The target

  • Know why you have arrays (multiple related elements)
  • Ability to create arrays (two ways)
  • The ability to get elements of an array (arR [subscript])
  • Ability to iterate over arrays (for loop)
  • Can add an element to an array
  • Can independently complete bubble sort cases

directory

  • The concept of arrays
  • Create an array
  • Gets the elements of an array
  • Through the array
  • New element in array
  • An array of cases

1. The concept of arrays

What is an array

A: An array is a collection of data, each of which is called an element. An array can hold any type of element. Arrays are an elegant way to store a set of data under the name of a single variable.

2. Create an array

There are two ways to create arrays in JS:

  • Create an array using new
  • Create arrays with array literals (common)
  • Array literals are square brackets []
  • Declaring an array and assigning a value to it is called array initialization
  • Arrays can hold any type of data, such as strings, numbers, booleans, etc.
var arr = new Array(); // Create an empty arrayCopy the code
var arr = []; // Create an empty arrayCopy the code

3. Get the array element

3.1 Index of array

Index (subscript) : The index used to access the elements of an array (array subscripts start at 0).

An array can be accessed, set, or modified by an index. We can obtain the elements of an array by name.

Access here means get

4. Go through the numbers

How do you get them all out?

As you can see from the code, the code repeats as each element is fetched from the array, except that the index values are increasing

The answer is loop

Traversal: The process of going through each element of the array once (similar to our morning roll call).

Var arr = [6]; for (var i =0; i<6; i++) { console.log(arr[i]); }Copy the code

4.1 Array length

Array name.length is used to access the number of elements in the array (array length).

Dynamically monitor the number of array elements

Exercise: Find the largest value in an array

Set a Max value, take the first element and compare it with the next element

Specific steps:

  • Declare a variable Max that holds the largest element.
  • The default maximum value can be the first element in the array.
  • Iterate through the array and compare each element to Max.
  • If the array element is greater than Max, store the array element in Max, otherwise proceed to the next round of comparison.
  • Final output Max
Var arr =,6,1,77,52,25,7 [2]; var max=arr[0] ; for (var i=1 ; i <arr.length; i++) { if(max <arr[i] ) { max = arr[i]; }} console.log(' Max is '+ Max);Copy the code

5. Add an element to the array

You can add array elements by length and index number

5.1 Adding an array element by Changing the Length

  • You can modify the length to expand the array
  • The length attribute is read-write

The default value of a declared variable is undefined.

5.2 Adding an Array Element by modifying an Array index

  • You can append array elements by changing the array index
  • You cannot assign to the name of the array directly, otherwise the previous data will be overwritten

Practice finding numbers greater than 10

The first way is to declare a variable j, and let the variable j++

Var arr = [2, 0, 6,1,77,0,52,0,25,7]; var newarr = []; var j =0 ; for (var i = 0 ; i < arr.length ; i++) { if ( arr[i]>=10) { newarr[j] = arr[i]; j++; } } console.log(newarr);Copy the code

Second, use newarr.length

newarr[newarr.length] = arr[i];
Copy the code

The key is that the new array index starts at 0

6. Array cases

6.1 Deleting a specified array element

Requirement: remove the zeros from the array [2,0,6,1,77,0,52,0,25,7] to form a new array that does not contain zeros.

Simple array de-duplication

Analysis:

  • A new array is required to hold the filtered data.
  • Iterate through the original array and add data that is not 0 to the new array (remember to accept data in the array name + index format).
  • The number in the new array is incremented by length.
Var arr =,0,6,1,77,0,52,0,25,7 [2]; var newarr = []; for (var i = 0 ; i<arr.length ; i++) { if (arr[i] ! =0 ){ newarr [ newarr.length]=arr [i]; } } console.log(newarr);Copy the code

6.2 Flipping an Array

Requirement: Store the contents of array [‘red’, ‘green’; ‘blue’, ‘pink ‘, ‘Purple ‘] in reverse.

Output :[‘purple’, ‘pink ‘; ‘blue’, ‘green’; ‘red’]

Ideas:

Reverse the contents of array [“red ‘, ‘green’, ‘blue’, ‘pink ‘, ‘Purple ‘]

  1. Declare a new array, newArr
  2. Return the 4th element of the old array index (arr.leng-1) and the 0th element of the new array index (newarr.length).
  3. We’re going to go down I —
Var arr =,0,6,1,77,0,52,0,25,7 [2]; var newarr = []; for (var i = arr.length-1 ; i>=0 ; i--) { newarr[newarr.length]= arr[i] ; } console.log(newarr);Copy the code

6.3 Bubble Sort

Bubble sort: is an algorithm that displays a series of data in a certain order (from smallest to largest or from largest to smallest).

For example, we can sort the elements in the array [5,4,3,2,1] from smallest to largest, with output: 1,2,3,4,5

  1. So the total number of trips we need to take is the outer for loop

Array length minus 1 arr. Length – 1 2. So the number of swaps per trip we use the inner for loop

There are four exchanges on the first trip

The second trip is three times

On the third trip, I switch twice

There will be one exchange on the fourth trip

The length is the length of the array minus the number of times, but our number of times starts at zero, so eventually arr. Length -i -1 3. Just swap two variables

var arr = [5, 4, 3, 2, 1]; for (var i = 0; i <= arr.length - 1; I++) {// for (var j = 0; j <= arr.length - i - 1; If (arr[j] > arr[j + 1]) {var temp = arr[j]; arr[j] = arr[j + 1]; arr[j + 1] = temp; } } } console.log(arr);Copy the code