Random numbers in the output - c++

Question

I've been coding in c++ for over a day now and I am currently stuck on this particular problem. I want to enter a integer and produce every prime numbers that are smaller than the given integer but the output always has random numbers that aren't prime. Here's the code:
#include <iostream>
#include <math.h>
using namespace std;
int check(int x){
bool prime = true;
if (x <= 1){
prime = false;
return 0;
}
for (int i = 2; i < x; i++){
if (x % i == 0){
prime = false;
return 0;
}
}
if (prime = true){
return x;
}
}
int main(){
int r;
cout << "Enter range: ";
cin >> r;
int primes[r];
int n = 0;
for (int i = 0; i < r; i++){
if (check(i) != 0){
primes[n] = i;
n++;
}
else{
}
}
for (int i : primes){
cout << i << endl;
}
}
Help me fix this problem. Thanks.

Answer 1

The problem in your code is that you check numbers in the range [0,r] and assume to find r prime numbers in that range. Thats of course not correct. For example when in the range [0,r] there are p prime numbers then your output will print the p prime numbers and then r-p uninitialized values (thats undefined behavior, the output of your code could be anything).
Change the last loop to
for (int i=0; i<n; ++i) {
cout << primes[i] << endl;
}
To see only entries of primes that you actually assigned a value in the loop before.
Also please consider that int primes[r]; is not standard C++. See here: Why aren't variable-length arrays part of the C++ standard?. Use std::vector for dynamically sized arrays. Actually in your code you do not need an array at all. You can simply print the prime numbers in the same loop where you check them:
if (check(i) != 0) { std::cout << i << "\n"; }
Moreover, your check is a little too complicated. You are using a flag that you don't need. The last if is wrong, because prime = true is assignment not comparison. However, that doesnt really matter because at that point prime is always true (if not you already returned), and prime=true does evaluate to true. The function can look like this:
int check(int x){
if (x <= 1){
return 0;
}
for (int i = 2; i < x; i++){
if (x % i == 0){
return 0;
}
}
return x;
}
This will also prevent the compiler from issuuing a warning for not returning from all branches. Note that you only need to check factors <= sqrt(x) because when there is a factor bigger than that there must also be a factor smaller than that.