js数组元素的添加和删除一直比较迷惑,今天l于扑ֈ详细说明的资料了Q先l个我测试的代码^-^
var arr = new Array();
arr[0] = "aaa";
arr[1] = "bbb";
arr[2] = "ccc";
//alert(arr.length);//3
arr.pop();
//alert(arr.length);//2
//alert(arr[arr.length-1]);//bbb
arr.pop();
//alert(arr[arr.length-1]);//aaa
//alert(arr.length);//1
var arr2 = new Array();
//alert(arr2.length);//0
arr2[0] = "aaa";
arr2[1] = "bbb";
//alert(arr2.length);//2
arr2.pop();
//alert(arr2.length);//1
arr2 = arr2.slice(0,arr2.length-1);
//alert(arr2.length);//0
arr2[0] = "aaa";
arr2[1] = "bbb";
arr2[2] = "ccc";
arr2 = arr2.slice(0,1);
alert(arr2.length);//1
alert(arr2[0]);//aaa
alert(arr2[1]);//undefined
shiftQ删除原数组W一,q返回删除元素的|如果数组为空则返?/span>undefined
var a = [1,2,3,4,5];
var b = a.shift(); //aQ[2,3,4,5] bQ?
unshiftQ将参数d到原数组开_q返回数l的长度
var a = [1,2,3,4,5];
var b = a.unshift(-2,-1); //aQ[-2,-1,1,2,3,4,5] bQ?
注:?span style="font-family: 'Times New Roman';">IE6.0下测试返回值MؓundefinedQFF2.0下测试返回gؓ7Q所以这个方法的q回g可靠Q需要用q回值时可用splice代替本方法来使用?/span>
popQ删除原数组最后一,q返回删除元素的|如果数组为空则返?span style="font-family: 'Times New Roman';">undefined
var a = [1,2,3,4,5];
var b = a.pop(); //aQ[1,2,3,4] bQ?//不用q回的话直接调用可以了
pushQ将参数d到原数组末尾Qƈq回数组的长?/span>
var a = [1,2,3,4,5];
var b = a.push(6,7); //aQ[1,2,3,4,5,6,7] bQ?
concatQ返回一个新数组Q是参数添加到原数l中构成?/span>
var a = [1,2,3,4,5];
var b = a.concat(6,7); //aQ[1,2,3,4,5] bQ[1,2,3,4,5,6,7]
splice(start,deleteCount,val1,val2,...)Q?/span>?span style="font-family: 'Times New Roman';">start位置开始删?span style="font-family: 'Times New Roman';">deleteCount,q从该位|v插入val1,val2,...
在清I数l时Q只需传?span style="font-family: 'Times New Roman';">startIndex?/span>
如果不删除所有元素,再传?span style="font-family: 'Times New Roman';">deleteCount参数?/span>
spliceq具有先删除后添加的功能Q即先删除几个元素,然后在删除的位置再添加若q元素,删除与添加的元素的个数没有必ȝ{,q时?span style="font-family: 'Times New Roman';">deleteCount也是要用到的?/span>
var a = [1,2,3,4,5];
var b = a.splice(2,2,7,8,9); //aQ[1,2,7,8,9,5] bQ[3,4]
var b = a.splice(0,1); //同shift
a.splice(0,0,-2,-1); var b = a.length;//?span style="font-family: 'Times New Roman';">unshift
var b = a.splice(a.length-1,1);//?span style="font-family: 'Times New Roman';">pop
a.splice(a.length,0,6,7); var b = a.length; //?span style="font-family: 'Times New Roman';">push
reverseQ将数组反序
var a = [1,2,3,4,5];
var b = a.reverse(); //aQ[5,4,3,2,1] bQ[5,4,3,2,1]
sort(orderfunction)Q?/span>按指定的参数Ҏ(gu)l进行排?/span>
var a = [1,2,3,4,5];
var b = a.sort(); //aQ[1,2,3,4,5] bQ[1,2,3,4,5]
slice(start,end)Q?/span>q回从原数组中指定开始下标到l束下标之间的项l成的新数组
var a = [1,2,3,4,5];
var b = a.slice(2,5); //aQ[1,2,3,4,5] bQ[3,4,5]
join(separator)Q?/span>数l的元素lv一个字W串Q以separator为分隔符Q省略的话则用默认用逗号为分隔符
var a = [1,2,3,4,5];
var b = a.join("|"); //aQ[1,2,3,4,5] bQ?1|2|3|4|5"
再给个利用数l模?span style="font-family: 'Times New Roman';">javaStringBuffer处理字符串的Ҏ(gu)Q?/span>
/**
* 字符串处理函?/span>
*/
function StringBuffer() {
var arr = new Array;
this.append = function(str) {
arr[arr.length] = str;
};
this.toString = function() {
return arr.join("");//?span style="font-family: 'Times New Roman';">appendq来的数l?span style="font-family: 'Times New Roman';">ping成一个字W串
};
}
今天在应用中H然发现join是一U把数组转换成字W串的好Ҏ(gu)Q故装成对象用了Q?/span>
/**
*把数l{换成特定W号分割的字W串
*/
function arrayToString(arr,separator) {
if(!separator) separator = "";//separator为null则默认ؓI?/span>
return arr.join(separator);
}
/**
* 查找数组包含的字W串
*/
function arrayFindString(arr,string) {
var str = arr.join("");
return str.indexOf(string);
}
如果仅仅是查找数据库中name不重复的字段Q很Ҏ(gu)
- SELECT min(`id`),`name`
- FROM `table`
- GROUP BY `name`;
但是q样q不能得到说有重复字D늚id倹{(只得C最的一个id|
查询哪些字段是重复的也容?/p>
- SELECT `name`,count(`name`) as count
- FROM `table`
- GROUP BY `name` HAVING count(`name`) >1
- ORDER BY count DESC;
但是要一ơ查询到重复字段的id|必M用子查询了,于是使用下面的语句来实现MySQL大表重复字段查询?/p>
- SELECT `id`,`name`
- FROM `table`
- WHERE `name` in (
- SELECT `name`
- FROM `table`
- GROUP BY `name` HAVING count(`name`) >1
- );
但是q条语句在mysql中效率太差,感觉mysqlq没有ؓ子查询生成零时表?/p>
于是使用先徏立零时表
- create table `tmptable` as (
- SELECT `name`
- FROM `table`
- GROUP BY `name` HAVING count(`name`) >1
- );
然后使用多表q接查询
- SELECT a.`id`, a.`name`
- FROM `table` a, `tmptable` t
- WHERE a.`name` = t.`name`;
l果q次l果很快出来了?/p>
?distinct去重?/p>
- SELECT distinct a.`id`, a.`name`
- FROM `table` a, `tmptable` t
- WHERE a.`name` = t.`name`;