(C++) Program to print all prime numbers in the a Fibonacci sequence - c++

Question

can someone help me out? I've been trying to get this program to print and add all prime numbers in the Fibonacci Sequence below 1000. Just typing the regular Fibonacci code works fine and will list the numbers 1 - 987.
However, the moment I put in a prime number checker it all of a sudden stops at 5 (printing "1 1 2 3 5" which is technically correct since they all fall under what a prime is (though 1 doesn't count). However I'm looking to see ALL prime numbers from 1 - 987 in the sequence, and no matter what I do I can't seem to get it to work.
My code's down below, don't mind the lack of a main function, I'm making this function as a part of a bigger program, but it can stand on its own. Currently testing it by just calling it in the main function.
#include <iostream>
using namespace std;
void primethousand() {
int fibo = 1;
int nacci = 1;
int fibonacci = 0;
int fibosum = 0; //used to get the sum of all printed numbers later, once this issue is fixed.
int pchk = 0; /*primecheck, used to check if a number is a prime or not. 1 means not prime, 0 means prime*/
cout << "\nPrime Fibonacci #s under 1000: \n\n";
for (int ctr = 1; fibonacci < 987; ctr++) {
if (ctr == 1) {
cout << fibo << " ";
continue;
} else if (ctr == 2) {
cout << nacci << " ";
continue;
}
fibonacci = fibo + nacci;
fibo = nacci;
nacci = fibonacci;
//cout << fibonacci << " ";
for (int chr = 2; chr < fibonacci; chr++) {
if (fibonacci % chr == 0) {
pchk = 1;
}
}
if (pchk == 0) {
cout << fibonacci << " ";
}
}
}

Answer 1

You should break up the big task into smaller tasks by using functions.
Additionally, the fibonacci sequence is growing strongly exponential. So, there are not so many numbers that can be calculated in C++ standard data types. For example, even the biggest 8 byte unsigned long long or uint64_t can hold only the 94th element of the Fibonacci series.
For Fibonaccis below 1000, it will be just 16 elements.
So, we can easily precalculate all vaues during compile time (so, not during runtime). This will be the fastest possible solution. Also the compile time will be very short. And the memory consumption will be very low.
Please see:
#include <iostream>
#include <array>
#include <cstdint>
// For fibonacci number < 1000, 16 of the series elements will be sufficient
constexpr std::size_t ArraySize{ 16 };
// Calculate all 16 needed fibonacci number during compile time
consteval auto CreateFibonacciNumberArray() {
std::array<std::uint64_t, ArraySize> fs{ 1, 1 };
for (std::size_t i{ 2 }; i < ArraySize; ++i)
fs[i] = fs[i - 1] + fs[i - 2];
return fs;
}
// This is an array with the 16 fibonacci numbers. It is an compile time array
constexpr auto FIB = CreateFibonacciNumberArray();
// Compiletime function to calculate, if fibonacci numbers are prime
constexpr bool isPrime(const std::uint64_t number) {
if (number % 2 == 0 or number <= 2) return false;
for (std::uint64_t i = 3; (i * i) <= number; i += 2)
if (number % i == 0) return false;
return true;
}
// Create a compile time array, to indicate, if a fibnacci number is prinme
consteval auto IsPrime(const std::array<std::uint64_t, ArraySize>& FIB) {
std::array<bool, ArraySize> primeFibonacci{};
for (std::size_t i{}; i < ArraySize; ++i)
primeFibonacci[i] = isPrime(FIB[i]);
return primeFibonacci;
}
// Boolean compile time array that shows, if a fibonacci number is prime
constexpr auto FibIsPrime = IsPrime(FIB);
int main()
{
for (std::size_t i{}; i < ArraySize; ++i) {
std::cout << FIB[i];
if (FibIsPrime[i]) std::cout << "\tis prime";
std::cout << '\n';
}
}

Answer 2

It looks like once pchk is set to 1, you never set it back to zero, so further primes are never noticed..