Inverting Axis

[2386programming]

Hello, I am trying to find a way for when our driver starts reversing with our mecanum drive robot, the rotation axis gets inverted.

[kinganu123]

Show us what you have first so we can suggest different approaches on how you can accomplish it.

[Oromus]

I'd suggest multiplying the axis you want to invert by -1. Example code:

if (reversing) { //Put whatever code you use to detect if you're reversing here
     myMotor.set(joystick.getYAxis() * -1); //Set a motor to the inverse of the Y axis.
}

[notmattlythgoe]

Originally Posted by Oromus

I'd suggest multiplying the axis you want to invert by -1. Example code:

if (reversing) { //Put whatever code you use to detect if you're reversing here
     myMotor.set(joystick.getYAxis() * -1); //Set a motor to the inverse of the Y axis.
}

You can also just place a - in front of it.

if (reversing) { //Put whatever code you use to detect if you're reversing here
     myMotor.set(-joystick.getYAxis()); //Set a motor to the inverse of the Y axis.
}

[Ether]

Originally Posted by notmattlythgoe

You can also just place a - in front of it.

if (reversing) { //Put whatever code you use to detect if you're reversing here
     myMotor.set(-joystick.getYAxis()); //Set a motor to the inverse of the Y axis.
}

Is this just a matter of aesthetics, or does it actually generate more efficient code?

[GeeTwo]

Originally Posted by notmattlythgoe

You can also just place a - in front of it.

if (reversing) { //Put whatever code you use to detect if you're reversing here
     myMotor.set(-joystick.getYAxis()); //Set a motor to the inverse of the Y axis.
}

Originally Posted by Ether

Is this just a matter of aesthetics, or does it actually generate more efficient code?

I'm not sure about Java, but I do know that with traditional compilers, it is more efficient - changing the sign on a number is a much simpler operation, usually done in a single clock cycle. Unless the compiler is doing serious optimization, multiplying by -1 is a full floating-point multiply.

[Ether]

Originally Posted by GeeTwo

Unless the compiler is doing serious optimization, multiplying by -1 is a full floating-point multiply.

Why would you consider that to be a serious -- rather than a simple and obvious -- optimization?

[notmattlythgoe]

Originally Posted by Ether

Is this just a matter of aesthetics, or does it actually generate more efficient code?

That is a question I do not know the answer to. I'm assuming that it is just aesthetic. You are technically using the unary minus operator in both situations because the - in front of the 1 is technically inverting the value of 1.

It is however better practice to place the - in front of the variable instead of multiplying by -1.

[Ether]

Originally Posted by notmattlythgoe

It is however better practice to place the - in front of the variable instead of multiplying by -1.

I share your opinion on this matter¹, but to move out of the realm of individual opinion: Is there a widely accepted code formatting standard that states this explicitly?

¹for aesthetic reasons and to discourage the compiler from generating a floating-point multiply

[notmattlythgoe]

Originally Posted by Ether

I share your opinion on this matter¹, but to move out of the realm of individual opinion: Is there a widely accepted code formatting standard that states this explicitly?

¹for aesthetic reasons and to discourage the compiler from generating a floating-point multiply

It is known.

I do not know of any source that explicitly states it, and I haven't found one in a quick Google search. It's just the way I learned and it is the way I do it at work.

[GeeTwo]

Originally Posted by Ether

Why would you consider that to be a serious -- rather than a simple and obvious -- optimization?

It's simple and obvious to a human, of course. If things were obvious to computers, programming them would be much easier.

OBTW, you pass the Turing test.

[Ether]

Originally Posted by GeeTwo

It's simple and obvious to a human, of course. If things were obvious to computers...

You are claiming that particular optimization isn't obvious to a compiler. In fact, you implied it would be difficult (serious optimization). Please explain your reasoning for that.

[GeeTwo]

I did a bit of Googling, and it does seem that several optimizers will generate the same code for both cases. Optimizers appear to have come a ways since I last paid close attention.

It still makes sense to me to ask for -x (or 0-x) rather than -1 * x. Provided that the code is equally (or arguably better) readable by people, why not give the computer the simplest directions?

[Ether]

Originally Posted by GeeTwo

It still makes sense to me to ask for -x...rather than -1 * x

Nobody is arguing otherwise.

..why not give the computer the simplest directions?

Because "simple" is subjective and a matter of personal preference.

That's why I asked if this (very reasonable, in my opinion) use of the unary operator in this case might be part of a widely-accepted Java coding standard that could be referenced in support of the recommendation.

[GeeTwo]

Originally Posted by GeeTwo

Provided that the code is equally (or arguably better) readable by people, why not give the computer the simplest directions?

For an example where the simpler code is not easier to read:

Logically, you want to do a conditional:

if (c is between a and b, and you don't know whether a is larger or smaller than b) then...

Human simpler (use this!):

if ((a<b) && (b<c) || (a>b) && (b>c)) then..

Probably machine quicker, but don't use:

if ((a-b)*(b-c) > 0) then..