Primed for Action

3 minutes to read

We are given the following website to write a coding solution:

Primed for Action 1

Problem

We need to write a program that is able find two prime numbers in a list of numbers and multiply these two. For example:

"2 6 7 18 6" -> "14"

Solution

Since we are allowed to use C, C++, Python and Rust, let’s write a solution on every language, just because.

C

#include <stdbool.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

bool is_prime(int n) {
  if (n == 2) {
    return true;
  } else if (n < 2 || n % 2 == 0) {
    return false;
  }

  for (int i = 3; i < n / 2; i += 2) {
    if (n % i == 0) {
      return false;
    }
  }

  return true;
}

int main() {
  // take in the numbers
  char input[4096] = { 0 };
  fgets(input, sizeof(input), stdin);
  int numbers[1024] = { 0 };

  char* token = strtok(input, " ");
  int i = 0;

  while (token != NULL) {
    numbers[i++] = atoi(token);
    token = strtok(NULL, " ");
  }

  // calculate answer
  int answer = 1;

  while (--i) {
    if (is_prime(numbers[i])) {
      answer *= numbers[i];
    }
  }

  // print answer
  printf("%d\n", answer);

  return 0;
}

C++

#include <iostream>
#include <sstream>
#include <vector>

bool is_prime(int n) {
  if (n == 2) {
    return true;
  } else if (n < 2 || n % 2 == 0) {
    return false;
  }

  for (int i = 3; i < n / 2; i += 2) {
    if (n % i == 0) {
      return false;
    }
  }

  return true;
}

int main() {
  // take in the numbers
  std::string s;
  std::vector<int> numbers;
  int n;
  
  std::getline(std::cin, s);
  std::istringstream iss(s);

  while (iss >> n) {
    numbers.push_back(n);
  }

  // calculate answer
  int answer = 1;

  for (int n : numbers) {
    if (is_prime(n)) {
      answer *= n;
    }
  }

  // print answer
  std::cout << answer << std::endl;

  return 0;
}

Python

def is_prime(n: int) -> bool:
    if n == 2:
        return True
    if n < 2 or n % 2 == 0:
        return False
    for i in range(3, n // 2, 2):
        if n % i == 0:
            return False
    return True


# take in the number
numbers = list(map(int, input().split()))

# calculate answer
primes = [n for n in numbers if is_prime(n)]
answer = primes[0] * primes[1]

# print answer
print(answer)

Rust

use std::io;

fn is_prime(n: &i32) -> bool {
    if *n < 2 || *n % 2 == 0 {
        return false;
    } else if *n == 2 {
        return true;
    }

    for i in (3..=n / 2).step_by(2) {
        if n % i == 0 {
            return false;
        }
    }

    return true;
}

fn main() {
    // take in the numbers
    let mut input = String::new();

    io::stdin()
        .read_line(&mut input)
        .expect("Failed to read line");

    let numbers: Vec<i32> = input
        .split_whitespace()
        .map(|s| s.parse().expect("Please enter a valid number"))
        .collect();

    // calculate answer
    let answer: i32 = numbers.into_iter().filter(is_prime).product();

    // print answer
    println!("{}", answer);
}

Flag

Using any of these programs, we are able to solve the chalenge and capture the flag:

HTB{pr1m3_Pr0}