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What are the common if statements in computer programming?
- 青灯夜游Original
- 2023-01-29 16:31:508878browse
The common if statement in computer programming is a conditional judgment statement. The if statement is a selective branch structure. It selects the execution path based on clear conditions, rather than strictly following the order. In actual programming, the appropriate branch statement must be selected according to the program flow. It changes the execution according to the result of the condition. program; the simple syntax of the if statement is "if (conditional expression) {//code to be executed;}".
The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.
The common if statement in computer programming is a conditional judgment statement.
Conditional judgment statement
The conditional judgment statement is a selection branch structure. It selects the execution path based on clear conditions, rather than Execute strictly in sequence. In actual programming, appropriate branch statements must be selected according to the program flow. It is a program that changes execution according to the results of conditions.
The flow chart is as follows:
Conditional judgment statement is a frequently used statement form in the program development process. It is the same as most programming languages. In JavaScript There are also conditional judgment statements. The so-called conditional judgment refers to the program performing different operations based on different conditions, such as displaying different content based on age, and judging whether the operation is successful or failed based on a Boolean value of true or false, etc.
if statement
The if statement is the simplest conditional judgment statement in JavaScript. The syntax format is as follows:
if(条件表达式){
// 要执行的代码;
}When the conditional expression is established, that is, the result is a Boolean value true, the code in { } will be executed.
The sample code is as follows:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>JavaScript</title>
</head>
<body>
<script type="text/javascript">
var age = 20;
if(age >= 18){ // 如果 age >= 18 的结果为 true，则执行下面 { } 中的代码
alert("adult");
}
</script>
</body>
</html>The running result is as shown below:
if else statement
The if else statement is an upgraded version of the if statement. It can not only specify the code to be executed when the expression is true, but also the code to be executed when the expression is not true. The syntax format is as follows :
if(条件表达式){
// 当表达式成立时要执行的代码
}else{
// 当表达式不成立时要执行的代码
}The sample code is as follows:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>JavaScript</title>
</head>
<body>
<script type="text/javascript">
var now = new Date(); // 获取当前的完整日期
var dayOfWeek = now.getDay(); // 获取一个 0-6 之间的数字，用来表示当前是星期几，0 表示星期日、1 表示星期一、以此类推
if (dayOfWeek > 0 && dayOfWeek < 6) { // 判断：如果当前是星期一到星期五中的一天，则输出“Have a nice day!”，若不是则输出“Have a nice weekend!”
alert("Have a nice day!");
} else {
alert("Have a nice weekend!");
}
</script>
</body>
</html>The running result is as shown below:
##if else if else statement
Both if and if else statements have only one conditional expression, and if else if else statement is their more advanced form, in if else if else statement allows you to define multiple A conditional expression, and executes the corresponding code according to the result of the expression. The syntax format is as follows:if (条件表达式 1) {
// 条件表达式 1 为真时执行的代码
} else if (条件表达式 2) {
// 条件表达式 2 为真时执行的代码
}
...
else if (条件表达式N) {
// 条件表达式 N 为真时执行的代码
} else {
// 所有条件表达式都为假时要执行的代码
}
Tip: During the execution of the if else if else statement, when a established conditional expression is encountered , the code in the following { } will be executed immediately, and then the entire if else if else statement will be exited. If there is a valid conditional expression in the subsequent code, it will not be executed.The sample code is as follows:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>JavaScript</title>
</head>
<body>
<script type="text/javascript">
var now = new Date(); // 获取当前的完整日期
var dayOfWeek = now.getDay(); // 获取一个 0-6 之间的数字，用来表示当前是星期几，0 表示星期日、1 表示星期一、以此类推
if(dayOfWeek == 0) { // 判断当前是星期几
alert("星期日")
} else if(dayOfWeek == 1) {
alert("星期一")
} else if(dayOfWeek == 2) {
alert("星期二")
} else if(dayOfWeek == 3) {
alert("星期三")
} else if(dayOfWeek == 4) {
alert("星期四")
} else if(dayOfWeek == 5) {
alert("星期五")
} else {
alert("星期六")
}
</script>
</body>
</html>The running results are as shown below:
Notes
When using nested if else, if there is only one line of statement, it should be wrapped in curly brackets to avoid condition ambiguity. For example, the following nested if else can easily lead to misunderstanding:if(0)
if(1)
console.log(1);
else
console.log(0);For the above code, the JavaScript interpreter will interpret it according to the following logical level based on the proximity principle: if(0)
if(1)
console.log(1);
else
console.log(0);So using braces can avoid many problems: if(0){
if(1) console.log(1);
}else{
console.log(0);
}[Recommended learning: javascript advanced tutorial]
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