质数

质数（素数）：除本身的绝对值外，不可能被大于1的整数除尽的数

• 双for实现

  function test(n) {
    var count = 0;
    if (n < 2) {
      return count;
    }
      
    for (var i = 2; i <= n; i++) {
      for (var j = 2; j < i; j++) {
        if (i % j == 0) {
          count++;
          break;
        }
      }
    }
    return n - count - 1;
  }
    
  var dt_start = new Date();
  var rst = test(1000000);
  console.log((new Date()) - dt_start, '运行时间');
  console.log(rst, '素数个数');
  

• 筛选法 ```javascript

function test2(n) { var count = 0; if (n < 2) { return 0; } var remove_obj = {}; for (var i = 2; i < n; i++) { if (remove_obj[i] == 0) { continue; } var tmp_value = 2; while ((tmp_value * i) <= n) { if (remove_obj[tmp_value * i] == 0) { tmp_value++; continue; } remove_obj[tmp_value * i] = 0; tmp_value++; count++; } } return n - count - 1; }

var dt_start = new Date(); var rst = test2(); console.log((new Date()) - dt_start, ‘运行时间ms’); console.log(rst, ‘素数个数’); ```