functions arguments

[Liberum]

Hi,

I want to pass a pointer to a function that will change from float**
ptr to int** ptr during a series of calls and back again.
In the declaration: int f(float** prt);
I have to write float but I want it to be flexible here.
There is the trivial solution of sending in two pointers but I wonder
if there is a better way?

/Sheldon

[Eric Sosman]

Re: functions arguments



Liberum wrote On 10/31/06 11:42,:

Hi,
>
I want to pass a pointer to a function that will change from float**
ptr to int** ptr during a series of calls and back again.
In the declaration: int f(float** prt);
I have to write float but I want it to be flexible here.
There is the trivial solution of sending in two pointers but I wonder
if there is a better way?

What are you trying to do? A pointer will not "change"
from one type to another; every expression in C has a type
that is determined at compile time and never changes. It
is possible to convert a pointer value of one type to a
pointer value of a different type, subject to constraints,
but it's not clear that's what you need -- in particular,
the double indirection seems likely to run afoul of some
of those pesky constraints.

So, again: What are you trying to do? Try to explain
the ultimate objective, not the immediate low-level task.

--
Eric.Sosman@sun .com

[Sheldon]

Re: functions arguments


Eric Sosman skrev:

Liberum wrote On 10/31/06 11:42,:

Hi,

I want to pass a pointer to a function that will change from float**
ptr to int** ptr during a series of calls and back again.
In the declaration: int f(float** prt);
I have to write float but I want it to be flexible here.
There is the trivial solution of sending in two pointers but I wonder
if there is a better way?

>
What are you trying to do? A pointer will not "change"
from one type to another; every expression in C has a type
that is determined at compile time and never changes. It
is possible to convert a pointer value of one type to a
pointer value of a different type, subject to constraints,
but it's not clear that's what you need -- in particular,
the double indirection seems likely to run afoul of some
of those pesky constraints.
>
So, again: What are you trying to do? Try to explain
the ultimate objective, not the immediate low-level task.
>
--
Eric.Sosman@sun .com

I am extracting data of different types, i.e., float, int, and these
are stored in hdf files. The original function had the target array
sent in as a pointer of type float, but not all the data are stored as
floats. Instead of creating another function for int datatypes I would
like to modify the original function to handle this problem.

/Sheldon

[Frederick Gotham]

Re: functions arguments

Liberum:

I want to pass a pointer to a function that will change from float**
ptr to int** ptr during a series of calls and back again.
In the declaration: int f(float** prt);
I have to write float but I want it to be flexible here.
There is the trivial solution of sending in two pointers but I wonder
if there is a better way?

Not quite sure what you're trying to do. Maybe you could do:

typedef enum Type { Int, Float } Type;

void SetToFive(Type const t,void *const p)
{
switch (t)
{
case Int: *(int*)p = 5; return;
case Float: *(float*)p = 5; return;
}
}

Or maybe even:

typedef enum Type { Int, Float } Type;

typedef union PtrToIntOrFloat {
int *pi;
float *pf;
} PtrToIntOrFloat ;

void SetToFive(Type const t,PtrToIntOrFlo at const p)
{
switch (t)
{
case Int: *p.pi = 5; return;
case Float: *p.pf = 5; return;
}
}

--

Frederick Gotham

[Eric Sosman]

Re: functions arguments



Sheldon wrote On 10/31/06 13:08,:

Eric Sosman skrev:
>
>

>>Liberum wrote On 10/31/06 11:42,:
>>

>>>Hi,
>>>
>>>I want to pass a pointer to a function that will change from float**
>>>ptr to int** ptr during a series of calls and back again.
>>>In the declaration: int f(float** prt);
>>>I have to write float but I want it to be flexible here.
>>>There is the trivial solution of sending in two pointers but I wonder
>>>if there is a better way?

>>
> What are you trying to do? A pointer will not "change"
>>from one type to another; every expression in C has a type
>>that is determined at compile time and never changes. It
>>is possible to convert a pointer value of one type to a
>>pointer value of a different type, subject to constraints,
>>but it's not clear that's what you need -- in particular,
>>the double indirection seems likely to run afoul of some
>>of those pesky constraints.
>>
> So, again: What are you trying to do? Try to explain
>>the ultimate objective, not the immediate low-level task.
>>
>>--
>>Eric.Sosman@s un.com

>
>
I am extracting data of different types, i.e., float, int, and these
are stored in hdf files. The original function had the target array
sent in as a pointer of type float, but not all the data are stored as
floats. Instead of creating another function for int datatypes I would
like to modify the original function to handle this problem.

I'm still failing to see the Big Picture. If "hdf files"
refer to one of the Hierarchical Data Formats, why can't you
use their (http://www.hdfgroup.org) free software to do what
you need? (That's just a guess, of course: other candidate
HDF's include Hubble Deep Field, Human Development Foundation,
Housing Development Fund, Howard Douglas & Farnell Insurance,
and the Hereditary Disease Foundation. The shortage of TLA's
means that you're SOL trying to use them to convey sense. ;-)

Anyhow, there seems to be a whole pile of software for
dealing with "hdf files." What are you trying to do that's
beyond its capabilities?

--
Eric.Sosman@sun .com

[Keith Thompson]

Re: functions arguments

"Sheldon" <shejo284@gmail .comwrites:

Eric Sosman skrev:

>Liberum wrote On 10/31/06 11:42,:

[snip]

Please use a consistent name in your posts. You've posted in this
thread as "Liberum" and "Sheldon"; that makes it difficult to follow
the discussion. Thanks.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Sheldon]

Re: functions arguments

Eric Sosman skrev:

Sheldon wrote On 10/31/06 13:08,:

Eric Sosman skrev:

>Liberum wrote On 10/31/06 11:42,:
>
>>Hi,
>>
>>I want to pass a pointer to a function that will change from float**
>>ptr to int** ptr during a series of calls and back again.
>>In the declaration: int f(float** prt);
>>I have to write float but I want it to be flexible here.
>>There is the trivial solution of sending in two pointers but I wonder
>>if there is a better way?
>
What are you trying to do? A pointer will not "change"
>from one type to another; every expression in C has a type
>that is determined at compile time and never changes. It
>is possible to convert a pointer value of one type to a
>pointer value of a different type, subject to constraints,
>but it's not clear that's what you need -- in particular,
>the double indirection seems likely to run afoul of some
>of those pesky constraints.
>
So, again: What are you trying to do? Try to explain
>the ultimate objective, not the immediate low-level task.
>
>--
>Eric.Sosman@su n.com

I am extracting data of different types, i.e., float, int, and these
are stored in hdf files. The original function had the target array
sent in as a pointer of type float, but not all the data are stored as
floats. Instead of creating another function for int datatypes I would
like to modify the original function to handle this problem.

>
I'm still failing to see the Big Picture. If "hdf files"
refer to one of the Hierarchical Data Formats, why can't you
use their (http://www.hdfgroup.org) free software to do what
you need? (That's just a guess, of course: other candidate
HDF's include Hubble Deep Field, Human Development Foundation,
Housing Development Fund, Howard Douglas & Farnell Insurance,
and the Hereditary Disease Foundation. The shortage of TLA's
means that you're SOL trying to use them to convey sense. ;-)
>
Anyhow, there seems to be a whole pile of software for
dealing with "hdf files." What are you trying to do that's
beyond its capabilities?
>
--
Eric.Sosman@sun .com

Thanks!