javascript - 质数测试 javascript 的问题

Question

我是一名初学者，正在尝试完成素数测试，但遇到了问题。这是我所拥有的:

var n = Number(prompt("Input the number you want to check for prime:"));
var i;


if (n < 2) {
    alert(n + " is not a prime number.");
}
for (var i = 2; i <= Math.sqrt(n); i++) {
    if (n % i === 0) {
        alert(n + " is not a prime number.");
        break;
    }

    else {
        alert(n + " is a prime number."); 
        break;
    }
}

它运行正常，除了如果我输入 3 或 2 时不会弹出警报，并且任何带有 3 的数字都会作为素数返回，即使它不是素数。除此之外，我的所有测试都有效。

Answer 1

啊啊，你的问题是你构建循环的方式。无论检查是否完成，你都会爆发。

var n = Number(prompt("Input the number you want to check for prime:"));
var i;
var isPrime = true;

if (n < 2) {
    isPrime = false;
} else if (n < 4 && n >= 2) {
    // isPrime is already true
} else if (n % 2 === 0) {
    isPrime = false; // no divisor of 2 can be prime
} else {
  var sqrtN = Math.sqrt(n);
  for (var i = 3; i <= sqrtN; i = i + 2) {
      if (n % i === 0) {
          // Only break out of the loop if a match is found
          isPrime = false;
          break;
      }
  }
}
if (isPrime) {
    alert(n + " is a prime number.");
} else {
    alert(n + " is not a prime number.");
}

或者，也许是更有组织的解决方案:

function isPrime (n) {
  n = parseInt(n);
  var i;
  if (Number.isNaN(n)) {
    return false;
  } else if (n < 2) {
    // 1 is not prime
    return false;
  } if (n < 4) {
    // 2 and 3 are prime, so why not skip checking them?
    return true;
  } else if (n % 2 === 0) {
    // No number divisible by 2 is prime.
    return false;
  } else {
    // This won't change, so calculate it once as suggested by Weather Vane.
    var sqrtN = Math.sqrt(n);
    // 4, 6, 8... All divisible by 2, and would be caught by initial check.
    for (i = 3; i < sqrtN; i = i + 2) { 
      // Not a prime if it's evenly divisible. 
      if (n % i === 0) {
        return false; 
      }
   }
   // Otherwise prime.
   return true; 
  }
}