How to get the first element in a string?

Question

Hello,

I'm trying to figure out a way to check a string's first element if it's either a number or not.

if not(myString[0] in [0..9]) then //Do something

The problem is that I get an error "Element 0 inaccessible - use 'Length' or 'SetLength"

Another way came to my head from my C-like exprieince - convert the first element of the string to char and check the char,but there is no difference in the compile errors.

if not(char(myString[0]) in [0..9]) then //Do something

How do I accomplish it?

Answer 1

Strings are 1-based:

if not (myString[1] in ['0'..'9']) then // Do something

Answer 2

Delphi strings use a 1-based index, so just rewrite to

if not(myString[1] in ['0'..'9']) then //Do something

Also take note of the quotes around the 0..9, otherwise you would be comparing characters to integers.

Answer 3

Pascal and Delphi indexes string from 1. This is a legacy from time where zero byte contained length, while next 255 (index 1 to 255) contained actual characters. Joel Spolsky wrote quite good article on string issues:
http://www.joelonsoftware.com/articles/fog0000000319.html

Answer 4

if not(myString[0] in [0..9]) then //Do something

If you're using Delphi 2009, the TCharacter class in Character.pas has functions like IsDigit to help simplify these kinds of operations.

Once you fix the indexing, of course. :)

Answer 5

The simplest way to check to see if the first character of string is an integer, and then dispatch:

var
  iResult : integer;
begin
  if TryStrToInt( mySTring[1], iResult) then
    begin
      // handle number logic here iResult = number
    end
  else
    begin
      // handle non number logic here
    end;
end;

Answer 6

I use a utility function to test the entire string:

function IsNumeric(const Value: string): Boolean;
var
  i: Integer;
begin
  Result := True;
  for i := 1 to Length(Value) do
    if not (Value[i] in ['0'..'9','.','+','-']) then
    begin
      Result := False;
      Break;
    end;
end;

The above code is for Delphi versions prior to 2007. In 2007 and 2009, you could change the integer variable i to a character c, and use for c in Value instead.

To test for integers only, remove the '.' from the set of characters to test against.

Answer 7

This is incorrect. ISO strings and older Pascal's also started at one. It is just a general convention, and afaik the s[0] thing is a result of that being vacant, and cheap to code in the UCSD bytecode interpreter. But that last bit is before my time, so only my guessing.

It results from the Pascal ability to have arbitrary upper and lower bounds, which provides for more typesafety accessing arrays.

Really old Pascal strings (till early eighties) strings were even worse than C ones btw. Multiple conventions were in used, but all were based on static arrays (like early C), but they were typically space padded, so you had scan back from the end till the spaces ended.

(removed the legacy tag, since being 1 based is not legacy. Accessing s[0] as length IS legacy, but that is not what the question is about)