Write a function that returns a pointer to the maximum value using pointers c++

Question

This is the problem that I'm trying to solve for class in C++.

Write a function that returns a pointer to the maximum value of an array of floating-point data: double* maximum(double* a, int size). If size is 0, return nullptr.

The issues I'm having are that:

1. The final output is not the correct location for the maximum value in the array.

2. An error that says: "cannot convert 'double**' to 'double*' in the initialization".

3. If I use nullptr at any point in this code, CodeBlocks gives me an error.

#include <iostream>
using namespace std;

// return pointer to location from function
double * maximum(double* a, int size)
{
    double maxVal = a[0]; // this is the starting max value
    double* max_pos = &a; // points to the value in a[0]
    // initialis]ze both variables
    for(int i = 0; i < size; i++){

        if(a[i] > maxVal){
            maxVal = a[i];
            cout << max_pos << endl;
            max_pos = &a[i];
        }
    }
    // return address
    return max_pos;
}

int main()
{
    double myarr[5];
    int i = 0;
    int arrSize = 5;

    cout << "Input 5 floating point values for your array" << endl;

    for(i = 0; i < arrSize; i++){ // loop to input values

        cin >> myarr[i];

    }
    for(int j = 0; j < arrSize; j++){
        cout << "Location for " << myarr[j] << " = " << &myarr[j] << endl;
    }
    double* maxNum = maximum( myarr, arrSize);     
    cout << &maxNum << endl;


    return 0;
}

This is the output I'm getting after finding max_pos:

Answer 1

The code you showed has a few mistakes in it:

1. using namespace std; is bad!

2. you are not following your instructions to return nullptr when size is 0.

3. you are trying to initialize max_pos (a double*) with &a (a double**), which is a compiler error.

4. you are passing &maxNum (a double**) to std::cout, printing the address of the maxNum variable itself, not the address that it is pointing to (the found array element). You need to pass maxNum (a double*) if you want to print the address of the found element, or pass *maxNum (a double) if you want to print the value of the found element.

Try something more like this instead:

#include <iostream>

// return pointer to location from function
double* maximum(double *a, int size)
{
    if (size == 0) return 0;

    // initialize both variables
    double* max_pos = a; // points to the value in a[0]
    double maxVal = *max_pos; // this is the starting max value
    std::cout << "max_pos = " << max_pos << " (" << maxVal << ")" << std::endl;

    for(int i = 1; i < size; ++i){
        if (a[i] > maxVal){
            max_pos = &a[i];
            maxVal = *max_pos;
            std::cout << "max_pos = " << max_pos << " (" << maxVal << ")" << std::endl;
        }
    }

    // return address
    return max_pos;
}

int main()
{
    const int arrSize = 5;
    double myarr[arrSize];

    std::cout << "Input " << arrSize << " floating point values for your array" << std::endl;

    for(int i = 0; i < arrSize; ++i) { // loop to input values
        std::cin >> myarr[i];
    }

    for(int j = 0; j < arrSize; ++j) {
        std::cout << "Location for " << myarr[j] << " = " << &myarr[j] << std::endl;
    }

    double* maxNum = maximum(myarr, arrSize);
    std::cout << "maxNum = " << maxNum << " (" << *maxNum << ")" << std::endl;

    return 0;
}

Live Demo

And then, you can throw it all away and use STL algorithms instead, like std::max_element():

#include <iostream>
#include <algorithm>    
#include <iterator>

int main()
{
    const int arrSize = 5;
    double myarr[arrSize];

    std::cout << "Input " << arrSize << " floating point values for your array" << std::endl;

    // loop to input values
    std::copy_n(std::istream_iterator<double>(std::cin), arrSize, myarr);

    for(int i = 0; i < arrSize; ++i) {
        std::cout << "Location for " << myarr[i] << " = " << &myarr[i] << std::endl;
    }

    double *maxNum = std::max_element(myarr, myarr + arrSize);
    std::cout << "maxNum = " << maxNum << " (" << *maxNum << ")" << std::endl;

    return 0;
}

Live Demo