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why does the cube root of -8 = NaN? #635

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yeled opened this issue Dec 6, 2023 · 4 comments
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why does the cube root of -8 = NaN? #635

yeled opened this issue Dec 6, 2023 · 4 comments
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bug Something isn't working done Issues that are done on dev (will be in next release) user Reported by an actual user, yay!

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@yeled
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yeled commented Dec 6, 2023

I was reading https://forum.swissmicros.com/viewtopic.php?f=17&t=2820 and tried the cube root of -8 on a DM32, DM42, C47 and then I tried it on the DB48X (on a DM32)

All of them give either -2 or an imaginary number.

Image

-8
3
1/x

(ignore my mislabled stickers)
@c3d
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c3d commented Dec 11, 2023 •
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The NaN result is a result of not catching numerical errors correctly for binary operations. You should get an Argument outside domain error.

I filed a separate bug on this specific problem (#639)

@c3d c3d added bug Something isn't working user Reported by an actual user, yay! labels Dec 11, 2023
@c3d c3d mentioned this issue Dec 11, 2023
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@c3d
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c3d commented Dec 11, 2023

Please note that there is a specific CBRT function that computes the cube root, and will give you the -2 result. I know that this is not what you were asking, but it's worth pointing out.

@c3d
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c3d commented Dec 11, 2023

I was reading https://forum.swissmicros.com/viewtopic.php?f=17&t=2820 and
tried the cube root of -8 on a DM32, DM42, C47 and then I tried it on the
DB48X (on a DM32)

All of them give either -2 or an imaginary number.

Just curious, have you found a calculator that actually returns -2 for (-8)^(1/3)?

HP41: Data error
HP15C: Error 0
HP48: Complex number (principal solution)
HP50: Complex number.

It seems to me like Thomas Okken's answer in that thread is correct: -8^(1/3) is generally understood in terms of exp and log. With a fraction type, we could probably correctly define the expression a^(b/c), though. Let me think about that.

@yeled
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yeled commented Dec 11, 2023

in my collection:

TI Galaxy 67: -2
C47: -2

@c3d c3d added the done Issues that are done on dev (will be in next release) label Dec 20, 2023
c3d added a commit that referenced this issue Dec 24, 2023
@c3d
arithmetic: Detect non-finite results
e8bc918 
When we have an arithmetic operation that fails, e.g `(-8)^0.3`,
the Intel decimal library reports a `NaN`, and we must convert that
into an error.

Fixes: #639
Fixes: #635

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
c3d added a commit that referenced this issue Dec 25, 2023
@c3d
Release 0.5.2 "Christmas Eve": Reaching hard limits on DM42
1283577 
This release was a bit longer in coming than earlier ones, because we are about
to reach the limits of what can fit on a DM42. This release uses 711228 bytes
out of the 716800 (99.2%).

Without the Intel Decimal Library code, we use only 282980 bytes. This means
that the Intel Decimal Library code uses 60.2% of the total code space. Being
able to move further requires a rather radical rethinking of the project, where
we replace the Intel Decimal Library with size-optimized decimal code.

As a result, release 0.5.2 will be the last one using the Intel Decimal Library,
and is release in parallel with 0.6.0, which switches to a table-free and
variable-precisions implementation of decimal code that uses much less code
space. The two releases should otherwise be functionally identical

**New features**

* Shift and rotate instructions (#622)
* Add `CompatibleTypes` and `DetsailedTypes` setting to control `Type` results
* Recognize HP-compatible negative values for flags, e.g. `-64 SF` (#625)
* Add settings to control multiline result and stack display (#634)

**Bug fixes**

* Truncate to `WordSize` the small results of binary operations (#624)
* Fix day-of-week shortcut in simulator
* Avoid double-evaluation of immediate commands when there is no help
* Generate an error when selecting base 1 (#628)
* Avoid `Number too big` error on based nunbers
* Correctly garbage-collect menu entries (#630)
* Select default settings that allow solver to find solutions (#627)
* Fix display of decimal numbers (broken by multi-line display)
* Fix rendering of menu entries for `Fix`, `Std`, etc
* Detect non-finite results in arithmetic, e.g. `(-8)^0.3`m (#635, #639)
* Fix range-checking for `Dig` to allow `-1` value
* Accept large values for `Fix`, `Sci` and `Eng` (for variable precision)
* Restore missing last entry in built-in units menu (#638)
* Accept `Hz` and non-primary units as input for `ConvertToUnitPrefix` (#640)
* Fix LEB128 encoding for signed value 64 and similar (#642)
* Do not parse `IfThenElse` as a command
* Do not consider `E` as a digit in decimal numbers (#643)
* Do not parse `min` as a function in units, but as minute (#644)

**Improvements**

* Add `OnesComplement` flag for binary operation (not used yet)
* Add `ComplexResults` (-103) flag (not used yet)
* Accept negative values for `B→R` (according to `WordSize`)
* Add documentation for `STO` and `RCL` accessing flash storage
* Mention `True` and `False` in documentation
* Rename `MaxBigNumBits` to `MaxNumberBits`
* Return HP-compatible values from `Type` function
* Minor optimization of flags implementation
* Catalog auto-completion now suggests all possible spellings (#626)
* Add aliases for `CubeRoot` and `Hypothenuse`
* Align based number promotion rules to HP calculators (#629)
* Expand the range of garbage collector integrity check on simulator
* Show command according to preferences in error messages (#633)
* Avoid crash in `debug_printf` if used before font initialization
* Update performance data in documentation
* Add ability to disable any reference to Intel Decimal Floating-point library
* Simplify C++ notations for safe pointers (`+x` and `operartor bool()`)
* Fix link to old `db48x` project in `README.md`

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
c3d added a commit that referenced this issue Dec 25, 2023
@c3d
Release 0.5.2 "Christmas Eve": Reaching hard limits on DM42
6c94f3d 
This release was a bit longer in coming than earlier ones, because we are about
to reach the limits of what can fit on a DM42. This release uses 711228 bytes
out of the 716800 (99.2%).

Without the Intel Decimal Library code, we use only 282980 bytes. This means
that the Intel Decimal Library code uses 60.2% of the total code space. Being
able to move further requires a rather radical rethinking of the project, where
we replace the Intel Decimal Library with size-optimized decimal code.

As a result, release 0.5.2 will be the last one using the Intel Decimal Library,
and is release in parallel with 0.6.0, which switches to a table-free and
variable-precisions implementation of decimal code that uses much less code
space. The two releases should otherwise be functionally identical

**New features**

* Shift and rotate instructions (#622)
* Add `CompatibleTypes` and `DetsailedTypes` setting to control `Type` results
* Recognize HP-compatible negative values for flags, e.g. `-64 SF` (#625)
* Add settings to control multiline result and stack display (#634)

**Bug fixes**

* Truncate to `WordSize` the small results of binary operations (#624)
* Fix day-of-week shortcut in simulator
* Avoid double-evaluation of immediate commands when there is no help
* Generate an error when selecting base 1 (#628)
* Avoid `Number too big` error on based nunbers
* Correctly garbage-collect menu entries (#630)
* Select default settings that allow solver to find solutions (#627)
* Fix display of decimal numbers (broken by multi-line display)
* Fix rendering of menu entries for `Fix`, `Std`, etc
* Detect non-finite results in arithmetic, e.g. `(-8)^0.3`m (#635, #639)
* Fix range-checking for `Dig` to allow `-1` value
* Accept large values for `Fix`, `Sci` and `Eng` (for variable precision)
* Restore missing last entry in built-in units menu (#638)
* Accept `Hz` and non-primary units as input for `ConvertToUnitPrefix` (#640)
* Fix LEB128 encoding for signed value 64 and similar (#642)
* Do not parse `IfThenElse` as a command
* Do not consider `E` as a digit in decimal numbers (#643)
* Do not parse `min` as a function in units, but as minute (#644)

**Improvements**

* Add `OnesComplement` flag for binary operation (not used yet)
* Add `ComplexResults` (-103) flag (not used yet)
* Accept negative values for `B→R` (according to `WordSize`)
* Add documentation for `STO` and `RCL` accessing flash storage
* Mention `True` and `False` in documentation
* Rename `MaxBigNumBits` to `MaxNumberBits`
* Return HP-compatible values from `Type` function
* Minor optimization of flags implementation
* Catalog auto-completion now suggests all possible spellings (#626)
* Add aliases for `CubeRoot` and `Hypothenuse`
* Align based number promotion rules to HP calculators (#629)
* Expand the range of garbage collector integrity check on simulator
* Show command according to preferences in error messages (#633)
* Avoid crash in `debug_printf` if used before font initialization
* Update performance data in documentation
* Add ability to disable any reference to Intel Decimal Floating-point library
* Simplify C++ notations for safe pointers (`+x` and `operartor bool()`)
* Fix link to old `db48x` project in `README.md`

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
@c3d c3d closed this as completed in d6496c9 Dec 25, 2023
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