用于检查素数的 C++ 代码不起作用

我在使用此 C++ 代码时遇到问题。整数num是一个我想检查它是否是质数的数字。然而这个程序总是返回 false。这可能很简单，但我找不到任何东西。

for(int i=2;i<num;i++){ //primes are allowed to be divided by 1 so we start at 2
        if(num % i == 0){ //can be divided by a number other than itself or 1 so we trip out
            return false;
        } else if(i == num){ //if we've already done checks as high as possible and not tripped out yet then report success
            return true;
        }
}

i == num永远不会发生，因为你的循环条件是i<num. Try:

for(int i=2;i<num;i++){ //primes are allowed to be divided by 1 so we start at 2
    if(num % i == 0){ //can be divided by a number other than itself or 1 so we trip out
        return false;
    } else if(i == num-1){ //if we've already done checks as high as possible and not tripped out yet then report success
        return true;
    }
}

如下文所指出的，else这里的条件是多余的，你只需要检查从2到sqrt(num)- 因为其余因素已经检查过。

根据您想要解决问题的复杂程度，还可以进行更多改进。现实中大多数方法都使用概率算法。