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Substitute with fold=true does not preserve rationals #1299

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hersle opened this issue Oct 7, 2024 · 4 comments · May be fixed by JuliaSymbolics/SymbolicUtils.jl#668
Open

Substitute with fold=true does not preserve rationals #1299

hersle opened this issue Oct 7, 2024 · 4 comments · May be fixed by JuliaSymbolics/SymbolicUtils.jl#668

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@hersle
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hersle commented Oct 7, 2024

It is annoying that

@variables x
substitute(1//2 * cos(x), x => 0)

gives 0.5 and not 1//2.

The reason is natural: cos(0) evaluates to 1.0 (not 1), giving (1//2) * 1.0, which is converted to 0.5.
Is there a way to fix this?
@ChrisRackauckas
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@shashi is this intended?

@hersle hersle mentioned this issue Oct 8, 2024
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@shashi
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shashi commented Oct 25, 2024

well it's incidental like the OP said. Maybe fold could call a narrow_type function which turns floats into integers if is_integer is true.

@hersle
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hersle commented Oct 29, 2024

How about if SymbolicUtils.simplify() by default uses the rule

using SymbolicUtils
r = @rule (~x::SymbolicUtils._isinteger) => Int(~x)

Then

r(7.0) # outputs 7

If you think this would work, can you point me to where I should add it for it to work on nested expressions? Like

@syms x
r(7.0*x) # should output 7x

I am not familiar enough with Symbolics to know how it should be added.

@ChrisRackauckas
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That's a nice idea. It would probably go in https://github.com/JuliaSymbolics/SymbolicUtils.jl/blob/master/src/simplify_rules.jl

@hersle hersle linked a pull request Oct 29, 2024 that will close this issue
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Simplify numbers with integer values to integers hersle/SymbolicUtils.jl
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@shashi @ChrisRackauckas @hersle