First, the flow control judgment statement if
switch
while loop statement
do while
for
Sequence structure, selection structure, loop structure
Second, if statement (the statement in parentheses is true when the braces are executed)
<script type="text/javascript"> var age=20; if(age>18){ console.log("成年人"); } else{ console.log("小孩"); } </script>
Exercise 1: Exam Results
<script type="text/javascript">
var cj=80;
if(cj>=60){
console.log("Pass");
}
else if(cj>=80){
console.log("良好");
}
else if(cj>=90){
console.log("Excellent");
}
else {
console.log("Failed");
}
</script>
Exercise 2: Input a number from the keyboard to determine whether it is an even number and output yes otherwise no
<script type="text/javascript">
/ / Input a number from the keyboard, to determine whether it is an even number is to output yes or no
//Define the input data storage in the receiving keyboard
var num;
//Use the prompt() function to receive the integer input by the user
num=+(window.prompt("Please enter a number"));
//validity verification
if(isNaN(num)){
alert("The input is not a number")
}
else{
// Determine if the user input is an even number
if(num%2===0){
alert("yes");
}
else{
alert("no");
}
}
</script>
Optimized:
<script type="text/javascript">
/ / Input a number from the keyboard, to determine whether it is an even number is to output yes or no
//Define the input data storage in the receiving keyboard
var num;
//Use the prompt() function to receive the integer input by the user
num=+(window.prompt("Please enter a number"));
//validity verification
if(isNaN(num)){
alert("The input is not a number")
}
else{
(num%2===0)?alert("yes"):alert("no");
}
</script>
Exercise 3: Receive an integer input from the user and output it is the day of the week
<script type="text/javascript">
//Receive the integer input by the user and output it is the day of the week
//Define the receiving data variable
var day;
//Receive data
day=+(window.prompt("Please enter an integer between 1 and 7"));
// Determine the day of the week
if(day===1){
console.log("Week 1");
}
else if(day===2){
console.log("Week 2");
}
else if(day===3){
console.log("Week 3");
}
else if(day===4){
console.log("Week 4");
}
else if(day===5){
console.log("Week 5");
}
else if(day===6){
console.log("Week 6");
}
else if(day===7){
console.log("Week 7");
}
else{
console.log("未知");
}
</script>
Exercise 4: Receive the integer month input by the user, and output the season according to the month
<script type="text/javascript">
//1, define variables
var month;
//2, receive input
month=+(window.prompt("Please enter 1-12 numbers"));
//3, verify the data
if(isNaN(month)){
alert("input error");
}
else{
//4, judge the season
if(month>=3 && month<=5){
alert("春");
}
else if(month>=6 && month<=8){
alert("夏");
}
else if(month>=9 && month<=11){
alert("秋");
}
else if(month===12 || month===1 || month===2) {
alert("冬");
}
else{
alert("error");
}
}
</script>
Exercise 5: Input 2 integers from the keyboard, sort and output
<script type="text/javascript">
/ / Input 2 integers from the keyboard, sort and output
//define variable
var num1, num2;
//Receive user input
num1=+(window.prompt("Please enter the first number"));
num2=+(window.prompt("Please enter the second number"));
// compare two numbers
if(num1>num2){
console.log(num1);
}
else if(num1===num2){
console.log("equal");
}
else{
console.log(num2);
}
</script>
Optimized:
<script type="text/javascript">
/ / Input 2 integers from the keyboard, sort and output
//define variable
var num1, num2;
//Receive user input
num1=+(window.prompt("Please enter the first number"));
num2=+(window.prompt("Please enter the second number"));
// compare two numbers
console.log(num1>num2?num1:num2);
</script>
Advantages of ternary operator: Simplified code. Disadvantage: must output deterministic results
Exercise 6: Enter three numbers on the keyboard, sort them from largest to smallest and then output
<script type="text/javascript">
//Enter three numbers in the keyboard, sort them from large to small and then output
//define three variables
var num1, num2, num3;
// receive three variables
num1=+(window.prompt("Enter the first number"));
num2=+(window.prompt("Enter the second number"));
num3=+(window.prompt("Enter the third number"));
//sort three numbers
if(num1>=num2 && num2>=num3){
console.log(num1,num2,num3);
}
else if(num1>=num3 && num3>=num3){
console.log(num1,num3,num2);
}
else if(num2>=num1 && num1>=num3){
console.log(num2,num1,num3);
}
else if(num2>=num3 && num3>=num1){
console.log(num2,num3,num1);
}
else if(num3>=num1 && num1>=num2){
console.log(num3,num1,num2);
}
else if(num3>=num2 && num2>=num1){
console.log(num3,num2,num1);
}
</script>
This comparison is the most primitive we want to optimize:
selection sort
<script type="text/javascript">
//Enter three numbers in the keyboard, sort them from large to small and then output
//define three variables
var num1, num2, num3;
// receive three variables
num1=+(window.prompt("Enter the first number"));
num2=+(window.prompt("Enter the second number"));
num3=+(window.prompt("Enter the third number"));
//Sort three numbers using selection sort
var temp;
if(num1>num2){
temp=num1;
num1=num2;
num2=temp;
}
if(num1>num3){
temp=num1;
num1=num3;
num3=temp;
}
if(num2>num3){
temp=num2;
num2=num3;
num3=temp;
}
console.log(num3,num2,num1);
</script>
bubble sort
<script type="text/javascript">
//Enter three numbers in the keyboard, sort them from large to small and then output
//define three variables
var num1, num2, num3;
// receive three variables
num1=+(window.prompt("Enter the first number"));
num2=+(window.prompt("Enter the second number"));
num3=+(window.prompt("Enter the third number"));
//Sort three numbers using bubble sort
var temp;
if(num1>num2){
temp=num1;
num1=num2;
num2=temp;
}
if(num2>num3){
temp=num2;
num2=num3;
num3=temp;
}
if(num1>num2){
temp=num1;
num1=num2;
num2=temp;
}
console.log(num3,num2,num1);
</script>
Exercise 7: The realization of rock-paper-scissors
<script type="text/javascript">
//The implementation of rock paper scissors defines scissors 0 rock 1 paper 2
//1, define variables, players and computers
var com,play;
//2. The computer punches random() to generate random numbers between 0 and 1
com=Math.floor(Math.random()*3);
//3, the player punches
play=+(window.prompt("Please input: 0 rock scissors 1 cloth 2"));
//4, judge the winner and loser
if(play<0 || play>2){
window.prompt("Please re-enter: scissors 0 rock 1 cloth 2");
}
else{
if( play===0 && com===2 ||
play===1 && com===0 ||
play===2 && com===1 )
{
alert("Congratulations you won");
}
else if(
play===0 && com===1 ||
play===1 && com===2 ||
play===2 && com===0
){
alert("Sorry you lost");
}
else{
alert("平");
}
}
</script>