C / C++ / MFC

24-Jan-04 4:42

Is there anyway (meaning, any tricks of the trade) by which one can set the size of an array at run time?

I am dealing with a 2-dimensional array, and it just seems a waste to set its size at the maximum, only to know that several times during the running of the program, less than 1/10 its size will be used. (Translated another way: From about a megabyte maximum, to a few dozen bytes minimum.)

Thanks for any insight.

Unsure | :~

William

Fortes in fide et opere!

Jörgen Sigvardsson24-Jan-04 5:09

24-Jan-04 5:09

No (unless you can settle with a reallocation of the array). Use a std::vector if you need dynamicity in your "arrays".

--
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WREY24-Jan-04 5:56

24-Jan-04 5:56

Thanks for replying.

Vector seemed to be the only alternative I know about, but thought someone might know of a clever way to get around the compiler's insistence for a constant integral expression.

William

Fortes in fide et opere!

PJ Arends24-Jan-04 9:35

PJ Arends

24-Jan-04 9:35

std::vector also does a lot of reallocations when it grows, it is just hidden from you. Although you can reduce the number of reallocations needed by using the vector::reserve() function.

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MrNiceBerG24-Jan-04 5:25

MrNiceBerG

24-Jan-04 5:25

I'm far from an expert but I do have an idea. When you figure out how much of the array is used for that preticular part of your progam, put those numbers into Variables. From there put the variable names into the line where you specify the size of the array. I have a feeling I have no idea what I'm talking about but it seemed like a good idea. I hope it helps you.

Jörgen Sigvardsson24-Jan-04 5:59

24-Jan-04 5:59

That only works for stack based arrays. And arrays on the stack is a bad coding practice (except for really small, fixed size arrays). Stack space is precious while heap space is not (not as precious as stack space anyway)

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WREY24-Jan-04 6:04

24-Jan-04 6:04

Thanks for replying.

It's a good thought, but if you're serious about maximizing reuse of your code, you cannot have duplicate codes doing the same thing at different locations in your program just so you can take advantage of what you're suggesting.

William

Fortes in fide et opere!

Michael Dunn24-Jan-04 6:13

Michael Dunn

24-Jan-04 6:13

If you're thinking of something like this:

size_t x, y;
//... set x and y somehow ...

int bigarray[x][y];

then no, that is illegal because array sizes must be known at compile time. (gcc allows this, but it's a non-standard extension.)
You can build your own array at runtime with lots of new calls though.

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Jörgen Sigvardsson24-Jan-04 9:42

24-Jan-04 9:42

Really?! Bummer. I was so positive that C++ is allowed to create variable length arrays on the stack. I just verified it in VS.NET too. Sigh | :sigh:

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C'est normal!

Inam24-Jan-04 6:57

Inam

24-Jan-04 6:57

If u are dealing with the array like this int arr[x][y] at compile time than by using pointers u can do it at runtime like for one dimensional array int *arr=new int[x];
where x can have any value similarly one can do it for two dimensional array.

Inam

WREY25-Jan-04 0:22

25-Jan-04 0:22

Thanks for replying.

If you are referring to:

int** ConstructMatrix(const int sz)
{
   int** pMtrx = new int *(Matrix)[sz];
   return pMtrx;
}

Then by passing a value which you are going to assign sz at run time, forget it. It doesn't work.

Frown | :(

William

Fortes in fide et opere!

wb25-Jan-04 4:48

25-Jan-04 4:48

Im sure, there is no way, because the compiler
needs the arraysize to make the pointer arithmetic right. I spent one or two days on it and found
no solution Blush | :O

then I used a normal array (one dimension)
and made the arithmetics myself.

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Gerald Schwab25-Jan-04 16:29

Gerald Schwab

25-Jan-04 16:29

Look into _alloca().
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclib/html/_CRT__alloca.asp[^]

toxcct25-Jan-04 22:58

toxcct

25-Jan-04 22:58

if you want to allocate a table (whatever its size and its dimension) depending on the datas inside, you don't have other ways to use dynamic allocation. if you are in C, use malloc(), calloc(), realloc(), but be careful, they are non-reantrant functions. if you program in C++, prefer new operator, and even if it sizes you source by adding code lines on the treatment of the memory, you win in the place your datas take in memory...

TOXCCT

WREY26-Jan-04 3:58

26-Jan-04 3:58

Thanks for replying.

The problem is NOT with the allocation of memory. Operator 'new' makes easy job of that (providing there is enough space on the heap). The problem is getting around the compiler's insistence that the size of the array be known at compile time. That IS the problem!!

The main objective is NOT whether memory is to be statically allocated, or dynamically allocated. The main problem is with the SIZE of the memory I wish to allocate, which the compiler is insisting it MUST know at compile time.

I am trying to find out if there is a way I can let the compiler know that information at run time, which so far it is saying, "NO!!"

William

Fortes in fide et opere!

toxcct26-Jan-04 6:35

toxcct

26-Jan-04 6:35

could you please show me a line you wrote to which the compiler says "NO"...

i don't really understand why it would answer like that...

thanks

TOXCCT

WREY26-Jan-04 10:14

convert .net project to vs6 project

26-Jan-04 10:14

It's a figure of speech I was using.

William

Fortes in fide et opere!

sstoyan24-Jan-04 3:28

sstoyan

24-Jan-04 3:28

hi i read in an article that there is a utility here on codeproject to convert vs.net projects to vs6 project, but i cant find it.
could somebody point me to it ?

thanx

Re: convert .net project to vs6 project

Jörgen Sigvardsson24-Jan-04 5:11