How do you return a vector from a class member function

[megaLU]

Hello,

I'm just wondering how you return a vector from a member function. I've looked online but had no luck. At the moment my function is set to long double, and when I tried setting it to std::vector it came up with lots of errors. Here is my code:

public:
long double evalx(vector<long double>, vector<long double>, vector<long double>);


.....
......



long double IFS::evalx(vector<long double> x, vector<long double>a, vector<long double>b)
{

    a[0] = 1.0;
    b[0] = 1.0;
    for(int i=1; i<=1; ++i)
    {
        a[i] = (matrix[0]*a[i-1] + matrix[1]*b[i-1] + matrix[2]);
    }
    x.push_back(a[i+1]);
    return x;

}

It is telling me
error:: cannot convert 'std::vector<long double>' to 'long double' in return
which makes sense, I just don't know how to tell it to return a vector. Sorry I'm still very new to c++. Any help would be gratefully received. Thank you.

[daviddoria]

The type before the function name is the return type:

std::vector<long double> yourFunction()
{
  vector<long double> a;
  return a;
}

[megaLU]

Oh right, nice one thanks a lot.
Is there any way of keeping my class in there though? As in the IFS bit. Because otherwise it doesn't know what I'm talking about when I call matrix[]..?

[daviddoria]

Certainly:

std::vector<long double> IFS::yourFunction()
{
  vector<long double> a;
  return a;
}

[megaLU]

Brilliant, thanks very much, that has saved me a lot of time that might have been spent guessing the different possible combinations, compiling, working out errors, guessing again etc. Cheers.

[megaLU]

Oh dear sorry sorry sorry, wish i understood c++ more.

When I try to call it i get a warning that c++ has encountered a problem and has to close. if i have

std::vector<long double> IFS::yourFunction(vector<long double> b)

can i not simply call

f.yourFunction(b);

?
Sorry.

[daviddoria]

That is fine, but you wont get the returned value.

Why are you passing a vector<long double> as a parameter?

The function should be defined like this:

std::vector<long double> IFS::yourFunction()
{

}

then you should call it with:

IFS yourClass;
std::vector<long double> returnedValue = yourClass.yourFunction();

[megaLU]

Oh man, i'm sorry to be annoying but i still get the generic warning myproject.exe c++ has encountered a problem and has to close, even though it all compiles fine. It only gets that error when i call it. Is there any obvious error here?

std::vector<long double> IFS::evalx()
{

    vector<long double>a;
    vector<long double>b;

    a[0] = 1.0;
    b[0] = 1.0;
    for(int i=1; i<=1; ++i)
    {
        a[i] = (matrix[0]*a[i-1] + matrix[1]*b[i-1] + matrix[2]);
    }

    return a;

}

and called with (when the error comes)

std::vector<long double> a = f_1.evalx();

i wish i didn't keep posting and managed to work it out myself, but i'm seriously running out of time. Thanks.

[daviddoria]

You can't do this:

vector<long double> a;
a[0] = 1.0;

'a' hasn't been given any memory.

You need to do:

vector<long double> a(1); // give 'a' exactly one element
a[0] = 1.0;

or much better:

vector<long double> a;
a.push_back(1.0); // extend 'a' as needed

It is starting to seem like you are just trying to get this assignment done instead of learning c++ - I hope this is not the case!

[megaLU]

It is only that I am at the stage of pulling all nighters and still might not finish. I definitely feel like i am learning a lot though. I really appreciate your help, thank you.

[megaLU]

The reason I was passing a vector as a parameter was because I am using this function in a for loop in int main. I don't want to create a new vector with each iteration of the for loop by declaring it inside the function. That is what I'd be doing isn't it?

[daviddoria]

Yes. In that case you want to pass the vector by reference, and not return anything:

void yourFunction(std::vector<long double> &yourVector) // note the &
{
 // modify yourVector

// don't return anything, the values have been modifying in the vector you have passed.
}

Then you call this with:

std::vector<long double> yourVector;
yourFunction(yourVector); // yourVector will be modified

David

[megaLU]

But if I don't return anything then my iterator has nothing to work with for the next step.. Oh no!

[daviddoria]

Why not? You just use the same array (it's values are now different). Post the loop if you still don't understand.

[megaLU]

Not to worry I have finally figured it out! I massively over complicated things. Thanks again.