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dynamic array declration

Posted on 2004-03-26
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Last Modified: 2013-12-14
i want to know if it is possible to declare an array of the type

int array[ length ]; where length is a variable.

i don't want to use malloc and calloc as they complicate the program.

i get an error message "Constant Expression Required"

is there any way of making the length constant so that the above statement is valid ??

i need this for a class called Timer

i want to declare it as

Timer T[5][length];
Where length is  a variable.

is it possible ??
( without using malloc and calloc as far as possible )

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Question by:radarsh
10 Comments
 
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by:
jkr earned 64 total points
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>>i want to know if it is possible to declare an array of the type
>>int array[ length ]; where length is a variable.

Not if you are using VC++. You might want to use std::vector, e.g.

#include <vector>


std::vector<int> array(lenght);
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by:Karl Heinz Kremer
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Your compiler tells you that it's not possible...

You are asking this in the C++ area, so why not just use a C++ feature? The STL vector is like an array, but it allocates it's memory dynamically in the background. This means that you don't have to take care of this yourself.

#include <vector>
using namespace std;

vector<Timer> T;

... then just use T like an array:
T[0] = ...
T[1] = ...
T[2] = ...

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by:GJanusz
GJanusz earned 62 total points
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I, too, would use vector.  How you use it is another matter, of course.  There are probably many ways.  Personally, working with a variable array of a constant array of class instances confuses me sometimes (hopefully not for the duration of this post).

I might consider coding it this way:

typedef struct {
      Timer      y[5];
} Timer5;
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by:GJanusz
Comment Utility
And the rest of my post ... (note that the typedef struct could be a class instead if you like)

vector<Timer5> T;
T.resize(nLength);

Then you can reference Timer instances with references that look like:
T[29].y[4]

In real life, I tend to run into situations more where I want to have a vector of vectors, in which case, I make my life simpler by defining something like this:

typedef vector<something> T_vSomething;

Then I can allocate a vector of a vector of somethings with:
vector<T_vSomething> myvv;
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by:itsmeandnobodyelse
itsmeandnobodyelse earned 62 total points
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The C++ equivalent of malloc is the new operator. You can do something like this:

class T
{
private:
    char* ps;
    int  i;
public:
    T() : ps(new char[10]), i(0) {}
    ~T() { delete [] ps; }
};

int main(int nArgs, char* szArgs[])
{

    typedef T T5[5];
    long len = 2;
    T5* pT5 = new T5[len];
   
    ...
    delete [] pT5;

    return 0;
}

Then you can have two dimensions - the first must be constant.

A dynamic container like vector is better, but vector<T> is an one-dimensional array.

Regards, Alex

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Expert Comment

by:fsugolf06
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heres a pretty simple solution here:

//first declare a pointer

int * array;

//then dynamically allocate the array like this:

array = new int[10][someVariable];

that should work
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by:Axter
Axter earned 62 total points
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>>array = new int[10][someVariable];

That works, but it leaves a potential for memory leaks just like malloc.

Here's another method:
http://axter.com/code/dynamic_2d_array.h

With the simple class in the above link, you can easily declare a 2-D array like this:

dynamic_2d_array<Timer> T[5][length];

For more information, and to see other methods for dynamically creating a 2D array, see the following link:
http://www.axter.com/faq/topic.asp?TOPIC_ID=60&FORUM_ID=4&CAT_ID=9
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