Loops

---

There are two kind of loops in Java, for and while.

For

The for loop has three sections:

for (int i = 0; i < 3; i++) {}

First section runs once when we enter the loop.

Second section is the gate keeper(门卫), if it returns true, we run the statements in the loop, if it returns false, we exit the loop. It runs right after the first section for the first time(第一次), then every time the loop is finished and the third section is run.

The third section is the final statement that will run every time(每次) the loop runs.

So in the case we have just seen, the loop will run 3 times. Here is the order of the commands:

int i = 0;
i < 3 // 0 < 3 = true
      // Inside of loop
i++ // i is now 1
i < 3 // 1 < 3 = true
      // Inside of loop
i++ // i is now 2
i < 3 // 2 < 3 = true
      // Inside of loop
i++ // i is now 3
i < 3 // 3 < 3 = false
      // Loop is done...

We can omit（省略） the first and third section of the loop (although it will be weird 不寻常的), and the loop will still work:

for (;i < 5;) {}

For cases where we want to use a loop that look like that, we use a while loop

While

The syntax is very similar to the previous for we looked at:

while (condition) {}

The condition will run for the first time when entering and every time the loop is done. If it returns false, the loop will not run.

If we want the loop to always run at least one（至少一次）, we can use do-while

do {

} while(condition);

Notice the ; in the end of the do-while.

Foreach 遍历数组

Another version of for, is the foreach. The keyword we use is still for, but when we want to iterate on the elements inside an array we can simply use it:

int[] arr = {2, 0, 1, 3};
for (int el : arr) {
    System.out.println(el);
}

This is a short version and equivalent to:

int[] arr = {1, 9, 9, 5};
for (int i = 0; i < arr.length; i++) {
    int el = arr[i];
    System.out.println(el);
}

Notice that if you want to use the index of the element inside the loop, you have to use the longer version and can't use foreach.

break and continue

These two keywords help us control the loop from within it. break will cause the loop to stop and will go immediately to the next statement after the loop:

int i;
for (i = 0; i < 5; i++) {
    if (i >= 2) {
        break;
    }
    System.out.println("Yuhu");
}
System.out.println(i);
// Output:
// Yuhu
// Yuhu
// 2

continue will stop the current iteration and will move to the next one. Notice that inside a for loop, it will still run the third section.

int i;
for (i = 0; i < 5; i++) {
    if (i >= 3) {
        break;
    }
    System.out.println("Yuhu");
    if (i >= 1) {
        continue;
    }
    System.out.println("Tata");
}
System.out.println(i);
// Output
// Yuhu
// Tata
// Yuhu
// Yuhu
// 3

Exercise

Loop through and print out all even numbers(偶数), each in a separate line, from the numbers list in the same order they are received. Don't print any numbers that come after 237 in the sequence.（该序列237后面的数字就不用打印了）

public class Main {
    public static void main(String[] args) {
        int[] numbers = {
            951, 402, 984, 651, 360, 69, 408, 319, 601, 485, 980, 507, 725, 547, 544, 
            615, 83, 165, 141, 501, 263, 617, 865, 575, 219, 390, 984, 592, 236, 105, 942, 941, 
            386, 462, 47, 418, 907, 344, 236, 375, 823, 566, 597, 978, 328, 615, 953, 345, 
            399, 162, 758, 219, 918, 237, 412, 566, 826, 248, 866, 950, 626, 949, 687, 217, 
            815, 67, 104, 58, 512, 24, 892, 894, 767, 553, 81, 379, 843, 831, 445, 742, 717, 
            958, 609, 842, 451, 688, 753, 854, 685, 93, 857, 440, 380, 126, 721, 328, 753, 470, 
            743, 527};
        
        for (int i = 0; i < numbers.length; i++) {
            int el = numbers[i];

            if (el == 237) {
                break;
            }

            // using the division remainder operator %
            if (el % 2 == 0) {
                System.out.println(el);
            }

        }
    }
}