为了提供一个具体的例子，我们需要编写代码来验证一个数字是否是质数。现在有很多算法，例如：可以用埃拉托色尼的筛子(sieve of Eratosthenes)来分离质数和非质数。如果有很多验证数字，我们不希望对每一个数字都进行Eratosthenes筛选。我们想要做的是将所有质数一次制表，直到数字的上限，然后使用一个表查的方式，找来验证大量的数字。
"""
Generates C++ vector of prime numbers up to max_number
using sieve of Eratosthenes.
"""
import pathlib
import sys
# for simplicity we do not verify argument list
max_number = int(sys.argv[-2])
output_file_name = pathlib.Path(sys.argv[-1])
numbers = range(2, max_number + 1)
is_prime = {number: True for number in numbers}
for number in numbers:
current_position = number
if is_prime[current_position]:
while current_position <= max_number:
current_position += number
is_prime[current_position] = False
primes = (number for number in numbers if is_prime[number])
code = """#pragma once
#include <vector>
const std::size_t max_number = {max_number};
std::vector<int> & primes() {{
static std::vector<int> primes;
{push_back}
return primes;
}}
"""
push_back = '\n'.join([' primes.push_back({:d});'.format(x) for x in primes])
output_file_name.write_text(
code.format(max_number=max_number, push_back=push_back))
我们的目标是生成一个primes.hpp，并将其包含在下面的示例代码中:
#include "primes.hpp"
#include <iostream>
#include <vector>
int main() {
std::cout << "all prime numbers up to " << max_number << ":";
for (auto prime : primes())
std::cout << " " << prime;
std::cout << std::endl;
return 0;
}