C++ array size by user input - c++

Question

I'm trying to write a program that will have a user input size for an array, and then take values into that array. I initially tried
int sz = 51;
double Arr[sz];
Which led to compilation errors. Apparently dynamic allocation of the variable has to happen, and I'd rather avoid that if possible. So I modified my code (current state as shown below) which now only throws "expected primary-expression before ']' token". Is there a way to fix this and I'm just not seeing it, or do I need to use dynamic allocation?
Thanks for your time!
#include <iostream>
#include <iomanip> //for setprecision
using namespace std;
int sz = 51;
double n=0;
double Arr[0];
void get_input(double Arr[], int &sz){ //gets input
do{
cout<< "Enter size: "<< endl;
cin>> sz;
if (sz<0 || sz>50){
cout<< "Invalid size, enter a value between 0 and 50"<<endl;
}
}while(sz<0 || sz>50);
for( int i=0; i<sz; i++){
cin>> Arr[i];
}
}
double calcSum( double Arr[], int sz){ //finds sum
for(int i=0; i<sz; i++){
n+= Arr[i];
}
return(n);
}
void printArray(double Arr[], int sz){ //prints array elements
for(int i=0; i<sz; i++){
cout<< Arr[i]<< setprecision(2)<<" ";
if(i%7 == 0)
cout<< endl;
}
}
int main()
{
double Arr[sz];
get_input(Arr[], sz); //error here
printArray(Arr[], sz); //error here
return 0;
}

Answer 1

VLAs (e.g. Arr[sz]) are only supported as extensions in C++. They aren't part of the official language standard. You should use std::vector instead.

Answer 2

Just use a std::vector, there's a standard library in C++ for this reason.
Demo:
notes: you don't need the globals (they are shadowed by the locals and you pass them by reference anyways)
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#include <iostream>
#include <iomanip>
#include <vector>
using namespace std;
using array_t = std::vector<double>;
void get_input(array_t& Arr) { // gets input
size_t sz = 51; // unsigned types cannot be negative
do {
cout << "Enter size: " << endl;
cin >> sz;
if (sz > 50) {
cout << "Invalid size, enter a value between 0 and 50" << endl;
}
} while (sz > 50);
for (size_t i = 0; i < sz; ++i) {
double v;
if (cin >> v)
Arr.push_back(v);
else
std::cerr << "Error reading input\n";
}
//assert(sz = Arr.size());
}
double calcSum(array_t const& Arr) { // finds sum
double n = 0;
for (size_t i = 0; i < Arr.size(); ++i) {
n += Arr[i];
}
return n;
}
void printArray(array_t const& Arr) { // prints array elements
for (size_t i = 0; i < Arr.size(); ++i) {
cout << Arr[i] << setprecision(2) << " ";
if (i % 7 == 6)
cout << endl;
}
}
int main() {
array_t Arr;
get_input(Arr);
printArray(Arr);
std::cout << "\nSum: " << calcSum(Arr) << "\n";
}
When entering 3 1 2 3 you get:
Enter size: 3
1 2 3
1 2 3
Sum: 6