[fpc-pascal] Floating point question

[wkitty42 at windstream.net]

On 2/16/24 9:57 AM, James Richters via fpc-pascal wrote:
>> So you are saying when constant propagation is on, an expression should have a different result than with constant propagation off?
> 
> The result of math when using constants MUST be the same as the result of identical math using variables.
> 
> There should never be a difference if I did my formula with hard coded constants vs variables.
> 
>    Const_Ans = 2.00000010627116630224
>   Const_Ans1 = 2.00000010627116630224
>     Var_Ans1 = 2.00000000000000000000
> 
> This should not be happening.

i've been quietly reading this entire thread and wow... i mean i do fully 
understand the situation and both sides of the problem... looking at the 
numbers, alone, the difference hits me hard in my OCD and i do agree with the 
assessment that max precision should be used for the const math and the result 
reduced when possible... but then, on the other hand, it really really really 
depends, _a lot_, on exactly /what/ is being calculated and what the numbers 
represent...

for example, here on Earth, (7 decimal places) 0.0000001 degree latitude is 
""only"" 1cm... (8 decimal places) 0.00000001 degree latitude is ""only"" 1mm... 
longitude, on the other hand, is variable such that 7 decimal places at the 
equator is the same as latitude but as you move toward the poles, it changes 
such that 4 decimal places is 20cm...

for me, this can be important when it comes to placing scenery objects in a 
"true to life" simulator of the Earth, its terrain, and building/object 
placement... a building or road being 1cm out of position isn't noticeable... 
even 20cm out of place isn't really noticeable...

if this math is being used for measuring parsecs, yeah, i don't think i want to 
be on that space ship when it arrives at the wrong position in space... if this 
math is being used to measure angstroms, yeah... it kinda of matters... if this 
math is being used for cutting wood as a carpenter does, it doesn't matter so 
much... so please, put some perspective on this problem...

i'll have to look at my code for calculating a satellite's position in space in 
relation to Earth's center... especially depending on the satellite's altitude 
above the Earth as the higher the satellite is, the further off position the 
calculation could be... depending on what the calculation is being used for, it 
could be the difference between a good pass of two satellites crossing paths or 
it could be quite the ugly loss of two satellites plus a huge spew of debris 
from their impact...


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