Is it possible to 'compose' functions?

Ivan · #1 19-Jun-2009, 13:00

I have a template class Vector<T> that is inherited from the std::vector<T>,
but has extended functionality, for example, it has the component-wise
multiplication * of vectors.

I also (separately) have a function

CPP / C++ / C Code:

  double mean(Vector<double>& v){
...
};

that computes the mean of a Vector<double>.

Now if, for a Vector<double> b (of 100 fixed numbers),
I write cout<< mean ( (b*b))<<endl;
the compiler is not happy. (the code as below works)

CPP / C++ / C Code:

  Vector<double> a=b*b;
          cout<<mean(a)<<end;

Is there a way to be able to write cout<< mean ( (b*b))<<endl; ? (for example,
surely you can write cout<< sin (cos (3))<<endl; )

davekw7x · #2 19-Jun-2009, 14:39

Quote:

Originally Posted by Ivan

...
Is there a way to be able to write cout<< mean ( (b*b))<<endl; ?

Yes:

CPP / C++ / C Code:

//
// davekw7x
//
// My Vector template class definition stuff is in Vector.h
//
#include "Vector.h"

#include <iostream>
using namespace std;


double mean(const Vector<double> & v);
void print_Vec(const Vector<double> & v);

int main()
{
    int n = 4;

    Vector<double> b(n);

    for (int i = 0; i < n; i++) {
        b[i] = (i+1);
    }
    cout << "b         =";
    print_Vec(b);

    cout << "mean(b)   = " << mean(b) << endl << endl;

    cout << "b*b       =";
    print_Vec(b*b);

    cout << "mean(b*b) = " << mean(b*b) << endl;

    return 0;
}


void print_Vec(const Vector<double> & v)
{
    for (size_t i = 0; i < v.size(); i++) {
        cout << " " << v[i];
    }
    cout << endl;
}


//
// The definition of mean() can go here
//

Output:

Code:

b         = 1 2 3 4
mean(b)   = 2.5

b*b       = 1 4 9 16
mean(b*b) = 7.5

Quote:

Originally Posted by Ivan

compiler is not happy...

I respectfully suggest that for a reasonable expectation of meaningful help with fewer iterations, you might tell us exactly what messages the compiler gave you. Maybe someone can help you to understand what your grouchy old compiler is trying to tell you.

Also, you might tell us what compiler and what version you are using. Sometimes it makes a difference to people who are trying to help.

Regards,

Dave

Ivan · #3 19-Jun-2009, 15:26

Thanks a lot, davekw7x. Your solution works. I should have declared
double mean(const Vector<double> & v)

instead of

double mean(Vector<double> & v).

I respectfully suggest that for a reasonable expectation of meaningful help with fewer iterations, you might tell us exactly what messages the compiler gave you. Maybe someone can help you to understand what your grouchy old compiler is trying to tell you.

Also, you might tell us what compiler and what version you are using. Sometimes it makes a difference to people who are trying to help.

Regards,

Dave[/quote]

Point taken.

davekw7x · #4 19-Jun-2009, 16:51

Quote:

Originally Posted by Ivan

Thanks a lot, davekw7x. Your solution works. I should have declared
double mean(const Vector<double> & v)

instead of

double mean(Vector<double> & v).

The point is that when you use an expression as an argument to a function, the compiler creates a temporary (constant) object to feed to the function. I'm guessing the compiler message might have been related to "improper initialization of non-constant reference...blah-blah-blah..."

That message may not make much sense to us mere humans (See Footnote) until we have seen it a few times (or a few hundred times like some of us), but it is actually better than lots of compiler messages.

Regards,

Dave

Footnote:

"No one was born knowing this stuff, you know!"
---davekw7x

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Is it possible to 'compose' functions?

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