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What this article brings to you is a summary (code) of methods for operating arrays in js. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.
1. Find the position of the element item in the given array arr
function indexOf(arr, item) {
if (Array.prototype.indexOf){ //判断原型中是否支持该方法
return arr.indexOf(item);
} else {
for (var i = 0; i < arr.length; i++){
if (arr[i] === item){
return i;
}
}
}
return -1;
}2. Calculate the sum of all elements in the given array arr
//forEach遍历：
function sum(arr) {
var s = 0;
arr.forEach(function(val, idx, arr) {
s += val;
}, 0);
return s;
};
//eval:
function sum(arr) {
return eval(arr.join("+"));
};3. Remove all elements in the array arr whose values are equal to item. Do not modify the array arr directly. The result will be a new array
//splice()
function remove(arr,item){
var newarr = arr.slice(0);
for(var i=0;i4. Remove all elements in the array arr whose values are equal to item and operate directly on the given arr array. , and return the result
//splice
function removeWithoutCopy(arr, item) {
for(var i=0; i5. Add the element item at the end of the array arr. Do not modify the array arr directly. The result will be a new array
//普通的迭代拷贝
var append = function(arr, item) {
var length = arr.length,
newArr = [];
for (var i = 0; i < length; i++) {
newArr.push(arr[i]);
}
newArr.push(item);
return newArr;
};//使用slice浅拷贝+push组合
function append(arr, item) {
var newArr=arr.slice(0);
newArr.push(item);
return newArr;
}6. Delete the last element of the array arr. Do not modify the array arr directly, the result will be a new array
//利用slice
function truncate(arr) {
return arr.slice(0,-1);
}
//利用filter
function truncate(arr) {
return arr.filter(function(v,i,ar) {
return i!==ar.length-1;
});
}
//利用push.apply+pop
function truncate(arr) {
var newArr=[];
[].push.apply(newArr, arr);
newArr.pop(); return newArr;
}
//利用join+split+pop 注意！！！：数据类型会变成字符型
function truncate(arr) {
var newArr = arr.join().split(',');
newArr.pop();
return newArr;
}7. Add the element item at the beginning of the array arr. Do not modify the array arr directly. The result will be a new array
//利用concat
function prepend(arr, item) {
return [item].concat(arr);
}
//使用push.apply
function prepend(arr, item) {
var newArr=[item];
[].push.apply(newArr, arr);
return newArr;
}
//利用slice+unshift/splice
function prepend(arr, item) {
var newArr=arr.slice(0);
newArr.unshift(item);//newArr.splice(0,0,item);
return newArr;
}8. Delete the first element of the array arr. Do not modify the array arr directly, the result will be a new array
//利用slice
function curtail(arr) {
return arr.slice(1);
}
//利用filter
function curtail(arr) {
return arr.filter(function(v,i) {
return i!==0;
});
}
//利用push.apply+shift
function curtail(arr) {
var newArr=[];
[].push.apply(newArr, arr);
newArr.shift();
return newArr;
}9. Merge array arr1 and array arr2. Do not modify the array arr directly. The result will be a new array
//利用concat
function concat(arr1, arr2) {
return arr1.concat(arr2);
}
//利用slice+push.apply
function concat(arr1, arr2) {
var newArr=arr1.slice(0);
[].push.apply(newArr, arr2);
return newArr;
}
//利用slice+push
function concat(arr1, arr2) {
var newArr=arr1.slice(0);
for(var i=0;i10. Add the element item at the index of the array arr. Do not modify the array arr directly, the result will be returned as a new array
//利用slice+splicefunction insert(arr, item, index) {
var newArr=arr.slice(0);
newArr.splice(index,0,item); return newArr;
}//利用push.apply+splicefunction insert(arr, item, index) {
var newArr=[];
[].push.apply(newArr, arr);
newArr.splice(index,0,item); return newArr;
}11. Count the number of occurrences of elements in the array arr whose value is equal to item
//filter()-->利用指定的函数确定是否在返回的数组中包含某一项
function count(arr, item) {
var count = arr.filter(function(a) {
return a === item; //返回true的项组成的数组
});
return count.length;
}
//map()-->对数组中的每一项进行给定函数，
//返回每次函数条用的结果组成的数组；
function count(arr, item) {
var count = 0;
arr.map(function(a) {
if(a === item) {
count++;
}
});
return count;
}//forEach()-->对数组中的每一项运行传入的函数
function count(arr, item) {
var count = 0;
arr.forEach(function(a) {
a === item ? count++ : 0;
});
return count;
}12. Find the repeated elements in the array arr
//两次遍历
function duplicates(arr) {
//声明两个数组，a数组用来存放结果，b数组用来存放arr中每个元素的个数
var a = [],b = [];
//遍历arr，如果以arr中元素为下标的的b元素已存在，则该b元素加1，否则设置为1
for(var i = 0; i < arr.length; i++){
if(!b[arr[i]]){
b[arr[i]] = 1;
continue;
}
b[arr[i]]++;
}
//遍历b数组，将其中元素值大于1的元素下标存入a数组中
for(var i = 0; i < b.length; i++){
if(b[i] > 1){
a.push(i);
}
}
return a;
}
//先排序，如果后一个与前一个相等且未保存，则保存。
function duplicates(arr) {
var a=arr.sort(),b=[];
for(var i in a){
if(a[i]==a[i-1] && b.indexOf(a[i])==-1) b.push(a[i]);
}
return b;
}
//先排序然后再判断function duplicates(arr) {
var new_arr = arr.sort();//先把arr排序
var res = [] ;//目标容器
for( var i = 0 ; i < new_arr.length ; i++){
if(new_arr[i] == new_arr[i+1] &&
new_arr[i] !=new_arr[i-1]){//判断是否重复,是否已经放入容器
res.push(new_arr[i]);
}
}
return res;13. Find the second power of each element in the array arr. Do not modify the array arr directly, the result will be a new array
function square(arr) {
//声明一个新的数组存放结果
var a = [];
arr.forEach(function(e){
//将arr中的每一个元素求平方后，加入到a数组中
a.push(e*e);
});
return a;
}
// 使用map
function square(arr) {
return arr.map(function(e) {
return e * e;
})
}
// ES6箭头函数版
const square = arr => arr.map(e => e * e);14. In the array arr, find all positions where the element whose value is equal to item appears
//forEach
function findAllOccurrences(arr, target) {
var a=[];
arr.forEach(function(e,index){
if(e==target)
a.push(index);
})
return a;
}
function findAllOccurrences(arr, target) {
var temp = [];
arr.forEach(function(val,index){
val !== target || temp.push(index);
});
return temp;
}//filter
function findAllOccurrences(arr, target) {
var result=[];
arr.filter(function(item,index){
return item===target&&result.push(index);
});
return result;
}Related recommendations:
Summary of js array operation methods (must read)
Explain the basic methods of Js array operation
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