JS foundation 3

Array

Definition ： A collection that stores an ordered sequence of numbers

Array creation

• Constructors ：
  new Array()
var arr=new array()
• Literal
  var arr=[]// An empty array
  notes ： Access array element index numbers - The array subscript （ Each array element corresponds to an index number , Array element index numbers are from 0 Start ）

Array common methods

 grammar ： Array name . Method ()

Array name	usage
push()	Array add elements
unshift()	Insert the first element
pop()	Delete the end of the element
shift()	Delete the front of the element
splice()	Delete any element of the array
sort()	Sort array
concat()	Array splicing
indexof（）	Returns the numeric index number
findeIndex	Returns the numeric index number
join()	Link each item of the array , Become a string
includes()	Check whether the array contains the contents in parentheses
reverse	Inversion array

notes ：
splice（1,2,3） Indicates that there are three values , The first one means to delete the element index number , The second represents the deletion length , The third replacement element
sort（function(a,b){return a-b}) Indicates normal sorting , That is, dictionary order
sort（function(a,b){return b-a}) Indicates reverse sort
indexof（） If the element does not exist , return -1
Sample code

<script> function scoreDuan() { 
    
  var numArr = [];
  var scoreArray = [98, 78, 89, 77, 69, 59, 85, 100, 93, 81];
   var a = 0,
       b = 0,
 	   c = 0,
       d = 0,
       e = 0,
       f = 0,
       g = 0,
       h = 0,
       j = 0,
       k = 0,
       m = 0;
    for (let i = 0; i < scoreArray.length; i++) { 
    
    //scoreArray.length The length of the array  if (scoreArray[i] == 100) { 
    
                    a = a + 1
       } else if (scoreArray[i] >= 90) { 
    
                    b = b + 1
       } else if (scoreArray[i] >= 80) { 
    
                    c = c + 1
       } else if (scoreArray[i] >= 70) { 
    
                    d = d + 1
       } else if (scoreArray[i] >= 60) { 
    
                    e = e + 1
       } else if (scoreArray[i] >= 50) { 
    
                    f = f + 1
       } else if (scoreArray[i] >= 40) { 
    
                    g = g + 1
       } else if (scoreArray[i] >= 30) { 
    
                    h = h + 1
       } else if (scoreArray[i] >= 20) { 
    
                    j = j + 1
       } else if (scoreArray[i] >= 10) { 
    
                    k = k + 1
       } else if (scoreArray[i] >= 0) { 
    
                    m = m + 1
       }


  }

     numArr.push(a, b, c, d, e, f, g, h, j, k, m)
     console.log(numArr)
        }
     scoreDuan() // The output is 1,2,3,2,1,1,0,0,0,0,0 function delArr() { 
    
   var scoreArray = [89, 78, 69, 89, 90, 69, 81];
      for (let i = 0; i < 7; i++) { 
    

         for (let k = i + 1; k < 7; k++) { 
    
          //k=i+1, Express K And i It's not equal , The array does not compare itself with itself  if (scoreArray[i] == scoreArray[k]) { 
    

               scoreArray.splice(i, 1)
               // If the arrays are equal, delete the array 
                    }
                }

            }

       console.log(scoreArray)
        }
 delArr() // The result is 89,78,69,90,81 function orderArr() { 
    
      var scoreArray = [98, 78, 86, 59, 94];
      scoreArray.unshift(90);
      // Add a... At the beginning 90
      for (i = 0; i < scoreArray.length; i++) { 
    
        if (scoreArray[i] < 60) { 
    
            scoreArray.splice(i, 1)
            // Delete less than 60 Number of numbers 
                }
            }
      console.log(scoreArray.sort())
      // Output... In sequence 
        }
     orderArr()
     // The result is 78,86,90,94,98
    </script>		

Storage between data types

1. Basic data variables are stored in the stack area
2. Reference variables are stored in the stack area
3. Complex data types are stored in the heap area
   

Traversal function

1. for loop

var arr=[98,78,58,90,95,49]
for(i=0;i<arr.length;i++){ 
    
    console.log(arr[i])
    // result 98,78,58,90,95,49
}    

2. forEach: Traverse the output

var arr =[98,78,58,90,95,49]; arr.forEach(function(item,index,arr){ 
    
            console.log(item,index)
        })
     // result 	98 0
  			78 1
	 		58 2
			90 3
			95 4
 			49 5

3. map： Traverse the output
   notes ： And forEach The difference is that a new array is output
4. filter: Filter by output condition

function fun1_1(){ 
    
    var arr =[98,78,58,90,95,49]; var newArr=arr.filter(function(item){ 
    
        return item>=60;
         // The judgment condition is greater than 60 Output        
            })
        console.log(newArr)
        }
  fun1_1()
  // result 98,78,90,95

5. find: You can also filter by conditions , But only the first value that satisfies the condition is returned

function fun2_1(){ 
    
     var arr =[98,78,58,90,95,49]; var newArr=arr.find(function(item){ 
    
          return item<60;
             })
          console.log(newArr)
   }
       fun2_1()
       // result 58  

6. some: As long as there is one in the array , return true

 function fun5_1(){ 
    
    var arr =[98,78,58,90,95,49]; let newArr=arr.some(function(item){ 
    
        return item<60;
         })
        console.log(newArr)
   }
     fun5_1()
     // The result is true

7. every: All the elements in the array satisfy , return true
8. reduce: Traversal array , Cumulative addition

 var arr=[10,20]; var sum=arr.reduce(function(s,item,index){ 
    
    		//s To save the sum 
            return s+item
    },0)//0 yes s Initial value 
        console.log(sum)
         result ：30

Bubble sort

If n Bubble sort by number , need n-1 Wheel bubbling , Make two bubbles each time , The former is bigger than the latter , Exchange order

// Bubble sort 
    var arr = [98, 86, 79, 69, 90, 82];
    var n = arr.length;
    for (let i = 0; i < n - 1; i++) { 
     
        //n-1 Secondary exchange 
        for (let j = 0; j < n - 1 - i; j++) { 
    
            if (arr[j] > arr[j + 1]) { 
    
               // The first number is greater than the second number 
               var stmp = arr[j];
               arr[j] = arr[j + 1];
               arr[j + 1] = stmp;
               // Exchange two numbers 
                }
            }
        }
        console.log(arr)
        // result [69, 79, 82, 86, 90, 98]

Selection sort

If n Number to select and sort , need n-1 Secondary ranking , Let's assume that the first number is the smallest number , Compare him with the last number , Exchange the position between the lowest decimal and the first number

     // Selection sort 
    var arr = [98, 86, 79, 69, 90, 82];
    var n = arr.length;
    for (i = 0; i < n-1; i++) { 
    
        let min = i;
        // Definition No arr[i] The number is the minimum 
        for (j = 1 + i; j < n; j++) { 
    
        if (arr[j] < arr[min]) { 
    
        // If arr[j] It's the smallest number , Assign index numbers to min
            min = j;

        }

    }
    let stmp = arr[i];
    arr[i] = arr[min];
    arr[min] = stmp;
    // The exchange index number is i and min Number of numbers 

}
console.log(arr)
// result [69, 79, 82, 86, 90, 98]