Question

I’m using g++ on fedora linux 13.
I’m just practicing some exercises from my c++ textbook
and can’t get this one program to compile. Here is the code:

double *MovieData::calcMed() {
        double medianValue;
        double *medValPtr = &medianValue;
        *medValPtr = (sortArray[numStudents-1] / 2);
        return medValPtr;
}

Here is the class declaration:

    class MovieData 
{
private:
    int *students;                  // students points to int, will be dynamically allocated an array of integers.
    int **sortArray;                // A pointer that is pointing to an array of pointers.
    double average;                 // Average movies seen by students.
    double *median;                 // Median value of movies seen by students.
    int *mode;                  // Mode value, or most frequent number of movies seen by students.
    int numStudents;                // Number of students in sample.
    int totalMovies;                // Total number of movies seen by all students in the sample.
    double calcAvg();               // Method which calculates the average number of movies seen.
    double *calcMed();              // Method that calculates the mean value of data.
    int *calcMode();                // Method that calculates the mode of the data.
    int calcTotalMovies();              // Method that calculates the total amount of movies seen.
    void selectSort();              // Sort the Data using selection sort algorithm.
public:
    MovieData(int num, int movies[]);       // constructor
    ~MovieData();                   // destructor
    double getAvg() { return average; }     // returns the average
    double *getMed() { return median; } // returns the mean
    int *getMode()  { return mode; }        // returns the mode
    int getNumStudents() { return numStudents; }    // returns the number of students in sample
};

Here is my constructor and destructor and selectSort():

MovieData::MovieData(int num, int movies[]) {
    numStudents = num;

    // Now I will allocate memory for student and sortArray:
    if(num > 0) {
        students = new int[num];
        sortArray = new int*[num];

        // The arrays will now be initialized:
        for(int index = 0;index < numStudents;index++) {
            students[index] = movies[index];
            sortArray[index] = &students[index];
        }
        selectSort();   // sort the elements of sortArray[] that point to the elements of students.
        totalMovies = calcTotalMovies();
        average = calcAvg();
        median = calcMed();
        mode = calcMode();
    }
}

// Destructor:
// Delete the memory allocated in the constructor.
MovieData::~MovieData() {
    if(numStudents > 0) {
        delete [] students;
        students = 0;
        delete [] sortArray;
        sortArray = 0;
    }
}

// selectSort() 
// performs selection sort algorithm on sortArray[],
// an array of pointers.  Sorted on the values its 
// elements point to.
void MovieData::selectSort() {
    int scan, minIndex;
    int *minElement;

    for(scan = 0;scan < (numStudents - 1);scan++) {
        minIndex = scan;
        minElement = sortArray[scan];
        for(int index = 0;index < numStudents;index++) {
            if(*(sortArray[index]) < *minElement) {
                minElement = sortArray[index];
                minIndex = index;
            }
        }
        sortArray[minIndex] = sortArray[scan];
        sortArray[scan] = minElement;
    }
}

The compiler is giving this error:

moviedata.cpp: In memberfunction
‘double * MovieData::calcMed()’:

moviedata.cpp:82: error: invalid
operands of types ‘int*’ and ‘double’
to binary ‘operator/’

I’m not sure what to make of this error, i’ve tried static casting the types with no luck, what does this error message mean?

Answer 1

sortArray[numStudents - 1] is a pointer to int, which can’t be on the left side of a division (when you remember pointers are addresses, this makes sense). If you post more of your code, we can help you correct it.

Perhaps you want something like:

int *MovieData::calcMed() {
      return sortArray[(numStudents - 1) / 2];
}

This returns the middle element in your array, which should be a pointer to the middle student. I’m not clear why you’re sorting lists of pointers (not the actual values), or why you’re returning a pointer here. The return value + 1 will be a pointer to the next value in students, which is not the next greater value numerically. So you might as well return the actual student (int from students). If you do this, you can also average the two middle elements when the count is even (this rule is part of the typical median algorithm).

Note that I changed the return type to int *, the type of sortArray’s elements. Also, your comment is incorrect. This is the median, not the mean.

Also, your selection sort is wrong. The inner loop should start at scan + 1.