return pointers from function. Value not updated for one pointer

Question

I have tried to obtain 2 pointers from a function and print it in main. the vague thing is one pointer seems to have recovered its values, while the other hasn't. And both the pointers, have the correct value inside the calling function, just before returning as well. Please tell me if you can identify any programmatic error that is preventing me from getting the right answer.

    #include<iostream>
    #include<fstream>
    #include<stdio.h>
    #include<string.h>
    #include<stdlib.h>

    using namespace std;
    double* readctrls()
    {
         fstream inputs;
         inputs.open("input_coods.txt");
         int nol = 0,i = 0;
         string line,temp,subtemptrans,subtemprots;
         while(getline(inputs,line))
         {
                ++nol;
         }
//      cout<<nol<<endl;
        inputs.close();
        inputs.open("input_coods.txt");
        string *lines = new (nothrow) string[nol];
        double* trans = new double[nol];
        double* rots = new double[nol];
        trans[0] =float(nol);
        for(int i = 0; i<nol ; i++)
        {
                getline(inputs,lines[i]);
        //      cout<<lines[i]<<endl;
                temp = lines[i];
//              cout<<temp<<endl;
                for(int j = 0; j<temp.length() ; j++)
                {
                        if(temp.at(j) == ' ')
                        {
                                subtemptrans = temp.substr(0,j);
                                subtemprots = temp.substr(j+1,temp.length()-j);
        //                      cout<<subtemprots<<endl;
                                *(trans+i+1) = ::atof(subtemptrans.c_str());
                                *(rots+i) = float(atoi(subtemprots.c_str()));
                        //      cout<<rots[i]<<endl;
                        }
                }
        }                       
        inputs.close();         
//      cout<<rots[2]<<endl;    
        return(rots,trans);                                    
}                               

int main()                              
{                               
        double *trans,*rots;                                   
        (rots,trans) = readctrls();                            
//      cout<<sizeof(trans)<<endl;
        for(int i=0;i<trans[0];i++)
        {                                                      
                cout<<*(trans+i)<<endl;
                cout<<*(rots+i)<<endl;
        }                                                      
}                       

The value of Trans is written fine in the memory and is perfectly retained from the main(). But the rots is giving garbage values of the order (e^-42). Please help me here.

Answer 1

C++ is neither Python nor Lua.

You can't return multiple values from a function.

return rots, trans;

This is the comma operator - evaluates its operands and yields the last (rightmost) one.

(rots, trans) = readctrls();

Likewise, this assigns to trans only, rots will be uninitialized.

Solution: you can either return a struct containing the two pointers, or pass them by reference, or whatever...

struct Foo {
    double *rots;
    double *trans;
};

Foo readctrls()
{
    // ...

    Foo r;
    r.rots = rots;
    r.trans = trans;
    return r;
}

or:

void readctrls(double *&r, double *&t)
{
    // ...

    r = rots;
    t = trans;
}

---

Other remarks:

1. Don't use raw arrays. std::vector<T> is generally preferred over T * in C++.

2. It's super wasteful to read the entire file just in order to count the lines, then read it once again to actually parse its contents. If you used an std::vector<double>, you could just vector.push_back(some_double); as you go along the lines, so you wouldn't have to walk through the file twice (you know, I/O is expensive, especially if the file is large).

3. You never delete the pointers that you allocate using new - here your program leaks memory.