2 stupid array questions!!

[prashna]

Hi Guru's,
Here are my questions...

1)Why does c allows an extra "," in array intialiser?Is there any
advantage of this?

ex: int arr[5]={1,2,3,4,5,};
^^Compiler does not give error for this!

2)How to determine the size of the array which is passes as a
parameter to the function?
ex :
void foo(int arr[])
{
/*I want determine the size of the array "arr" in this function.I
tried *using "sizeof" operator and realised it is not going to work
as "arr" will be *treated as pointer to an int!!!!!!*/
}

Thanks in advance

[Ivan Vecerina]

"prashna" <va******@redif fmail.com> wrote in message
news:d4******** *************** ***@posting.goo gle.com...
| 1)Why does c allows an extra "," in array intialiser?Is there any
| advantage of this?
|
| ex: int arr[5]={1,2,3,4,5,};
| ^^Compiler does not give error for this!
It makes things easier for code generators.
No real benefit besides that...

| 2)How to determine the size of the array which is passes as a
| parameter to the function?
| ex :
| void foo(int arr[])
| {
| /*I want determine the size of the array "arr" in this function.I
| tried *using "sizeof" operator and realised it is not going to work
| as "arr" will be *treated as pointer to an int!!!!!!*/
| }
The size of the array needs to be passed as a second parameter.
No other way in C (C++ allows what you need, through templates...).

hth,
Ivan
--
http://ivan.vecerina.com

[pete]

prashna wrote:

Hi Guru's,
Here are my questions...

1)Why does c allows an extra "," in array intialiser?Is there any
advantage of this?

ex: int arr[5]={1,2,3,4,5,};
^^Compiler does not give error for this!

2)How to determine the size of the array which is passes as a
parameter to the function?
ex :
void foo(int arr[])
{
/*I want determine the size of the array "arr" in this function.I
tried *using "sizeof" operator and realised it is not going to work
as "arr" will be *treated as pointer to an int!!!!!!*/
}

If you want foo to know the size,
then you have to give foo more information.

void foo(int arr[], size_t n_elem) /* number of elements */

void foo(int arr[], size_t bytes) /* size in bytes */

--
pete

[Andreas Kahari]

In article <d4************ **************@ posting.google. com>, prashna wrote:

Hi Guru's,
Here are my questions...

1)Why does c allows an extra "," in array intialiser?Is there any
advantage of this?

ex: int arr[5]={1,2,3,4,5,};
^^Compiler does not give error for this!
It simplifies code that generate C code.

Oh, and the 5 in [5] is not needed in your example.

2)How to determine the size of the array which is passes as a
parameter to the function?

[cut]
Change the interface of the function so that the caller also
sends the length of the array (i.e. add an extra int argument).
--
Andreas Kähäri

[Zygmunt Krynicki]

On Fri, 10 Oct 2003 11:48:00 +0200, Ivan Vecerina wrote:

| 2)How to determine the size of the array which is passes as a
| parameter to the function?
| ex :
| void foo(int arr[])
| {
| /*I want determine the size of the array "arr" in this function.I
| tried *using "sizeof" operator and realised it is not going to work
| as "arr" will be *treated as pointer to an int!!!!!!*/
| }
The size of the array needs to be passed as a second parameter.
No other way in C (C++ allows what you need, through templates...).

Offtopic since C++
I must disagree. Usage of a template in this case would be a total
overkill. I believe vector and other collections address this problem
better.

Regards
Zygmunt Krynicki

[Ivan Vecerina]

"Zygmunt Krynicki" <zyga@_CUT_2zyg a.MEdyndns._OUT _org> wrote in message
news:pan.2003.1 0.10.19.03.41.2 71708@_CUT_2zyg a.MEdyndns._OUT _org...

On Fri, 10 Oct 2003 11:48:00 +0200, Ivan Vecerina wrote:

| 2)How to determine the size of the array which is passes as a
| parameter to the function?
| ex :
| void foo(int arr[])
| {
| /*I want determine the size of the array "arr" in this function.I
| tried *using "sizeof" operator and realised it is not going to work
| as "arr" will be *treated as pointer to an int!!!!!!*/
| }
The size of the array needs to be passed as a second parameter.
No other way in C (C++ allows what you need, through templates...).

Offtopic since C++
I must disagree. Usage of a template in this case would be a total
overkill. I believe vector and other collections address this problem
better.

What exactly is overkill in the following examples ?

// returns the size of the C array passed as a param
// This is safer than the sizeof(a)/sizeof(a[0]) trick...
template <typename T, int N> inline
int arraySize(T (&)[N]) { return N; }

// fills an array with a specified value
template<typena me T, int N> inline
void fill( T (&array)[N], T const& value )
{ for(int i=0;i<N;++i) array[i]=value; }

If code duplication is of concern (in the second case), you could
choose to call a back-end function that takes a pointer and
an array size as parameters -- and benefit from the template still.
Regards,
Ivan
--
http://ivan.vecerina.com

[Default User]

Zygmunt Krynicki wrote:

Offtopic since C++
I must disagree. Usage of a template in this case would be a total
overkill. I believe vector and other collections address this problem
better.

Of course, that just hides the templates from the user. They're still
there.


Brian Rodenborn

[Zygmunt Krynicki]

On Fri, 10 Oct 2003 21:56:55 +0200, Ivan Vecerina wrote:

What exactly is overkill in the following examples ?

// returns the size of the C array passed as a param
// This is safer than the sizeof(a)/sizeof(a[0]) trick...
template <typename T, int N> inline
int arraySize(T (&)[N]) { return N; }
This can only be used in the scope of the declaration of the array you're
going to pass along.
// fills an array with a specified value
template<typena me T, int N> inline
void fill( T (&array)[N], T const& value )
{ for(int i=0;i<N;++i) array[i]=value; }
Don't reinvent the wheel: std::fill
If code duplication is of concern (in the second case), you could
choose to call a back-end function that takes a pointer and
an array size as parameters -- and benefit from the template still.

What is the benefit?
That I can type foo(a) instead of foo(a, n)?
I don't see that as a benefit but a inconveniance. It's illogical to
assume all routines need to operate on the whole set of data.
This is why most of STL routines use begin/end iterators as input.

Your code, interesting in its design, is not the kind of a solution
I would advice to anyone interested in C++, usage of plain C arrays
is a bad thing (to quote the appropriate FAQ) as it undermines your
code with all C-memory-handling related issues.

Regards
Zygmunt Krynicki

[Zygmunt Krynicki]

On Fri, 10 Oct 2003 19:51:08 +0000, Default User wrote:

Zygmunt Krynicki wrote:

Offtopic since C++
I must disagree. Usage of a template in this case would be a total
overkill. I believe vector and other collections address this problem
better.

Of course, that just hides the templates from the user. They're still
there.

I was not trying to say that templates are not present or that they are
bad in general. I was just objecting to using them in accompaniance with C
arrays as a method of obtaining the declared array size.

Regards

Zygmunt

[Default User]

Zygmunt Krynicki wrote:

Don't reinvent the wheel: std::fill
Did you forget which newsgroup you are on?

Your code, interesting in its design, is not the kind of a solution
I would advice to anyone interested in C++, usage of plain C arrays
is a bad thing (to quote the appropriate FAQ) as it undermines your
code with all C-memory-handling related issues.

Seems like it. This is comp.lang.c.

Brian Rodenborn