Strings and nested loops

English

A String is a row of characters, each with a position (an index).
The first index is 0, and the last is length() - 1.
In this lesson you walk through a String one character at a time, and put loops inside loops.

中文

字符串与嵌套循环

String 是一排字符,每个字符都有一个位置(索引 index)。
第一个索引是 0,最后一个是 length() - 1。
在这节课里,你会一个字符一个字符地遍历字符串,并把循环放进循环里。

English

Traversing a String

s.length() gives the number of characters.
s.charAt(i) gives the character at index i.
Loop i from 0 up to length() - 1 to visit every character.

中文

遍历字符串

s.length() 给出字符的数量。
s.charAt(i) 给出索引 i 处的字符。
让 i 从 0 循环到 length() - 1,就能访问每一个字符。

public class Main {
    public static void main(String[] args) {
        String s = "code";
        for (int i = 0; i < s.length(); i++) {
            System.out.println(s.charAt(i));
        }
    }
}

English

Counting characters

You can count how many times a character appears.
Use a counter that starts at 0.
A single character uses single quotes, like 'o'. Compare with == (chars, not Strings).

中文

数字符的个数

你可以数一个字符出现了多少次。
用一个从 0 开始的计数器。
单个字符用单引号,例如 'o'。用 == 来比较(比较的是字符 char,不是字符串)。

public class Main {
    public static void main(String[] args) {
        String s = "balloon";
        int count = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == 'o') {
                count++;
            }
        }
        System.out.println(count);   // 2
    }
}

English

Building a new String

Start with an empty String "" and add characters with +.
Reading from the end to the start reverses the String.
Each + makes a longer String.

中文

构建一个新字符串

从一个空字符串 "" 开始,用 + 添加字符。
从末尾读到开头,就能把字符串倒过来。
每个 + 都会生成一个更长的字符串。

public class Main {
    public static void main(String[] args) {
        String s = "code";
        String result = "";
        for (int i = s.length() - 1; i >= 0; i--) {
            result = result + s.charAt(i);
        }
        System.out.println(result);   // edoc
    }
}

English

Enhanced for (preview)

For walking a String you usually need the index, so a normal for is best.
But Java also has an enhanced for loop that visits items directly.
You will use it a lot with arrays in lesson 8. Here is a small preview with text.

中文

增强 for(预览)

遍历字符串时你通常需要索引,所以普通的 for 最合适。
但 Java 还有一种增强 for(enhanced for)循环,它直接访问每个元素。
你会在第 8 课和数组一起大量使用它。这里用文字做一个小预览。

public class Main {
    public static void main(String[] args) {
        String[] words = {"hi", "there", "!"};
        for (String word : words) {
            System.out.println(word);
        }
    }
}

English

Nested loops

A nested loop is a loop inside another loop.
The inner loop runs all the way for each step of the outer loop.
Below, the outer loop picks a row, and the inner loop prints stars in that row.

中文

嵌套循环

嵌套循环(nested loop)是一个循环里面又有一个循环。
对于外层循环的每一步,内层循环都会完整地跑一遍。
在下面,外层循环挑选一行,内层循环在那一行里打印星号。

public class Main {
    public static void main(String[] args) {
        for (int row = 1; row <= 3; row++) {
            String line = "";
            for (int col = 1; col <= row; col++) {
                line = line + "*";
            }
            System.out.println(line);
        }
    }
}

English

Common mistakes

Strings are immutable — store the result of s.substring(...) etc.
s.length() is a method (with ()), unlike an array's .length.

中文

常见错误

字符串不可变——要存下 s.substring(...) 等的结果。
s.length() 是方法(带 ()),不同于数组的 .length。

English

Now you try

Each task pre-fills the class skeleton — write your code inside main, or complete the method shown.
Press Run to compile and run, then Check answer.
Your code compiles and runs on the server, so even the first run is fast.

中文

现在轮到你

每个任务都已经填好了类的骨架 —— 把你的代码写在 main 里面,或者补全给出的方法。
按运行来编译并运行,然后按检查答案。
你的代码在服务器上编译并运行,所以第一次运行也很快。

Explore · ⁨探索⁩

Indexing a String · ⁨字符串索引⁩

Each char has an index from 0; substring takes a range · ⁨范围 (range)⁩. · ⁨每个字符有从 0 开始的索引;substring 取一段。⁩

Complete countChar(String s, char c) so it returns how many times c appears in s. Use charAt and ==. For example countChar("balloon", 'o') is 2. · ⁨完成 countChar(String s, char c),让它返回 c 在 s 中出现的次数。使用 charAt 和 ==。例如 countChar("balloon", 'o') 是 2。⁩

public class Text {
    // Count how many times c appears in s.
    public static int countChar(String s, char c) {
        return 0;   // change this
    }
}

Click Run to see the output here. · ⁨点击“运行”查看此处输出。⁩

Complete reverse(String s) so it returns s with the letters in the opposite order. For example reverse("code") is "edoc". Build a new String with +. · ⁨完成 reverse(String s),让它返回字母顺序相反的 s。例如 reverse("code") 是 "edoc"。用 + 构建一个新字符串。⁩

public class Reverser {
    // Return s with the letters in the opposite order.
    public static String reverse(String s) {
        return "";   // change this
    }
}

Click Run to see the output here. · ⁨点击“运行”查看此处输出。⁩

Use nested loops to print a triangle of stars: line 1 has one *, line 2 has two, line 3 has three, line 4 has four. Print each line with System.out.println. · ⁨用嵌套循环打印一个星号三角形:第 1 行有一个 *,第 2 行有两个,第 3 行有三个,第 4 行有四个。每行用 System.out.println 打印。⁩

public class Main {
    public static void main(String[] args) {
        // Print:
        // *
        // **
        // ***
        // ****
    }
}

Click Run to see the output here. · ⁨点击“运行”查看此处输出。⁩

Strings and nested loops · ⁨字符串与嵌套循环⁩