Val(octal[len],i,code); {This line converts string into integer.}
                  Result:=result+i*base; {Converts Octal into Decimal (the result).}
                  base:=base*8; {octal base increment by 8}
         end;
     Writeln('The decimal number is ',result); {the answer}
end;
 
 
      {This procedure works the conversion of dec to hex.}
Procedure convert(decimal: longint; Var i: integer; Var hexa: hexadecimal);
Begin
   i := 1;
   while decimal <> 0 do
         Begin
              hexa[i] := decimal mod 16;
              decimal := decimal div 16;
              i := i + 1;
         end;
end;
 
Procedure writetothescreen(i: integer; hexa: hexadecimal);
Var j: integer;
Begin
   write('The Hexadecimal is: ');
   {This is how you make a hex number}
      For j := (i - 1) downto 1 do
          Begin
               if hexa[j] < 10 then
            write(output, hexa[j])
               else
                   Begin
{Here, program checks for number larger than 10 to write a letter instead of number}
            Case hexa[j] of
               10: write(output, 'A');
               11: write(output, 'B');
               12: write(output, 'C');
               13: write(output, 'D');
               14: write(output, 'E');
               15: write(output, 'F');
            End; {End of Case}
         End; {End of ELSE}
   End; {End of FOR loop}
End; {End of Procedure}
 
 
Procedure BinToHex; {This contains all the other procedure to get the final answer
                    of Binary to hexadecimal.}
var
   decimal:longint;
   binary:string;
Begin
   decimal:=bintodec(binary);
   convert(decimal, i, hexa);
   writetothescreen(i, hexa);
End;
 
 
Procedure SelectOption;{here this is use to make your choice}
var
   option:integer;
   binary:string;
Begin
     Readln(option);
     If not option in [1..4] then
        writeln('invalid choice')
     Else
         If (option=1) Then
            begin
                 clrscr;
                 Writeln('**Binary to Decimal conversion**');
                 writeln;
                 decimal:=BinToDec(binary);
                 writeln ('The Decimal number is: ',decimal); {Display the Result.}
                 PressAnyKey;
                 SelectOption;
            end;
         If (option=2) then
            Begin
                 clrscr;
                 Writeln('**Octal to Decimal conversion**');
                 Writeln;
                 OctToDec;
                 PressAnyKey;
                 SelectOption;
            End;
         If (option=3) then
            Begin
                 clrscr;
                 Writeln('**Binary to Hexadecimal conversion**');
                 writeln;
                 BinToHex;
                 writeln;
                 PressAnyKey;
                 SelectOption;
            End;
         If(option=4) then
            Begin
                 clrscr;
                 Writeln('Press Alt+F4 to exit');
            end;
end;
 
 
Begin {The main Program}
     Menu;
     SelectOption;
 
End.{End the main program}