accessing a variable from another method

**larne** · 01-16-2009

Hey,
I have trouble accessing a matrix from another method. I have a matrix allocated like this in one method

Code:

DTM **dtmpixel = new DTM*[ny_dtm];
for (int k=0; k<ny_dtm; k++)
{
	dtmpixel[k] = new DTM[nx_dtm];
}

How can I access it from another method?
Regards

**laserlight** · 01-16-2009

You would pass the pointer to that second function.

**larne** · 01-16-2009

That sounds good =) How would I go about doing that?
Cheers

**laserlight** · 01-16-2009

Well, besides passing the pointer (of type DTM**), you would also need to pass the dimensions of the matrix (i.e., ny_dtm and nx_dtm) in order for that function to correctly access the elements of the matrix.

By the way, are you aware of containers like std::vector that would help you avoid having to manually manage memory?

**larne** · 01-16-2009

Thanks for your reply, I am aware of vectors, but I wanted to try the "normal way" first. Now it seems to work except for I dont know how to pass nx_dtm and ny_dtm. My function who creates the DTM** now returns it to main and the next method can "take it". But I can only return one value, right?

**cpjust** · 01-16-2009

In C++, vectors are the "normal way".

You can only return 1 value from a function, but you can pass pointer or reference parameters to a function and modify their values, thereby "returning" more than 1 value.

**larne** · 01-16-2009

Should I from say function a call function b with the pointer, and additional, dimension data as arguments instead of returning the data as I did now? And how could I create the same matrix in first post with vectors? I tried it but... it didnt work out =/
Thanks!

**laserlight** · 01-16-2009

Originally Posted by larne

Should I from say function a call function b with the pointer, and additional, dimension data as arguments instead of returning the data as I did now?

That would be one option. An even better option, moving towards creating a full blown matrix class, would be to define:

Code:

struct Matrix
{
    DTM** dtmpixel;
    size_t ny_dtm;
    size_t nx_dtm;
};

Then you can pass a Matrix pointer to the function that initialises the Matrix object (and later on this initialisation function would become a constructor when you decide to write a proper class).

Originally Posted by larne

And how could I create the same matrix in first post with vectors?

Code:

std::vector<std::vector<DTM> > dtmpixel(ny_dtm, std::vector<DTM>(nx_dtm));

Basically, you create a vector of ny_dtm number of vectors and initialise each of them with nx_dtm number of DTM objects.

**larne** · 01-16-2009

Perfect, thanks!

**larne** · 01-16-2009

there was no problem just changing the allocation to vector style and using the same code except for passing the vector back to main. Im not sure what´s going wrong here (but I did have some beers doing it). I tried putting receiving datatype as vector<DTM>. Is "vector" enough? No my compilation error is

Code:

error: conversion from `std::vector<std::vector<RADAR, std::allocator<RADAR> >, std::allocator<std::vector<RADAR, std::allocator<RADAR> > > >' to non-scalar type `std::vector<RADAR, std::allocator<RADAR> >' requested|

The vector I am passing on is vector<RADAR> containing instances of DTM. Or if if this is confusing how do I pass a vector to another function?
Any idea?

**laserlight** · 01-16-2009

What is your code?

**larne** · 01-16-2009

to make it short,
header:

Code:

// headers

// Constants
//#define M_PI  3.1415926535897932384626433832795

// classes
class DTM
{
...

};

class RADAR
{

	vector<DTM> radarpixel;

	public:
	void adddata(DTM pt)
	{
		radarpixel.push_back(pt);
	}
	int getsize()
	{
		return radarpixel.size();
	}
	DTM getdata(int n)
	{
		return radarpixel[n];
	}

};

// funcions
vector<RADAR> a();
void b(vector<RADAR>);

main:

Code:

#include "header.h"

int main()
{

	vector<RADAR> radar = a();

	b(radar);

	return 0;
}

function a.

Code:

vector<RADAR> a()
{
	...
    vector<vector<DTM> > dtmpixel(ny_dtm, vector<DTM>(nx_dtm));

    vector<vector<RADAR> > radarpixel(ny_radar, vector<RADAR>(nx_radar));
	
    radarpixel[tmplin][tmpcol+midcol].adddata(dtmpixel[y][x]);

    return radarpixel;
}

function b:

Code:

#include "header.h"

void b(vector<RADAR> radarpixel)
{
...
}

**laserlight** · 01-16-2009

Well, you declared radarpixel to be a vector<vector<RADAR> >, but then you return it from a(), whose return type is vector<RADAR>.

By the way, unless you really want a copy, it would usually be better to take the arguments by (const) reference.

**larne** · 01-16-2009

ok, being a rookie and all, how could I return a copy correctly? ...and I dont really understand (const) reference. Pass the pointer? In this case, how?
Thanks for your help

**laserlight** · 01-16-2009

Originally Posted by larne

ok, being a rookie and all, how could I return a copy correctly?

You're returning a copy correctly, but copying could be expensive if certain compiler applied optimisations are not applicable.

Originally Posted by larne

and I dont really understand (const) reference. Pass the pointer? In this case, how?

I mean:

Code:

void b(vector<RADAR>& radarpixel)
{
    // ...
}

or:

Code:

void b(const vector<RADAR>& radarpixel)
{
    // ...
}

If you don't understand the above syntax then you need to read a good introductory book on C++.

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