How to chane sign of var from + to -

AGCB

Probably a dumb question.
I want to enter a method w/ a negative value. The calling method does not support negative numbers .

IE I need to get -5 from +5
Simply negating subtracts from zero(256) and leaves 251.

I've tried every way I can think of. I'm using SPIN

Thanks
Aaron

Martin_H

Isn't negation a urinary operator? So you would use syntax like the following:

var := -var

Dave Hein

It sounds like you're trying to negate a byte variable, which has a range from 0 to 255. You need to use a long variable to support negative numbers.

AGCB

I still get 251

xnew:=5
xnew1:=-xnew

AGCB

I didn't read the last part of your post about the LONG var.

That did it!
Thanks much

Duane Degn

Martin_H wrote: »

Isn't negation a urinary operator? So you would use syntax like the following:

var := -var

In Spin you just need;

-var

Zetsu

Couldn't you also just multiply the value by (-1) ? and to go backwards do the same again ? In Lou of the (cleaner looking) negation operator obviously.

Duane Degn

Zetsu wrote: »

Couldn't you also just multiply the value by (-1) ? and to go backwards do the same again ? In Lou of the (cleaner looking) negation operator obviously.

Yes, but negatives only work with longs.

This one of the reasons from the sign extend operator "~" (when used on left side of a variable) in Spin. It can convert a 16-bit negative number (like those read from a sensor) into a full 32-bit negative which will behave properly when doing math in Spin.

In generally, I try to use longs if I'm going to being do much math with the variable.

localroger

When you store a negative value in a byte, it gets truncated. This leaves it a positive byte value when you treat it as a long (which all math operations do).

This is one of those very useful but weird and nonstandard things about Spin. Remember the hated ~ operator that clears or sets vars when used postfix? Used prefix, it extends the sign of a short variable to 32 bits for this sort of thing.

~bytevar = sign-extended byte variable value
~~wordvar = sign-extended word variable value

These take the high bit of the byte or word and extend that bit through the rest of the 32-bit value to make something that will work with signed 32-bit arithmetic.