How to master Java selection and loop structures

Selection structure

1. Simple if

int money=10; 
if(money>100){ 
System.out.println("money小于100"); }

The if in Java is actually the same as other programming languages. In the brackets after the if, write the data that needs to be judged. If If it matches, it will be executed.

2.Multiple if

Multiple if structure, there are many choices, but to choose one, you need to use it with else if

if (number==1) {
		System.out.println("你选择的是牙膏，10￥");
			shop = 10;
		}else if (number==2) {
			System.out.println("你选择的是毛巾，15￥");
			shop = 15;
		}else if (number==3) {
			System.out.println("你选择的是脸盆，20￥");
			shop = 20;
		}else {
			System.out.println("选择错误");
			a = false;
		}

When multiple judgments are needed, just After the closing parenthesis of the previous if, write else if to make the next judgment again, and the program will continue to compare. If all the ifs are executed and there is still no match, the else statement will be executed. For example, the above code can let everyone A deep understanding is a judgment on the value of number.

3. Nested if

As the name suggests, nested if means writing if inside if. When the program is executed, it first skips the parent if judgment and then enters the nested if , if there are multiple ifs nested inside, the execution process is the same.

if(a>0) {
	if (a<10) {
		if(a=7){
			System.out.println("a是7");
		}
	}
}

The code here, if a>0, matches, enters the second if, if a<10, matches, enters the third if, if a=7, the execution is successful and the output statement is run.

4.switch selection structure

switch: can also be used for multiple selections, but switch can only be used for equality judgment, not interval judgment

Syntax:

switch (a) {      //在switch后写入需要判断的值
case 1:				// 若判断值等于1
	System.out.println("a=1");   // 执行

	break;    //退出
case 2 :		// 若判断值等于2
	System.out.println("a=2");

	break;

case 3 :		// 若判断值等于3
	System.out.println("a=3");
	break;
default:     // 若都不符合
	break;  //退出，这里也可以自定义任何文本，例如输入错误等等
}

switch can only be used for equivalence judgment, not interval judgment

switch can only be used for equivalence judgment, not interval judgment

switch can only be used It is used for equivalence judgment and cannot be used for interval judgment

Focus on this three times! ! !

Loop structure

while loop

Judge first, then execute; first judge whether the condition is met, and if it is true, execute the loop body code, otherwise it will not execute

while(a<=10) {
			//若a<=10,则符合条件，进入到while内
			//这里可以写需要循环的语句
			//例如输入序号输入错误时，可以进入到while内
			
			System.out.println("a请重新输入：");

do-while loop

Execute first, then judge. Execute it once regardless of whether it is satisfied or not, and then judge whether a loop is needed.

do {
	System.out.println("程序已经运行");;
			
	}while(a==1);
			System.out.println("a是1");
		//当 程序开始执行后，不管怎么样，都会执行do后面的，然后在去while判断
		//过程和上面的while是一样的

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