c++ - 无法为整数类型的变量赋值

Question

我想找出1到100之间的质数。我遇到的问题是当程序将值2赋值给整数类型的变量j时。变量 j 的值不变。有谁知道为什么会这样？

Create a program to find all the prime numbers between 1 and 100.

One way to do this is to write a function that will check if a number is prime (i.e., see if the number can be divided by a prime number smaller than itself) using a vector of primes in order (so that if the vector is called primes, primes[0]==2, primes[1]==3, primes[2]==5, etc.). Then write a loop that goes from 1 to 100, checks each number to see if it is a prime, and stores each prime found in a vector. Write another loop that lists the primes you found. You might check your result by comparing your vector of prime numbers with primes. Consider 2 the first prime.

#include <iostream>
#include <string>
#include <vector>

int main()
{
    std::vector<int> primes = { 2 };
    for (int i = 2; i <= 100; ++i) 
        for (int j = 2; j < i; ++j) 
            if (i % j == 0)
                break;
            else
                primes.push_back(j);
    for (int i = 0; i < primes.size(); ++i)
        std::cout << primes[i] << ' ';
}

Answer 1

在你最初的实现中有一些错误:

• 您在 primes vector 中添加元素的时间过早
• 你对素数没有明确的定义
• 您正试图将所有内容放在一个函数中

一些更干净的代码看起来有点像这样:

#include <iostream>
#include <string>
#include <vector>

namespace {
    bool isPrime(const std::vector<int> &previousPrimes, int possiblePrime) {
        for (auto prevPrime : previousPrimes)
            if (possiblePrime % prevPrime == 0)
               return false;

        return true;
    }
}

int main()
{
    auto primes = std::vector<int>({2});
    for (int i = 3 /*2 is already a prime*/; i <= 100; ++i)
        if (isPrime(primes, i))
           primes.push_back(i);

    for (auto prime : primes)
        std::cout << prime << ' ';
    std::cout << std::endl;
}

但是，由于这道题看起来像是家庭作业，所以不要盲目照搬，先尝试理解这里使用的所有概念。