[fpc-pascal] Order of Precedence: FPC/Delphi vs Java
gtt at wolfgang-ehrhardt.de
gtt at wolfgang-ehrhardt.de
Thu Oct 4 10:01:41 CEST 2018
This is one of the most useless collection of floating point myths I
have seen since a long time.
> I don't know why you want to compare two floats, but you'd better
> use currency type. Float is for calculus, but comparing
> float1=float2 (or float1>float2) is rolling the dice. Obviously, the
> more precision, the less errors. But an accurate, intuitive result,
> is not guaranteed.
With rolling a dice you mean, that the comparisons are only
randomly correct or what)? Since the floating-point numbers
are well-defined and exact (yes they are, and truncation/rounding
errors are the results from former computations and/or
the rounding of non-representable numbers). All operations
are predictable, so there is no chance for random.
> People who play with maths may use something like
> function equal(const f1,f2:Extended; const Error:extended=1E-6):boolean;
> begin
> Result:=(abs(f1-f2)<error);
> end;
This function is non-sense and I doubt that 'math people' will use it.
First: it uses only absolute errors, so 1e-7 and 1e-4000 are equal.
Second: A tolerance of 1e-6 is ridiculous, given that the machine epsilon
for 80-bit extended is 1.0842e-19.
> What does java does? I don't know. Perhaps it just rounds the
> output, try System.out.println(ans==0.0). Perhaps it uses a high
> precision that *in this case* gets always 0.
As already said, you can get the same values as Java, if you use the
same data types (double) and the same precision (53-bit) with
Free Pascal (even with the X87 FPU)
5.1009900000000001E+004
5.1009900000000001E+004
5.1009900000000001E+004
---
0.0000000000000000E+000
0.0000000000000000E+000
0.0000000000000000E+000
0.0000000000000000E+000
0.0000000000000000E+000
> But the question is that floating point representation can't store
> accurately many numbers. To begin with, 0.1 in base 10, is periodic
> number in binary 0.0001...001..001... so it has to truncate it,
No is this only true if you use the truncate rounding mode, which
is not default (default is round-to-nearest/even).
> and when you truncate, the result depends of the order of
> operations, no matter the language or precision. So, it is matter of
> probability to get 1.0000 or 1.0001 or 0.9999
Yes, but it is predictable and deterministic and no matter of
probability.
To conclude: The reason for the differences is the usage of
intermediate elevated precision (and this can occur also
e.g. for C where it is also allowed to use intermediate
higher precision). For Delphi/Free Pascal this phenomenon
does not occur if you always compute with the precision
appropriate to the data type, as e.g. with 64-bit/SSE.
Regards,
Wolfgang
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