Strings and nested loops

Strings and nested loops

• 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.

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.

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));
        }
    }
}

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).

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
    }
}

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.

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
    }
}

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.

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

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.

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);
        }
    }
}

Common mistakes

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

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.