stachour@sctc.com (Paul Stachour) (11/29/90)
I have this "simple" problem. I have something that I need
its internal representation of. I need this to calculate a
checksum or a hash-value or ... I really don't even care what
the value is, or plan to do anything neat-o with it. I just
need it to be repeatable.
In the (simplified) example below, I am trying to get a hash-value
from several characters in a string. However, I seem to get problems
in doing this. In the example given, I get "constraint-error" on the
attempt to do the "conversion". (V_Name is a 60-char string with an
OK name in it, the first character is a "c".)
What am I doing wrong? How should I be doing it?
[Unchecked conversion between an 8-bit character and an
integer is not guarenteed either, is it? Or do I work
with some particular range of integer?]
-- ********************************************************** --
function Hash_Name(
The_Name : in V_Name )
return Positive is
C: STRING(1..1);
begin
C(1) := The_Name(1);
return POSITIVE'VALUE(C(1..1));
--1 return POSITIVE'VALUE(The_Name(1));
end;
-- ********************************************************** --
--
Paul Stachour Secure Computing Technology Corp
stachour@sctc.com 1210 W. County Rd E, Suite 100
Arden Hills, MN 55112
[1]-(612) 482-7467murphy@mips.COM (Mike Murphy) (11/30/90)
In article <1990Nov28.210856.14960@sctc.com> stachour@sctc.com (Paul Stachour) writes: > > In the (simplified) example below, I am trying to get a hash-value >from several characters in a string. However, I seem to get problems >in doing this. In the example given, I get "constraint-error" on the >attempt to do the "conversion". (V_Name is a 60-char string with an >OK name in it, the first character is a "c".) > >-- ********************************************************** -- >function Hash_Name( > The_Name : in V_Name ) > return Positive is > C: STRING(1..1); >begin > C(1) := The_Name(1); > return POSITIVE'VALUE(C(1..1)); >--1 return POSITIVE'VALUE(The_Name(1)); >end; >-- ********************************************************** -- The type'VALUE(string) attribute returns the type-value that the string image represents. Thus POSITIVE'VALUE("12") = 12, while POSITIVE'VALUE("cat") raises constraint_error, because "cat" is not the image of a positive number. To get a hash value, you want to use the 'POS attribute, e.g. CHARACTER'POS(The_Name(1)) = character'pos('c') = 99 To get a real hash value you probably want to do some arithmetic on the 'pos values of more than one character in the string. If you want to use unchecked_conversion, you could convert four characters at a time to integer (assuming 4-byte integers), or convert one character to a tiny_integer (or whatever 1-byte integer your implementation defines): function to_int is new unchecked_conversion (string, integer); function to_tint is new unchecked_conversion (character, tiny_integer); integer_obj := to_int(The_Name(1..4)); tiny_integer_obj := to_tint(The_Name(1)); -- this is essentially the same as doing a 'pos.