QList<struct>

[Axsis]

Hi
I am working in winХР, qt.4.3.3.
First will show my code:

#include <QVector>
#include <QList>
#include <QString>
#include <QImage>
//#include <list.h>
//#include <vector.h>
typedef struct{
		unsigned int x;
		unsigned int y;
		unsigned int z;
		unsigned short int A1;
		unsigned short int A2;
		unsigned short int A3;
		unsigned short int A4;
		unsigned short int A[1024];
	} dataPoint;
typedef struct{
		QVector<dataPoint>		dataVector;
		QImage				      preview;
		QString					author;
		QString					date;
		QString					time;
	} dataScan;
class DataStorage
{
	//Q_OBJECT
public:
	DataStorage();
	QVector<dataPoint>		      point;
	QList<dataScan>				dataContainer;
	QList<dataScan>::iterator	     dataContainerIterator;
};

#include <QVector>
#include <QList>
#include <QString>
#include <QImage>
//#include <list.h>
//#include <vector.h>
typedef struct{
		unsigned int x;
		unsigned int y;
		unsigned int z;
		unsigned short int A1;
		unsigned short int A2;
		unsigned short int A3;
		unsigned short int A4;
		unsigned short int A[1024];
	} dataPoint;
typedef struct{
		QVector<dataPoint>		dataVector;
		QImage				      preview;
		QString					author;
		QString					date;
		QString					time;
	} dataScan;
class DataStorage
{
	//Q_OBJECT
public:
	DataStorage();
	QVector<dataPoint>		      point;
	QList<dataScan>				dataContainer;
	QList<dataScan>::iterator	     dataContainerIterator;
};

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I need to get access to elements of structure dataContainer and dataVector which is an element of dataContainer. I tried various ways, but at execution the program gives out errors like "QVector: index out of range". When I used classical C++ list and vector I receive RuntimeError.
Below some ways with which I tried to get access to the necessary elements of structures
are shown. But any of them does not work.

DataStorage::DataStorage()
{
    const int nPoints = 360;
	int q=0;
	dataContainerIterator = dataContainer.begin();
	for (int i=0;i<=nPoints;i++)
	{
		q = qAbs(qRound(100*sin(i*3.14/180)));
		dataContainerIterator->dataVector[i].x << i;
		dataContainerIterator->dataVector[i].x = i;
		dataContainerIterator->dataVector.at(i).x << 1;
	}
}

DataStorage::DataStorage()
{
    const int nPoints = 360;
	int q=0;
	dataContainerIterator = dataContainer.begin();
	for (int i=0;i<=nPoints;i++)
	{
		q = qAbs(qRound(100*sin(i*3.14/180)));
		dataContainerIterator->dataVector[i].x << i;
		dataContainerIterator->dataVector[i].x = i;
		dataContainerIterator->dataVector.at(i).x << 1;
	}
}

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Where is my mistake?

[wysota]

Did you allocate enough space in the vector? You can't add data to an empty vector using the index operator.

[Axsis]

Did you allocate enough space in the vector? You can't add data to an empty vector using the index operator.

No, I don't know how much memory can be neccessary. If I do as it is shown below I receive errer: class QVector<dataPoint>' has no member named 'x'.

dataContainerIterator->dataVector.x << i;

dataContainerIterator->dataVector.x << i;

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As far as I know there is no necessity to allocate memory before add a data.

[wysota]

As far as I know there is no necessity to allocate memory before add a data.

You are not adding data. You are assigning data.

[Axsis]

Ok. It's clear. But how I can allocate memory for dataVector? I mean that it is an element of structure not a usual vector.

Solved...

[wysota]

Use QVector::resize or QVector::push_back.

[Axsis]

Use QVector::resize or QVector::push_back.

I know about this functions but I dont know how to use it. I mean that I have QVector<struct> and I don't know how to call QVector::push_back if I want to add .x, then .y ...

[wysota]

QVector<MyStruct> vec;
MyStruct s;
s.x = 7;
s.y = 8;
vec.push_back(s);

QVector<MyStruct> vec;
MyStruct s;
s.x = 7;
s.y = 8;
vec.push_back(s);

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[kenkit]

Hi, from the solution posted above. How do you access the items in the vectors struct.

Hi, from the solution posted above. How do you access the items in the vectors struct.
Not sure but would this work ?

cout<<vec.at(0).x;

cout<<vec.at(0).x;

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---

Added after 10 minutes:

Haha it works.
I wonder why none of this has been documented.

---

Added after 14 minutes:

Wait one problem, how do you modify the data.
I tried

m_programs.at(0).exitcode=1;

m_programs.at(0).exitcode=1;

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which fails to compile with

error: assignment of member 'program::exitcode' in read-only object

error: assignment of member 'program::exitcode' in read-only object

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Hope there is a cleaner way so I won't have to create a new data structure and assign it over again.

[Radek]

As to the list (vector or a similar structure), the index operator or the at() function can access only existing data. You need to insert the data into the list first and then you can reference them using [] or at(). When you create the vector, use append() (or push_back(), which is the same for STL compatibility). See Wysota's comments above. AFAIK, reserve() will not suffice because reserve() only reallocates space but it does not add any elements.

dataScan tmpstruct;
 
for( int i = 0; i < nmbOfElements; ++i )
{
  fill tmpstruct with new data;
  dataContainer.append(tmpstruct);
}

dataScan tmpstruct;

for( int i = 0; i < nmbOfElements; ++i )
{
  fill tmpstruct with new data;
  dataContainer.append(tmpstruct);
}

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if the data aren't too sequential, you can also insert() new data to the proper place but the element where you are inserting must already exist when you command insert(). In the worst case, you can prefill the list with placeholders (using append() ) and then use replace(). Again, the data at replace() position must already exist.

As to accessing the data: The index operator or the at() functions returns data of the type of the list element. You can process the data subsequently. Therefore:

cout << dataContainer.at(0).author; 
cout << dataContainer.at(0).datavector.at(0).x;
cout << dataContainer.at(0).datavector.at(0).A[15];

cout << dataContainer.at(0).author; 
cout << dataContainer.at(0).datavector.at(0).x;
cout << dataContainer.at(0).datavector.at(0).A[15];

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should be OK.

Actual size of the list (the biggest valid index into the list plus one) - use the size() function.

[anda_skoa]

Not sure but would this work ?

cout<<vec.at(0).x;

cout<<vec.at(0).x;

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Haha it works.
I wonder why none of this has been documented.

Of course this is documented.
Documentation for access to the vector element using QVector::at() and access to a struct member is documented in the C++ standard.

which fails to compile with

error: assignment of member 'program::exitcode' in read-only object

error: assignment of member 'program::exitcode' in read-only object

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Since you are obviously very new to C++: the const keyword on a type means that the object cannot be modified, it is a constant.

Hope there is a cleaner way so I won't have to create a new data structure and assign it over again.

Of course, there are accessors that return a non-const reference, the index operator for example

Cheers,
_

[lyesqiz]

As far as I know there is no necessity to allocate memory before add a data.