Re: [fpc-pascal] Strange "division by zero" error using variants
That's a funny interaction between Variants and Pascal's ambiguous
(logical or bitwise) "and" :) Both the operators and the types are
ambiguous -> making the boolean short-circuit evaluation not possible.
Indeed one workaround is to cast the variants to Booleans explicitly,
which makes it use logical "and" with expected result. I.e. this
doesn't raise "division by zero":
if Boolean(v1) and Boolean(v2 > 0) and Boolean((0+1) mod v2 = 0) then
Writeln ('ok');
Regards,
Michalis
wt., 24 maj 2022 o 22:52 Florian Klämpfl via fpc-pascal
napisał(a):
>
> Am 24.05.22 um 19:28 schrieb Thomas Kurz via fpc-pascal:
> > Dear all,
> >
> > please consider the following code:
> >
> >
> > program Project1;
> >
> > {$booleval off}
> >
> > var
> >v1, v2: variant;
> >a: boolean;
> >b: integer;
> >
> > begin
> >a := true;
> >b := 0;
> >// this works as expected:
> >if a and (b > 0) and ((0+1) mod b = 0) then Writeln ('ok');
> >
> >v1 := true;
> >v2 := 0;
> >// this gives a "division by zero":
> >if v1 and (v2 > 0) and ((0+1) mod v2 = 0) then Writeln ('ok');
> > end.
> >
> >
> > The "variant" variant results in a "division by zero". Obviously, it tries
> > to evaluate the modulo-expression even though this shouldn't happen because
> > complete boolean evaluation is disabled and the "if"-result is already
> > known to be false after checking "v2>0".
> >
> > Can anyone explain this strange behavior?
>
> When compiling the and expressions, the compiler does not know (in this
> example it could, but in general it cannot) what the variants contain so
> just the variant and-operator is executed which does not/cannot
> distinguish between the logical and bitwise and variant.
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Re: [fpc-pascal] Strange "division by zero" error using variants
Am 24.05.22 um 19:28 schrieb Thomas Kurz via fpc-pascal:
Dear all,
please consider the following code:
program Project1;
{$booleval off}
var
v1, v2: variant;
a: boolean;
b: integer;
begin
a := true;
b := 0;
// this works as expected:
if a and (b > 0) and ((0+1) mod b = 0) then Writeln ('ok');
v1 := true;
v2 := 0;
// this gives a "division by zero":
if v1 and (v2 > 0) and ((0+1) mod v2 = 0) then Writeln ('ok');
end.
The "variant" variant results in a "division by zero". Obviously, it tries to evaluate the
modulo-expression even though this shouldn't happen because complete boolean evaluation is disabled and the
"if"-result is already known to be false after checking "v2>0".
Can anyone explain this strange behavior?
When compiling the and expressions, the compiler does not know (in this
example it could, but in general it cannot) what the variants contain so
just the variant and-operator is executed which does not/cannot
distinguish between the logical and bitwise and variant.
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[fpc-pascal] Strange "division by zero" error using variants
Dear all,
please consider the following code:
program Project1;
{$booleval off}
var
v1, v2: variant;
a: boolean;
b: integer;
begin
a := true;
b := 0;
// this works as expected:
if a and (b > 0) and ((0+1) mod b = 0) then Writeln ('ok');
v1 := true;
v2 := 0;
// this gives a "division by zero":
if v1 and (v2 > 0) and ((0+1) mod v2 = 0) then Writeln ('ok');
end.
The "variant" variant results in a "division by zero". Obviously, it tries to
evaluate the modulo-expression even though this shouldn't happen because
complete boolean evaluation is disabled and the "if"-result is already known to
be false after checking "v2>0".
Can anyone explain this strange behavior?
Kind regards,
Thomas
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