local variables in a recursive program

[pereges]

In a recursive program, do you think it is necessary to add the static
keyword for every variable or do you think it is unnecessary ? Is
every call associated with seperate copy of variables ?

[Richard Heathfield]

Re: local variables in a recursive program

pereges said:

In a recursive program,

Presumably you mean a recursive function.

do you think it is necessary to add the static
keyword for every variable

No. In fact, to do so defeats the whole point of recursion.

In general, for any object use the smallest scope and the shortest lifetime
that you can get away with.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Gordon Burditt]

Re: local variables in a recursive program

>In a recursive program, do you think it is necessary to add the static

>keyword for every variable

It is probably necessary to *REMOVE* the static keyword for most
of the variables.

>or do you think it is unnecessary ? Is
>every call associated with seperate copy of variables ?

If you use static, you get *one* copy of the variable for all of the
calls *combined*. This is not what you want for a recursive program,
although static read-only variables are not a problem.

[Malcolm McLean]

Re: local variables in a recursive program


"pereges" <Broli00@gmail. comwrote in message
news:69ccb072-b245-4b6c-abfc-31589d11fbf7@c1 9g2000prf.googl egroups.com...

In a recursive program, do you think it is necessary to add the static
keyword for every variable or do you think it is unnecessary ? Is
every call associated with seperate copy of variables ?
>

It's very wise to make every variable that is not conserved across recursive
calls static. Otherwise you will get a separate instance on each call, which
could waste significant amounts of memory.
However in the nature of things, most variables will need to be local.
Otherwise you probably wouldn't be tackling the problem recursively.

--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

[Keith Thompson]

Re: local variables in a recursive program

gordonb.2o4kx@b urditt.org (Gordon Burditt) writes:

pereges <Broli00@gmail. comwrites:

>>In a recursive program, do you think it is necessary to add the static
>>keyword for every variable

>
It is probably necessary to *REMOVE* the static keyword for most
of the variables.
>

>>or do you think it is unnecessary ? Is
>>every call associated with seperate copy of variables ?

>
If you use static, you get *one* copy of the variable for all of the
calls *combined*. This is not what you want for a recursive program,
although static read-only variables are not a problem.

That may or may not be what you want. It depends on what you're
doing.

If you want a single copy of a variable for all invocations of a
function, use "static". If you want a separately allocated copy for
each invocation, don't use "static". Recursion has very little to do
with the choice.

If you find yourself using "static" a lot, re-think what you're doing;
the vast majority of variables declared within functions should *not*
need to be static.

Note: Gordon deliberately and rudely deleted the attribution line for
"pereges", as he always does. I've re-inserted it. I do not grant
permission to quote this or any other article I post to Usenet without
attribution.

--
Keith Thompson (The_Other_Keit h) <kst-u@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Malcolm McLean]

Re: local variables in a recursive program


"Keith Thompson" <kst-u@mib.orgwrote in message

If you want a single copy of a variable for all invocations of a
function, use "static". If you want a separately allocated copy for
each invocation, don't use "static". Recursion has very little to do
with the choice.
>

Conside this simple function

void zero(NODE *node)
{
int i;

for(i=0;i<node->N;i++)
node->x[i] = 0;
}

it would be very confusing to make i static, though almost harmless - you
gobble an extra few bytes of memory.

Now let's make it recursive

void zeror(NODE *node)
{
static int i;

if(!node)
return;
for(i=0;i<node->N;i++)
node->x[i] = 0;
zeror(node->left);
zeror(node-<right);
}

now there is a point in making i static. Unless the compiler is extremely
good it won't realise that a separate instance of i is not required for each
call. So we can handle a rather deeper tree than if i is automatic.

--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

[Willem]

Re: local variables in a recursive program

Malcolm wrote:
) void zeror(NODE *node)
) {
) static int i;
)
) if(!node)
) return;
) for(i=0;i<node->N;i++)
) node->x[i] = 0;
) zeror(node->left);
) zeror(node-<right);
) }
)
) now there is a point in making i static. Unless the compiler is extremely
) good it won't realise that a separate instance of i is not required for each
) call. So we can handle a rather deeper tree than if i is automatic.

If the compiler is even remotely decent it will put i in a register and
never allocate memory for it in the first place. Making it static might
prevent that from happening.


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

[CBFalconer]

Re: local variables in a recursive program

Malcolm McLean wrote:

>

.... snip ...

>
Conside this simple function
>
void zero(NODE *node) {
int i;
>
for (i = 0; i < node->N; i++) node->x[i] = 0;
}
>
it would be very confusing to make i static, though almost
harmless - you gobble an extra few bytes of memory.

No, it would be highly harmful. Consider:

NODE n1, n2;
...
zero(&n1);
...

/* some interrupt routine */
...
zero(&n2);
...

Would because of the 'static', require forbidding interrupts during
execution of zero(&n1).

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
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