Problem with precision of number

micro:bit
1

I hope there is a micro:bit expert out there who can help me with this…

I can’t get the “Show data” window (or the LED display) to show the correct number.

10^-4.5 = 0.00003162.

When I do a test and just write the calculation it works. But when -4.5 is stored in the variable “power” it doesn’t work. Then I just get the result 0.0001?!??? Going crazy here, anyone have an explanation and maybe even a solution?

There are pictures in the comment… (it is in Danish but I it is possible to understand)

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MicroBit 2

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Soon after my other reply I realized it may be easier to use if the power() were a function that can be used from blocks. Here’s a version that encapsulates the power() for floating point numbers as a function that’s callable from blocks: https://makecode.microbit.org/_URjXjd9YbKht

The function part can be copy/pasted from one project to another by copying the test in the JavaScript view or you could simply edit the code given in the links.
Bill

1 Like
3

I think this is what’s going on:

  • The. 10 ** -4.5 is all literals. The compiler is probably doing the computation simply replacing the expression with the final value at compile time. So this computation isn’t happening on the micro:bit.
  • The ** operator is probably really just Math.pow(). It looks like the simulator may use a full fledged Math.pow() (probably the JavaScript function), but the micro:bit version appears to only work with integers. So .5 part of 4.5 is lost. The result is 10 ** -4 = .0001.

TypeScript has access to Math.log() and Math.exp(), which work with floating point numbers. You can use a change of base to do the computation, like Received_Power = Math.exp(Power * Math.log(10)). (log(10) is constant, so it could also be expressed as Received_Power = Math.exp(Power * 2.30258509299))

Here’s a shared project that uses this approach. You’ll want to switch the the JavaScript view to edit/copy/paste the formula from this to your work: https://makecode.microbit.org/_Re7Pfme2mVHx . I’ve tested this on a micro:bit v2.00 and do get 0.000031622776601684.

I hope that helps,
Bill

1 Like
4

Thank you very much!!! This seems to solve it :slight_smile:

5

Out of curiosity, if I want a student to reproduce the function, can the “Math.exp(exponent *Math.log(base))” expression be made in “Blocks” tab, or do they need to go to the JavaScript tab?

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6

Hi @bibbib,

I don’t think you can use exp() or log() with the default blocks and I don’t see an extension that would help either.

Here are two options that you might consider if you want to simplify this for students:

Option 1: Just have them open this project and customize it via the Edit button: https://makecode.microbit.org/_4R13oKaYM0AK

Option 2: (Using the JavaScript tab)

  1. Go to the JavaScript tab.
  2. Paste in:
    function power (base: number, exponent: number) {
    return Math.exp(exponent * Math.log(base))
}
  3. Return to blocks.
  4. Expand the “Advanced” section at the bottom of the blocks toolboxes.
  5. Select the Functions toolbox.
  6. Drag out the oval call power block.

I hope that helps,

Bill

P.S. I’d love to hear about your assignment or how you’re using this in class. (And what ages / subjects / grades). There are some educators on the micro:bit Developer Slack forum that may be interested in your work (and sharing their work).