Web 基础学习之 JS ，循环练习

1.判断闰年 

var i;
for(i=1980;i<=2016;i++)
{
  if( i % 4 == 0 && i % 100 != 0 || i % 400 == 0)
  document.write( i + " ");
}

2.九九乘法表

var i, j;
for(i=0;i<10;i++)
{
  for(j=1;j<=i;j++){
    document.write( i + "*"+ j +"=" +i*j );
    document.write("  ");
  }
document.write("<br/>");
}

3.100-999的水仙花：

         第一种方法：

var i,a = 0,b =0,c = 0;
for(i = 100;i < 1000;i++){
   a = parseInt(i / 100);
   b = parseInt(i / 10 - a * 10);//b = parseInt(i /%100 /10);
   c = parseInt(i % 10);
  if( i==(a * a * a + b * b * b + c * c * c))
document.write( i + " ");
}

        第二种方法：

for(var i = 1;i < 10;i++){
     for(var j = 0;j < 10;j++){
        for(var k=0; k < 10;k++){
           if(i * 100 + j * 10 + k == i * i * i + j * j * j + k * k * k){
           document.write(i * 100 + j * 10 + k+" ");
          }
       }

    }
}

4. 1+3+5+7.......+99

var i,sum=0;
document.write("1+3+5+7.......+99的结果为:");
for(i = 0;i < 100;i++){
   if(i % 2 != 0 ){
   sum+=i;
   }
}
document.write(sum+ " ");

5.100以内的阶乘

var i,sum=1;
document.write("100以内的阶乘结果为:");
for(i = 1;i <= 100;i++){
   sum*=i;
}
document.write(sum+ " ");

6.打印三角形

for( var i = 0; i < 6; i++){
   for( var j = 0; j < 5 - i ;j++ ){
       document.write(" ");
   }
   for( var j = 0; j < i ;j++ ){
      document.write("*");
   }
document.write("<br/>");

}

7.菱形

for( var i = 0; i < 6; i++){
     for( var j = 0; j < 5 - i ;j++ ){
        document.write(" ");
     }
     for( var j = 0; j < 2*i+1 ;j++ ){
        document.write("*");
     }
document.write("<br/>");

}

for( var i = 0; i < 5; i++){    for( var j = 0; j < i+1;j++ ){       document.write(" ");    }    for( var j = 0; j < 2*(4-i)-1 ;j++ ){        document.write("*");    }document.write("<br/>");}

      8.空菱形

for( var i = 0; i < 6; i++){
   for( var j = 0; j < 5 - i ;j++ ){
      document.write(" ");
   }
   for( var j = 0; j < 2 * i + 1 ;j++ ){
     if(j == 0 || j == 2 * i)
        document.write("*");
     else{
     document.write(" ");
     }
   }
document.write("<br/>");

}

for( var i = 0; i < 5; i++){    for( var j = 0; j < i + 1;j++ ){      document.write(" ");    }    for( var j = 0; j < 2 * (4 - i)-1 ;j++ ){      if(j == 0 || j == 2 * (4 - i)-2)          document.write("*");     else{         document.write(" ");      }     }document.write("<br/>");}

9.判断今天是星期几

var now = new Date();
var day = now.getDay();
var str = " ";
switch(day){
<span style="white-space:pre"></span>case 1:
<span style="white-space:pre"></span>str = "星期一";
<span style="white-space:pre"></span>break;
<span style="white-space:pre"></span>case 2:
<span style="white-space:pre"></span>str = "星期二";
<span style="white-space:pre"></span>break;
<span style="white-space:pre"></span>case 3:
<span style="white-space:pre"></span>str = "星期三";
<span style="white-space:pre"></span>break;
<span style="white-space:pre"></span>case 4:
<span style="white-space:pre"></span>str = "星期四";
<span style="white-space:pre"></span>break;
<span style="white-space:pre"></span>case 5:
<span style="white-space:pre"></span>str = "星期五";
<span style="white-space:pre"></span>break;
<span style="white-space:pre"></span>case 6:
<span style="white-space:pre"></span>str = "星期六";
<span style="white-space:pre"></span>break;
<span style="white-space:pre"></span>case 7:
<span style="white-space:pre"></span>str = "星期天";
<span style="white-space:pre"></span>break;
}
document.write("今天是：" + str);

10.空三角

for( var i = 0; i < 6; i++){
    for( var j = 0; j < 5 - i ;j++ ){
       document.write(" ");
    }
    for( var j = 0; j < 2 * i + 1 ;j++ ){
        if(j == 0 || j == 2 * i)
           document.write("*");
<span style="white-space:pre"></span>else{
<span style="white-space:pre"></span>    document.write(" ");
<span style="white-space:pre"></span>}
     }
document.write("<br/>");
<span style="white-space:pre"></span>if(i == 5)
<span style="white-space:pre"></span>{
<span style="white-space:pre"></span>for( var j = 0; j < 2 * i + 1 ;j++ )
<span style="white-space:pre"></span>document.write("*");
        }

}

11.1-1/2+1/3-1/4+........-1/100 =  1+(-1/2)+1/3+(-1/4).....+(-1/100)

var sum = 0;
for(var i = 1;i <= 100; i++){
  if(i % 2 == 0){
<span style="white-space:pre"></span>sum = sum + -1 * (1 /i ) ;
   }
  else {
    sum = sum + 1 / i ;
  }
}
document.write(sum);