Array declaration doubt
Hi everybody,
I am new to this forum, so thank you for you replies
My doubt is about arrays, it is wrong to declare array's size using variables? if not (as I expect), why exactly?
a simple example is
where "a" is an integer variable.Code:int arr[a];
thank you
Not necessarily.Originally Posted by finalsayan
You would be making use of the variable length array language feature. However, this feature is relatively new, so it may not be available.
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can you make an example?
I guess that when the value of the varialble "a" is not know at compile time, then we could have problems, it is right? for example when the value of "a" is taken as a user input.Code:int arr[a];
I prefer to stay with classic C, without using these new features
thank you
While I can use a variable to size and array I get warnings which I make a habit of eliminating !
#defines ...is the only clean way I've seen in C.I don't know what the reasoning is but I would imagine it's because they want to keep a static object far away from dynamic objects.Code:/* I compile with full warnings : gcc -Wall -W -pedantic arr1.c -o arr1 */ #include <stdio.h> #define SZ1 256 static const int z = 256; int main(void) { int x = 256; static const int y = 256; char buff1[x]; /* warning: ISO C90 forbids variable-size array ‘buff’ */ char buff2[y]; /* warning: ISO C90 forbids variable-size array ‘buff’ */ char buff3[z]; /* warning: ISO C90 forbids variable-size array ‘buff’ */ char buff4[SZ1]; /* no warns */ return 0; }
Last edited by HowardL; 03-29-2010 at 09:13 AM.
Howard, try compiling with
and see what happens. (I.e., using the 1999 version instead of the 1990 version of the language.)Code:gcc -Wall -W -pedantic -std=c99 arr1.c -o arr1
-std=c99
I see , no warnings now, thanks.
Is that a good thing???
I wonder why the greybeards thought it was not so good?
Something in me trusts old school more than new school.
Of course it sure could be handy when jumping to a new block.
But I could see where it could be a problem if you don't keep your size variable straight while the array is still in scope and in use.
That could lead to REAL trouble!
Last edited by HowardL; 03-29-2010 at 09:45 AM.
Yes, if your intention is to use variable length arrays.
I cannot speak for any "greybeards", but I avoid VLAs in practice because I want my code to be more portable, and often to be compilable as C++.
Yeah, but this is the same when working with malloc() and company.
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It's not old school, vs new school, it's that it was a widely supported compiler extension that was added into C99. As long as the application will always be compiled in c99, this is not a problem. If compiled on an old compiler that does not have the extension, then there's a problem. If compiled in c90 mode then it issues a warning, because compiler extensions are not generally portable.
It is too clear and so it is hard to see.
A dunce once searched for fire with a lighted lantern.
Had he known what fire was,
He could have cooked his rice much sooner.
But then you KNOW your walking on eggshellsYeah, but this is the same when working with malloc() and company.
I don't know, I've gotten by pretty well without using them so far.
