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automatic typecasting is broken

 
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equack



Joined: 21 Sep 2009
Posts: 21

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automatic typecasting is broken
PostPosted: Tue Sep 07, 2010 2:15 pm     Reply with quote
Automatic typecasting is broken in version 4.112 (PIC18)

The following code gives the wrong value:
Code:

long scandex;
int thiskey;

scandex=thiskey*3;



To get the correct result you must do this:
Code:

long scandex;
int thiskey;

scandex=(long)thiskey*3;


If you're pulling your hair out maybe this is why.
SherpaDoug



Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA

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PostPosted: Tue Sep 07, 2010 2:25 pm     Reply with quote
Since thiskey is an int and 3 is an int I would expect the output value to wrap at 255 for thiskey values above 85. Is this what it does? What are you expecting? Can you give us some sample inputs and outputs?
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bkamen



Joined: 07 Jan 2004
Posts: 1615
Location: Central Illinois, USA

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Re: automatic typecasting is broken
PostPosted: Tue Sep 07, 2010 2:52 pm     Reply with quote
equack wrote:
Automatic typecasting is broken in version 4.112 (PIC18)

The following code gives the wrong value:
Code:

long scandex;
int thiskey;

scandex=thiskey*3;



To get the correct result you must do this:
Code:

long scandex;
int thiskey;

scandex=(long)thiskey*3;


If you're pulling your hair out maybe this is why.


I would expect (knowing what I know about C and microcontrollers) that if you take an INT8 and *3, that values over the size of the INT would roll over before they can be assigned to a larger INT.

One way to fix that is exactly what you've done.
The other is breaking down the equation to component steps.

scandex = thiskey;
scandex *= 3;

sometimes with the long stuff, it's easier to read... but anyway.

-Ben
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