Global variable-sized arrays

[Chris Johnson]

I've got a program that has two functions, and both need to access an
array. However, the size of the array is not even defined until I'm
partially through the main() function. Here's a quick demonstration of
what I need:

#include <stdio.h>
#include <stdlib.h>

void fubar(int,int);

// This is the array I want to make available to both functions
int equation[count];

int main()
{
int y, count = 0;
while(scanf("%d ",&y) != EOF)
count++;

fubar(y,count);

printf("%d\n",e quation[0]);
}

void fubar(int y, int count)
{
int i;
for(i = 0; i < count; i++)
equation[i] = y;
}

Effectively what I need is to be able to declare global variables within a
function. Is that possible? Or else is there a workaround?

[Jason Whitehurst]

Re: Global variable-sized arrays

Chris Johnson wrote:[color=blue]
> Effectively what I need is to be able to declare global variables
> within a function. Is that possible? Or else is there a workaround?[/color]

int *equation = NULL;

int main(void) {
..
..
..
equation = malloc(sizeof(i nt) * count);
..
..
..
}

--
Jason Whitehurst

[Jack Klein]

Re: Global variable-sized arrays

On Mon, 22 Nov 2004 22:09:55 -0500, "Jason Whitehurst"
<jasonmac@cc.ga tech.edu> wrote in comp.lang.c:
[color=blue]
> Chris Johnson wrote:[color=green]
> > Effectively what I need is to be able to declare global variables
> > within a function. Is that possible? Or else is there a workaround?[/color]
>[/color]

#include <stdlib.h>
[color=blue]
> int *equation = NULL;[/color]

Unnecessary for a pointer with static storage duration, which anything
defined at file scope has. Will be initialized to NULL by default.
[color=blue]
> int main(void) {
> .
> .
> .
> equation = malloc(sizeof(i nt) * count);[/color]

The recommend form by most CLC regulars is:

equation = malloc(sizeof *equation * count);

Saves editing and potential mistakes if it later turns out that the
type of the array needs to be changed to long, or float, or anything
other than int.

--
Jack Klein
Home: http://JK-Technology.Com
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http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

[Al Bowers]

Re: Global variable-sized arrays



Chris Johnson wrote:[color=blue]
> I've got a program that has two functions, and both need to access an
> array. However, the size of the array is not even defined until I'm
> partially through the main() function. Here's a quick demonstration of
> what I need:
>
> #include <stdio.h>
> #include <stdlib.h>
>
> void fubar(int,int);
>
> // This is the array I want to make available to both functions
> int equation[count];
>
> int main()
> {
> int y, count = 0;
> while(scanf("%d ",&y) != EOF)
> count++;
>
> fubar(y,count);
>
> printf("%d\n",e quation[0]);
> }
>
> void fubar(int y, int count)
> {
> int i;
> for(i = 0; i < count; i++)
> equation[i] = y;
> }
>
> Effectively what I need is to be able to declare global variables within a
> function. Is that possible? Or else is there a workaround?[/color]

First of all you do not need a global varialbe to do this. You can
use pointer arguments to modify variables.

Since you are modifying the array length, you can use dynamic
allocations to allocate the size as you need it.
For example, you could prototype the function fubar to:

int fubar(int **y, size_t *count, int value);

where y is a pointer to a pointer to the dynamic allocated array.
count is a pointer to a count of elements
and value is the value you wish to insert in the array.
The Function returns an int that will indicate success or failure
of the allocation.

Here is a working example using function realloc to do the allocations.

#include <stdio.h>
#include <stdlib.h>

int AddArray(int **array,size_t *count,int value);

int main(void)
{
int *equation = NULL, value;
size_t i, count = 0;
puts("Enter integer: (Enter 'q' to quit)");
while(1 == scanf("%d",&val ue))
if(!AddArray(&e quation, &count,value )) break;

for(i = 0; i < count;i++)
printf("equatio n[%u] = %d\n",i,equatio n[i]);
free(equation);
return 0;
}

int AddArray(int **array, size_t *count, int value)
{
int *tmp = realloc(*array, ((*count)+1)*(s izeof *tmp));
if(tmp)
{
*array = tmp;
(*array)[(*count)++] = value;
return 1;
}
return 0;
}


--
Al Bowers
Tampa, Fl USA
mailto: xabowers@myrapi dsys.com (remove the x to send email)

http://www.geocities.com/abowers822/

[Jason Whitehurst]

Re: Global variable-sized arrays

Jack Klein wrote:[color=blue]
> Unnecessary for a pointer with static storage duration, which anything
> defined at file scope has. Will be initialized to NULL by default.[/color]

Of course it will. It seems he is learning though - I figure best to show
him to just init to NULL. =)

[color=blue]
> equation = malloc(sizeof *equation * count);[/color]

either way.

--
Jason Whitehurst

[Mark A. Odell]

Re: Global variable-sized arrays

"Jason Whitehurst" <jasonmac@cc.ga tech.edu> wrote in
news:cnuggl$b6q $1@news-int2.gatech.edu :
[color=blue][color=green]
>> Unnecessary for a pointer with static storage duration, which anything
>> defined at file scope has. Will be initialized to NULL by default.[/color]
>
> Of course it will. It seems he is learning though - I figure best to
> show him to just init to NULL. =)
>
>[color=green]
>> equation = malloc(sizeof *equation * count);[/color]
>
> either way.[/color]

Either the maintenance nightmare way or the self-documenting,
automatically type correcting way?

--
- Mark ->
--