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Dumb question. but can you do this. (these are global variables by the way)

int size;
double Values[size];

If Im getting the SIZE from a file?

I know you probably can't, but maybe theirs some way to readjust the size based on the number I read in from the file (like say I read in 7, I know Values will have to be of Size 7).

The compiler complains, but Im wondering if their is some workaround. (these have to stay as global variables btw.

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    @Helper: Nothing ever needs to be a global. Well, "nothing ever" is a bit strong, but you get my point. Commented Nov 29, 2010 at 22:50
  • @John This was more of an ironic question. Sadly, it's hard to embed irony in textual form :-). Commented Nov 30, 2010 at 10:01

3 Answers 3

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No, you can only specify array sizes like this using constant integral expressions. Meaning known at compile-time.

You probably shouldn't be doing this, anyway. Instead, you should be using a vector.

If for whatever reason you can't use a vector (which I would seriously doubt), then your next option would be to use dynamically-allocated arrays. But please, for the love of all that is good int he world, use a smart pointer if you do this.

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6 Comments

You can also use the non-standard but apparently extremely common alloca() for stack-based dynamic arrays.
Ya I was going to use a vector....but for some reason I can't find for the world of me how to read just a single line into a vector. When i know how long the vector needs to be (after I read in the first value)
@Mercfh: Post a seperate question about that. I estimate you get an answer within 42 seconds. Maybe even from me.
Will do, i've been asking tons of questions today so I always feel bad when I have to ask another.....but I guess it's just one of those days.
@Eiko I know, I've literally been going from question to question today. I'll get one solved, but that presents another problem. Like I said one of those days
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You can, actually :)

C99 allows that. MSVC doesn't support C99, GCC does.

C++ compilers also tend to allow that as an extension (answer yourself if you're OK with depending on compiler extensions). The only requirement is that you need to know size at the moment of declaration of Values.

A std::vector is usually better (and safer and standarised and so on) anyway, unless you really really want that data on the stack.

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Does C99 really support global variable-length arrays?
I mentioned size being known at the moment of declaration, so for global arrays it would reduce to "known at compile time", imho - unless I've got something wrong here?
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You can do it if you allocate them dynamically:

double *Values = new double[size];
...

delete [] Values;

but not statically.

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How....would I go about doing that? Allocating them dynamically. Like lets say I say Size=10, can I just change the size later on in the program after I read in the number from the file
Sorry, I missed the bit about global variables. Don't do the new where you declare Values (so double *Values;), then set it wherever you read the size from the file (i.e. Values = new double[size];).

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