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digitalmars.D.learn - Alocating memory depending of a variable value INT variable

reply "Carlos" <checoimg gmail.com> writes:
Well in C I just declared an array during execution with an array 
with a multiplied variable for the size of the array.

Since I only need two spaces in the array for each line of 
process it was multiplied by two.

so it was like this :

scanf("%d", &Num);
int array[Num*2];

When I tried to do this on D, well it says the variable cannot be 
read at compile time.

I don't know how the C language implements this but the main 
thing is that it does.

So how can I manage memory depending on the user's input in D ?


Thank you for your attention.
Nov 19 2013
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 11/19/2013 03:16 PM, Carlos wrote:> Well in C I just declared an 
array during execution with an array with a

 multiplied variable for the size of the array.

 Since I only need two spaces in the array for each line of process it
 was multiplied by two.

 so it was like this :

 scanf("%d", &Num);
 int array[Num*2];


That is a VLA.

In D, you would normally use a slice. Below, 'a' and 'b' are two ways of 
having a slice with valid elements. On the other hand, 'c' is merely 
reserving space for that many elements:

import std.stdio;

void main()
{
     size_t num;
     write("How many? ");
     readf(" %s", &num);

     auto a = new int[num * 2];

     int[] b;
     b.length = num * 2;

     int[] c;
     c.reserve(num * 2);
}

Ali
Nov 19 2013
next sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Ali Çehreli:


 That is a VLA.


That are currently not present in D. The most common and safe 
alternatives in D are allocating the memory on the heap with 
'new', or over-allocating on the stack a fixed size and then 
slicing.


If the OP really wants to allocate on the stack, there is the 
alloca() function (untested, I don't remember the correct name 
for the memory error):


auto ptr = cast(int*)alloca(num * 2 * int.sizeof);
if (ptr == null)
     throw new outOfMemoryError("...");
auto array = ptr[0 .. num * 2];


Bye,
bearophile
Nov 19 2013
parent "Namespace" <rswhite4 googlemail.com> writes:
On Wednesday, 20 November 2013 at 00:02:42 UTC, bearophile wrote:

 Ali Çehreli:


 That is a VLA.


 That are currently not present in D. The most common and safe 
 alternatives in D are allocating the memory on the heap with 
 'new', or over-allocating on the stack a fixed size and then 
 slicing.


That's why I use a Stack and a Heap struct in combination with an 
Array struct:
Heap heap;
Stack stack;
byte[] arr = Array!byte(&stack, &heap).of(Num);
or even more naturally:
byte[] arr = Array!byte(&stack, &heap)[Num];

Stack has a buffer of e.g. 4096 and tries to allocate there, if 
it fails, it returns null. If this happens, the Heap struct 
allocates on the GC or C heap.
And if the Stack / Heap struct gets destroyed, the stored memory 
is freed.

But VLA's were really desirable. :(
Nov 20 2013
prev sibling parent "Carlos" <checoimg gmail.com> writes:
On Tuesday, 19 November 2013 at 23:34:48 UTC, Ali Çehreli wrote:

 On 11/19/2013 03:16 PM, Carlos wrote:> Well in C I just 
 declared an array during execution with an array with a

 multiplied variable for the size of the array.

 Since I only need two spaces in the array for each line of


 process it

 was multiplied by two.

 so it was like this :

 scanf("%d", &Num);
 int array[Num*2];


 That is a VLA.

 In D, you would normally use a slice. Below, 'a' and 'b' are 
 two ways of having a slice with valid elements. On the other 
 hand, 'c' is merely reserving space for that many elements:

 import std.stdio;

 void main()
 {
     size_t num;
     write("How many? ");
     readf(" %s", &num);

     auto a = new int[num * 2];

     int[] b;
     b.length = num * 2;

     int[] c;
     c.reserve(num * 2);
 }

 Ali



auto array = new int[Num * 2];

Well that works just perfect. Thanks a lot!
Nov 19 2013