How to use if with array elements in c++ - c++

Question

I want to receive only positive numbers from the user which are less than or equal to 4
Here is my code:
#include <iostream>
using namespace std;
int main()
{
unsigned int n;
cin>>n;
unsigned int arr[n];
for (int i =0 ;i<n ; i++){
if (arr[i]<=4)
cin>>arr[i];
}
}
What am I doing wrong with my code? It receives numbers which are greater than 4 .

Answer 1

The behaviour of your code is undefined. You are reading elements of the array before writing to them.
And then you don't check the value inputted by the user.
Note also that variable length arrays are not standard C++ (unsigned int arr[n]; declares such an array). Use a std::vector instead?
Really the best way to sort out these problems is to use a good line-by-line debugger.

Answer 2

#include <iostream>
using namespace std;
int main() {
unsigned int input = 0u; // Holds user input
unsigned int nextPos = 0u; // Next free position in our array
unsigned int arraySize = 0u; // The size of the dynamic array
unsigned int* array = nullptr; // Pointer to our dynamic array
// Ask the user for the array size
cout << "Please Enter The Array Length" << endl;
cin >> arraySize;
// using array = int[arraySize] wouldn't work because the compiler won't know what value the number is at compile time.
// instead we use the 'new' keyword to allocate some memory while the program is running
// this is called dynamic allocation
array = new unsigned int[arraySize];
// The reason it's a while loop instead of a for loop is to only insert numbers into the array
// when the pass the rule if input <= 4u (the u part just means it's unsigned)
while(nextPos < arraySize) {
// Here we ask for an input and store it in the same named input variable
cout << "Please Enter A Valid Number" << endl;
cin >> input;
// Does the input pass the check?
if (input <= 4u) {
// If so store it in the current position in the array and increment
// the line below is the same as doing
// array[nextPos] = input;
// nextPos = nextPos + 1u;
array[nextPos++] = input;
} else {
// When the input doesn't meet the rule, complain about an invalid number
cout << input << " is an invalid number" << endl;
}
}
// A rule in C++ is that we need to manually release any memory we allocate using new
// it would be 'delete variable' for objects or 'delete[] variable' for arrays
delete[] array;
}