The below writes: -9223372036854775808
Shouldn't absolute return a positive number?
program Project1;
begin
writeln( abs(low(int64)) );
end.
Seems
writeln( abs(low(longint)) );also returns negative...
Mailing lists / fpc-devel / 2024-04
wrong result for abs(low(int64))
5 messages · 4 people ·
- Martin Frb 2
- Bart 1
- J. Gareth Moreton 1
- Kai Burghardt 1
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Ahoy, hoy,
Martin Frb wroteThe below writes: -9223372036854775808 Shouldn't absolute return a positive number?
No, see https://wiki.freepascal.org/Integer#characteristics
Sincerely yours,
Kai Burghardt
The below writes: -9223372036854775808 Shouldn't absolute return a positive number?…
Essentially, an arithmetic overflow is happening. Since the largest Int64 possible is 9,223,372,036,853,775,807, going one above that (the result to abs(low(int64))) wraps back around to -9,223,372,036,853,775,808.
Internally, you can think about negating (positing?) a negative number as inverting the bits and then adding one (aka. two's complement), so low(int64) is 1000...000, which inverted becomes 0111...111, and then adding one results in 1000...000 again.
Kit
Martin Frb wrote 20 lines
The below writes: -9223372036854775808
Shouldn't absolute return a positive number?
program Project1;
begin
writeln( abs(low(int64)) );
end.
Seems
writeln( abs(low(longint)) );
also returns negative...
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J. Gareth Moreton wrote 9 lines
Essentially, an arithmetic overflow is happening. Since the largest
Int64 possible is 9,223,372,036,853,775,807, going one above that (the
result to abs(low(int64))) wraps back around to
-9,223,372,036,853,775,808.
Internally, you can think about negating (positing?) a negative number
as inverting the bits and then adding one (aka. two's complement), so
low(int64) is 1000...000, which inverted becomes 0111...111, and then
adding one results in 1000...000 again.
I got that bit...
Initially I was surprised to find "abs" to return a signed number. After all, it will never be negative.
But, in the meantime I recalled that unsigned numbers will cause other issues when used in expressions.
"abs(-1)-2" would behave different.
The below writes: -9223372036854775808 Shouldn't absolute return a positive number?…
Show quoted text 8 lines
The below writes: -9223372036854775808
Shouldn't absolute return a positive number?
program Project1;
begin
writeln( abs(low(int64)) );
end.
See https://gitlab.com/freepascal.org/fpc/source/-/issues/40694 There were several commits about Abs w.r.t. this bug lately.
Bart