An array passed to a function gives nan elements (C++) - c++

Question

I have a function that takes in an array and defines a new array made up of a subset of the original array.
#include<iostream>
#include<cmath>
using namespace std;
double * subarray(double *array){
double *sub= new double[100];
for (int i=0; i<10; i++){
sub[i]=array[i];
}
return sub;
}
int main(){
double *x=new double[100];
double *y=new double[10];
for(int j=0; j<100; j++){
x[j]=sin(j*3.14/2.0);
}
y=subarray(x);
for(int k=0; k<10; k++){
cout<<y[k]<<endl;
}
return 0;
}
When I run this code some of the elements of the sub array come out as nan.

Answer 1

calling subarray is creating a new double[100] then passing that back out. But it's a completely separate array to y = new double[10]. Whenever you call "new" it's making a new thing, separate to all other "new" things. y gets the address of the "new double[100]" but it loses track of the address of the "new double[10]" you made in main, which is a memory leak. If you're going to return a pointer to an new'ly allocated item, start the pointer that's going to store it as a nullptr, and call "delete" or "delete []" when you're done, depending on whether it was a "new[]" thing or just "new".
#include<iostream>
#include<cmath>
using namespace std;
double * subarray(double *array) {
double *sub = new double[10]; // don't allocated more than you want to use
for (int i = 0; i < 10; i++) {
sub[i] = array[i];
}
return sub;
}
int main() {
double *x = new double[100];
double *y = nullptr; // don't set this if you're going to overwrite it
for (int j = 0; j < 100; j++) {
x[j] = sin(j*3.14 / 2.0);
}
y = subarray(x);
for (int k = 0; k < 10; k++) {
cout << y[k] << endl;
}
delete [] x; // if you make new things, delete them afterwards
delete [] y;
std::cin.get();
return 0;
}
I don't get any NaNs when I run this, but you are allocating a lot of memory that never gets set, inside the function. You're allocating 100 spaces but only writing to 10. This isn't necessarily an error, but it's not efficient.