C++ User defined function to populate an array

Question

I'm just using cout to check if the function worked correctly. The cout that's in the function works and the output is 17 which is the first number in the text file. However, the cout in the main() function outputs 0. Why is this?

#include <iostream>
#include <fstream>
#include <cmath>
#include <iomanip>

using namespace std;

double arrayPop();

int main()
{
    double X[10];
    double Y[10];

    arrayPop();

    cout<<Y[0];

    return 0;
}

double arrayPop()
{
    string horAxis, vertAxis;

    ifstream input;
    input.open("lin_reg-2.txt");

    input>>horAxis;
    double X[10];
    for (int i = 0; i<10; i++)
    {
        input>>X[i];
    }

    input>>vertAxis;
    double Y[10];
    for (int i = 0; i<10; i++)
    {
        input>>Y[i];
    }

    cout<<Y[0]<<endl;
}

Because in the main() function you declared some vectors with no initialization. There is no relation between the X and Y arrays in main() and arrayPop(). You should pass them by parameters with the function. — Mojtaba Ahmadi
– Mojtaba Ahmadi, Commented Nov 23, 2020 at 22:34

gsamaras · Accepted Answer · 2020-11-23 22:26:07Z

1

You need to pass your array as a parameter to the method arrayPop().

Then you would have something like this:

#include <iostream>
#include <fstream>

using namespace std;

void arrayPop(double X[], double Y[], const int size);

int main()
{
    const int size = 10;
    double X[size];
    double Y[size];

    arrayPop(X, Y, size);

    cout<<Y[0];

    return 0;
}

void arrayPop(double X[], double Y[], const int size)
{
    string horAxis, vertAxis;

    ifstream input;
    input.open("lin_reg-2.txt");

    input >> horAxis;
    for (int i = 0; i < size; i++)
    {
        input >> X[i];
    }

    input >> vertAxis;
    for (int i = 0; i < size; i++)
    {
        input >> Y[i];
    }

    cout<<Y[0]<<endl;
}

PS: I suggest you read about Variable Scope in C++.

edited Nov 23, 2020 at 22:26

answered Nov 23, 2020 at 22:19

gsamaras

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2 Comments

keyrimee Over a year ago

This made so much sense and helped a lot. Thanks a bunch.

sergiom Over a year ago

Hi keyrimee, if you think this answers your question, then you may consider to mark it as the accepted answer

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