Recently I ran across an interesting phenomenon where an obvious string was converted into a number by Delphis Functions: Val, IntToStr and InToStrDef.

Hi folks,

since quite some time I am using the following function to check whether a submitted string is a number or "just" a string with some non-numeric characters.

function IsInteger(const Str: String): Boolean;
var
  E, I: Integer;
begin
  Val(Str, I, E);
  Result := E = 0;
end;

It worked fine, or so it seemed, until the other day a rather rare situation occered.

The function returned True on the value of "x56e", which wasn't quite the thing I expected to happen. After a while of searching for the reason it became rather obvious. Delphi tries to interpret strings starting with either "0x", "x", or "$" as hexa-decimal numbers.

Well for strings starting with "$" I knew it would, in fact I wrote an articel about it, but the fact that "0x" and "x" are interpreted as well, was a newie for me.

Therefore "x56e" is equal to "0x56e", which is equal to "$056E" which is "Delphi"-hexadecimal to 1390 (decimal).

Anyway, I looked into the source code of the Val function, and here is the place where I found the solution:

@@endBlanks:
        MOV     CH,0
        CMP     BL,'-'
        JE      @@minus
        CMP     BL,'+'
        JE      @@plus
        CMP     BL,'$'
        JE      @@dollar

--->    CMP     BL, 'x'
        JE      @@dollar
--->    CMP     BL, 'X'
        JE      @@dollar
--->    CMP     BL, '0'
        JNE     @@firstDigit
        MOV     BL, [ESI]
        INC     ESI
--->    CMP     BL, 'x'
        JE      @@dollar
--->    CMP     BL, 'X'
        JE      @@dollar
        TEST    BL, BL
        JE      @@endDigits
        JMP     @@digLoop


The new version of my function converts the string to a number and back - if incoming string and resulting string are equal, it was a number, just the way I was it expecting to work.

function IsInteger(const Str: String): Boolean;
begin
  Result := IntToStr(StrToIntDef(Str, -1)) = Str;
end;

THANKS MAGNUS - THERE IS STILL A BUG IN THE PREVIOUS SOLUTION - SEE COMMENTS FOR A SOLUTION THAT ALMOST WORKS - you did forget about empty strings, though ;-)

Therefore, I will allow myself to add your solution to the article, slightly editied. Thanks again!

function IsInteger(Str: String) : boolean;
var
  I, L: Integer
begin
  Result := False;
  L := Length(Str);
  if L = 0 then
    // empty string
    Exit;
  if L > 1 then
    // first character for strings longer than one char
    if not (Str[1] in ['+', '-', '0'..'9']) then
      Exit
    else
  else
    // first character for strings exact one char long
    if not (Str[1] in ['0'..'9']) then
      Exit;  
  for I := 2 to length(Str) do
    // check remaining characters
    if not (Str[I] in ['0'..'9']) then
      Exit;
  Result := True;
end;


I hope, this will help you a little bit, once you are going to check for a numeric value.

By the way, no book I know of pays respect to this effect, a nice on it is, though ;-)

Content Ace