Problems with for loop c++ - c++

Question

i used to code in javascript, but my new school force me to learn c++.
Im kind new in this language, and here's the problem:
In javascript i can write such a code:
for(let i = 0; i < 10; i++){
var variable[i] = i+3;
}
for(let j = 0; j < 10; j++){
console.log(variable[j]);
}
You may say 'Why don't you just write the code into 1 for loop', but that's only example.
And now i'm trying to rewrite above code to cpp:
int n,k,w;
cin>>n>>k;
for(int i = 0; i < n; i++){
int w[i];
cin>>w[i];
}
//some code here
for(int i = 0; i < n; i++){
cout<<w[i];
}
And here's the question. How can i cout all variables w with index i, cause im getting an error [Error] invalid types 'int[int]' for array subscript.

Answer 1

What you probably want is:
#include <iostream>
#include <vector>
int main() {
std::vector<int> vec;
int size = 0;
std::cin >> size;
for(int i = 0; i < size; i++){
int number = 0;
std::cin >> number;
vec.push_back(number);
}
for(int i : vec){
std::cout << i << " ";
}
}
std::vector<int> is a class designed to provide an interface to resizable array. The push_back() function appends the vector with given argument.
The last loop, which is called a ranged-based for(), is used to print all elements of the vector. You can replace it with plain old for() loop with indexing, since std::vector supports operator [], but if ranged-based approach is sufficient, it should be preferred.
EDIT: I don't know JavaScript, but I assume (from your example) that variables declared inside loops are visible everywhere. This is not the case in C++. Variables' visibility is dependent on the scope they are declared in. If you want your list/array/vector/any other container to be visible to those two for() loops, you have to declare it outside them - like in my example above.
EDIT2: While you should almost always use std::vector for such tasks, one could argue that they want to disable resizing the container. In this case, we are left with simple dynamic allocation. We reach for <memory> library, since we shouldn't manage it ourselves:
#include <iostream>
#include <memory>
int main() {
int size = 0;
std::cin >> size;
auto arr = std::make_unique<int[]>(size);
for(int i = 0; i < size; i++){
int number = 0;
std::cin >> number;
arr[i] = number;
}
for(int i = 0; i < size; i++){
std::cout << arr[i] << " ";
}
}
For auto, either read here or imagine that it's just a magic type that is (almost always) correct. It's like var in Python or JavaScript (but later on its type cannot be changed).
For std::unique_ptr<T[]>, either read here or imagine that it's just a dynamically allocated array that automatically delete[]s itself. If you did not learn about dynamic allocation yet, simply ignore what that means and wait until it's introduced.
Notice that we also got rid of the ranged-based for() loop. Unfortunately, it does not work with plain, dynamically allocated arrays.

Answer 2

Not all compilers support VLA so stick to the Standards; always specify a constant size for arrays. If you need some dynamically changed size then consider using another type of containers like: std::vector.
Also why you re-declared int w inside the for loop? It is local to for loop and changes to it won't affect the outer one.
You get a compile time error in the second loop that complains that w is not an array. To solve it make int w[] outer and on top of the two loops.
int n, k;
cin >> n >> k;
int w[n];
for(int i = 0; i < n; i++)
cin >> w[i];
//some code here
for(int i = 0; i < n; i++)
cout << w[i];
The alternative to VAL use std::vector:
std::vector<int> w;
int n, k;
std::cin >> n;
for(int i(0); i!= n; ++i){
std::cin >> k;
w.push_back(k);
}
for(auto i(0); i != w.length(); ++i)
std::cout << w[i] << ", ";