QList += operator

[alrawab]

i got error when try :

QList<double> zz;
double r=.05;
for (int k = 0; k < 3; ++k)
                {
        zz[0] +=r;
 
}

QList<double> zz;
double r=.05;
for (int k = 0; k < 3; ++k)
                {
        zz[0] +=r;

}

To copy to clipboard, switch view to plain text mode 

how to solve this
thanks

[Lykurg]

In general it would be helpfull if you would actually paste the error message!

As far as I can see, you have to fill the list first. [] can only be used if that index allready exists.

[alrawab]

you have to fill the list first. []
yes i did that by :

zz<<0.0<<0.0<<0.0<<0.0

but that is idiot

[wysota]

What (who?) is idiot?

[alrawab]

There is no need to inquire not my nature abuse literature with one .
I meant the Behavior of the QList
that is all

[wysota]

What behaviour of QList? That you can't operate on items you don't have? Every list works this way. It's a list with finite number of elements.

[alrawab]

but in c# for example you don't obligate to initialize initial values :

doble zz=3;
double[] foo = new double[4];
for (int k = 0; k <10; ++k)
{
foo[0] += zz;


}

this is similar to QList and works fluently without any initialization

[wysota]

No, you are wrong. This is the initialization of the array:

double[] foo = new double[4];

double[] foo = new double[4];

To copy to clipboard, switch view to plain text mode 

You explicitly set the array size to four elements filled with possibly random data.

The equivalent for Qt (with the difference that each cell is initialized to 0.0) is:

QVector<double> foo(4);

QVector<double> foo(4);

To copy to clipboard, switch view to plain text mode 

[alrawab]

yes but this not applicable For QList as i know ?
is there any replacement for zz<<0.0<<0.0<<0.0<<0.0;
it's looks ugly

[amleto]

use QVector

[wysota]

yes but this not applicable For QList as i know ?

It's a list. Initializing it like I did with a vector is slow and unnecessary.

is there any replacement for zz<<0.0<<0.0<<0.0<<0.0;

e.g.

for(int i=0;i<4;++i) zz.append(0);

for(int i=0;i<4;++i) zz.append(0);

To copy to clipboard, switch view to plain text mode 

By the way, if you know your array of doubles is going to be always of size 4, use a regular C array instead.

double zz[4];

double zz[4];

To copy to clipboard, switch view to plain text mode 

[ChrisW67]

If the object is to always have a fixed number of elements then you have better choices than QList as others have pointed out. If your QList must start with a certain number of elements but may grow, and you really find the simple initialisation loop so distasteful, then I guess you can use QVector to initialise your QList:

QList<double> stuff = QList<double>::fromVector(QVector<double>(4, 0.0));

QList<double> stuff = QList<double>::fromVector(QVector<double>(4, 0.0));

To copy to clipboard, switch view to plain text mode 

I don't think this is an improvement over the other options. Ultimately expecting C++ and Qt to behave like C# and its library is like tilting at windmills.

[wysota]

With C++11 and Qt 5 one can also use initializer lists.

QList<double> xxx = { 0.0, 0.0, 0.0, 0.0 }; // with C++11 enabled

QList<double> xxx = { 0.0, 0.0, 0.0, 0.0 }; // with C++11 enabled

To copy to clipboard, switch view to plain text mode