Process control: Branch (condition), loop
1. Branch statement
1.1 conditional statements
if statement
The statement executes code only when the specified condition is true
if (condition)
{
When the condition is true Code executed when
}
if... else statement
Use the if... else statement to execute code when the condition is true and other code when the condition is false.
if (condition)
{
When the condition is true Code executed when
}
else
{
When the condition is not true Code executed when
}
If... else if... else statement
Use the if... else if... else statement to select one of multiple code blocks for execution.
if (condition1)
{
statement1
When condition 1 is true Code executed when
}
else if (condition2)
{
statement2
When condition 2 is true Code executed when
}
else
{
statement3
When neither condition 1 nor condition 2 is true Code executed when
}
switch Statements
Used to perform different actions based on different conditions
Use the switch statement to select one of multiple code blocks to execute
switch(n)
{
case 1:
Execute code block 1
break;
case 2:
Execute code block 2
break;
default:
And case 1 and case 2 Code not executed at the same time
}
default keyword
Use the default keyword to specify what to do when a match does not exist:
var d = new Date().getDay();
// Use the switch case statement to make a judgment
switch (obj) {
case 0:
console.log("Sunday");
break;
case 1:
console.log("Monday");
break;
// case 2:
// console.log("Tuesday");
// break;
case 3:
console.log("Wednesday");
break;
case 4:
console.log("Thursday");
break;
case 5:
console.log("Friday");
break;
case 6:
console.log("Saturday");
break;
// When all conditions are not met, execute default
default:
console.log("Looking forward to the weekend");
break;
}
be careful:
-
break in case code block cannot be omitted
-
default can be placed anywhere in the code, break cannot be omitted, and break can be omitted in the last position;
-
"Variable vs. constant"===“
Recommendations:
switch is recommended for branch control for equivalence comparison, and If is recommended for non equivalence judgment
Circular statement
Loop can specify the number of times the code block is executed.
Different types of loops
JavaScript supports different types of loops:
- for - cycles a block of code a certain number of times
- for/in - loop through the properties of the object
- while - loop the specified code block when the specified condition is true
- do/while - also loop the specified code block when the specified condition is true
for loop
The following is the syntax of the for loop:
for (*Statement 1*; *Statement 2*; *Statement 3*)
{
*Executed code block*
}
for (var i=0; i<5; i++)
{
console.log(i);
}
Statement 1 is executed before the start
Statement 2 defines the conditions for running a cyclic code block
Statement 3 executes after the loop code block has been executed
Algorithm case of for loop:
1-100 even sum
var result = 0;
for(var i=0;i<=100;i++){
if(i%2==0){
result += i;
}
}
console.log(result);
Bubble sorting
//1. Compare two adjacent elements. If the former is larger than the latter, exchange the position.
//2. After the first round of comparison, the last element is the largest element.
//3. At this time, the last element is the largest, so the last element does not need to participate in the size comparison.
// First cycle
/* 19<11 false [11,19,20,6,10]
19<20 true [11,19,20,6,10]
20<6 false [11,19,6,20,10]
20<10 false [11,19,6,10,20]*/
// Create a new method bubble sort
function bSort(arr) {
// Define a variable to receive the length of the array
var len=arr.length;
// for loop to realize bubble sorting
for(var i=0;i<len-1;i++)
{
for(var j=0;j<len-1;j++)
{
// Compare the front and back elements if the front large swap position
if(arr[j]>arr[j+1]){
// Create an intermediate quantity for value conversion
var x=arr[j];
arr[j]=arr[j+1];
arr[j+1]=x;
}
}
}
// After sorting, the arr is returned
return arr;
}
var arr=[19,11,20,6,10];
// console.log(bSort(arr));
Special for loop conditions
// There are commas in the comma effect condition. Finally, the rightmost condition (the condition closest to the semicolon) is used as the termination condition
for(i=0,j=0;i<10,j<18;i++,j++)
{
x=i+j;
}
console.log(x);//34
Keyword break
If you want to exit before all iterations, you can use break. When the break is executed, it will immediately jump out of the loop body.
The difference between the keyword continue and break is that continue does not jump out of the loop. Instead, immediately end the current cycle and enter the next cycle.
For... in -- enhanced for loop
for... in is used to traverse the properties of an array or object
For (custom variable name in array / object){
Execute code
}
"Custom variable name" is used to specify the element index of the array or the attribute of the object
// for...in... loop
for (var arrIndex in bSort(arr)) {
console.log(arrIndex + '---' + arr[arrIndex]);
}
while Loop
The while loop executes the code block when the specified condition is true
grammar
while (*condition*)
{
*Code to execute*
}
while (i<5)
{
console.log(i);
i++;
}
// Find the sum of 1-100
var result=0;
// Starting condition
var i=1;
// Cycle end condition
while(i<=100)
{
result+=i;
// The starting condition is self increasing or self decreasing, which is written at the end
i++;
}
console.log(result);
do-while Loop
The do/while loop executes the code once before checking whether the condition is true. Therefore, if the condition is false, it will also execute once. If the condition is true, the loop will be repeated.
do
{
Code to execute
}
while (condition);
// Find the sum of 1-100
var result = 0;
var i = 1;
do{
result += i;
i++;
} while(i<=100);
console.log(result);
recursion
A process or function has a method to call itself directly or indirectly in its definition or description. It usually transforms a large and complex problem layer by layer into a smaller problem similar to the original problem. The recursive strategy can describe the multiple repeated calculations required in the problem-solving process with only a small number of programs, which greatly reduces the amount of code of the program.
//The function calls itself
//A jump out condition is required
// Recursive factorization
function fc(n) {
// Jump out condition
if(n==1)
{
return 1;
}
// Factoring with recursion
return fc(n-1)*n;
}
console.log(fc(10));
Algorithm instance expansion:
1. Factorial realization:
var n = 1;
// 1.while loop
var m1=1;
while (n <= 10) {
var m1 = m1 * n
n++;
}
console.log(m1);
// 2. Do while loop
var m2=1;
do{
var m2=m2*n;
n++;
}while(n<=10)
// 3.for loop
var m3=1;
for(var n=1;n<=10;n++){
var m3=m3*n;
}
console.log(m);
//Recursive Method
function fc(n) {
// Jump out condition
if(n==1)
{
return 1;
}
// Factoring with recursion
return fc(n-1)*n;
}
console.log(fc(10));
2.99 implementation of multiplication table
// Using the for loop
for (var n = 1; n <= 9; n++) {
for (var m = 1; m<=n ; m++) {
var result = n * m;
document.write(m + '*' + n + '=' + result+' ');
}
document.write("<br>");
}
// Using do/while loop
var n = 9;
do {
var m = 1;
do {
var result = n * m;
document.write(m + '*' + n + '=' + result + ' ');
m++;
} while (m <= n)
n--;
document.write('<br>');
} while (n >= 1)
// Using while loop
var n = 9;
while (n >= 1) {
var m = 1;
while (m <= n) {
var result = n * m;
document.write(m + '*' + n + '=' + result + ' ')
m++;
}
n--;
document.write('<br>');
}
3. Three digits of non repeating numbers composed of 1, 2, 3 and 4
var arr = [1, 2, 3, 4]
var sum =0;
for (var i = 0; i < arr.length; i++) {
for (var j = 0; j < arr.length; j++) {
if (i != j) {
for (var k = 0; k < arr.length; k++) {
if (j != k && i!=k) {
var result = arr[i] * 100 + arr[j] * 10 + arr[k];
console.log(result);
sum++;
}
}
}
}
}
console.log(sum);
4. Print out 101-200 prime numbers
var count = 0;
for (var i = 101; i <= 200; i++) {
// var flag = true;
for (var j = 2; j < i; j++) {
var result = i % j;
if (result == 0) {
// flag = false;
break;
}
}
if (result != 0) {
console.log(i);
count++;
}
}
console.log(count);
5. 100-1000 daffodils, e.g. 153 = 111 + 555 + 333
for (var i = 100; i < 1000; i++) {
// Hundreds
var num1=parseInt(i/100) ;
// Ten digits
var num2=parseInt((i%100)/10);
// Single digit
var num3=parseInt(i%100%10);
var Num=Math.pow(num1,3)+Math.pow(num2,3)+Math.pow(num3,3);
if(Num==i)
{
console.log(Num);
}
}
6.1-1000 perfect number, for example, 6 = 1 + 2 + 3; 1, 2 and 3 are factors of 6;
for(var i=1;i<=1000;i++)
{
var sum=0;
for(var j=1;j<i;j++)
{
if(i%j==0)
{
sum+=j;
}
}
if(sum==i){
console.log(i);
}
}