[9fans] fun question

[9fans] fun question

[ron minnich]

I ask this every time I hit this piece of code, I think. But what the heck:

  if(seed <- 0)
    seed = seed + m_31;

what's wrong with this code?

ron

Re: [9fans] fun question

[erik quanstrom]

On Thu Jan 22 15:55:09 EST 2009, rminnich@gmail.com wrote:
> I ask this every time I hit this piece of code, I think. But what the heck:
>
>   if(seed <- 0)
>     seed = seed + m_31;
>
> what's wrong with this code?

supposing the code does what it is supposed
to on some machines, i would guess that this
could be a problem for sign-magnitude or one's-comp.
machines, since 0 and -0 are different.  if that's
the case, one would guess that the object is to clear
sign bit and -0 is the failure case.

then again, that's a very wild guess and one
would assume that sign = -sign would be
better.

-0 is a strange construction and the spacing
is misleading. it's interesting that atoi(2) and
the differ on "- 0".

- erik

Re: [9fans] fun question

[Russ Cox]

On Thu, Jan 22, 2009 at 12:53 PM, ron minnich <rminnich@gmail.com> wrote:
> I ask this every time I hit this piece of code, I think. But what the heck:
>
>  if(seed <- 0)
>    seed = seed + m_31;
>
> what's wrong with this code?

<- is a unary operator.

russ

Re: [9fans] fun question

[Pietro Gagliardi]

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On Jan 22, 2009, at 5:01 PM, Russ Cox wrote:
> <- is a unary operator.


okay, what does it do? (unless you meant -> in C++)


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Re: [9fans] fun question

[Russ Cox]

>> <- is a unary operator.

> okay, what does it do? (unless you meant -> in C++)

it receives from a channel.

russ

Re: [9fans] fun question

[erik quanstrom]

On Thu Jan 22 17:55:55 EST 2009, rsc@swtch.com wrote:
> >> <- is a unary operator.
>
> > okay, what does it do? (unless you meant -> in C++)
>
> it receives from a channel.

i assumed that ron was talking about c.
in c, "<- 0" tokenizes as "<", "-", and "0".
"-" is taken to be a unary operator on "0".

even gcc does this correctly:

; cat > x.c
#include <u.h>
#include <libc.h>

void
x(int seed)
{
	int m_31;

	m_31 = 1<<31;
	if(seed <- 0)
		seed = seed + m_31;
	print("seed %d\n", seed);
}

void
main(void)
{
	x(-5);
	exits("");
}
<eot>; 9c x.c
; 9l x.o
; ./a.out
seed -2147483648

- erik

Re: [9fans] fun question

[ron minnich]

Erik got it. That code originally was written for a 1s complement
machine, the CDC 7200 I believe. I get a kick out of it every time I
see something and remember machines with + and - 0 :-)

ron

Re: [9fans] fun question

[Pietro Gagliardi]

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On Jan 22, 2009, at 6:01 PM, erik quanstrom wrote:

> i assumed that ron was talking about c.

Yeah, that's what I thought. Sorry, Russ.

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