How to reverse a String with Stringbuilder and for loop

Question

I managed to reverse a String making it a char array, also with .reverse method on a Stringbuilder, managed to do it with another String and a for loop (I read that this isn't a good idea because it hurts the performance), but i cant figure out how to reverse a string with a Stringbuilder and for loop. It gives me error - "The left-hand side must be a variable" . Also tried to set a length for the Stringbuilder, but looks like I am missing something. I tried to set indexes outside the brackets of the Stringbuilder, because they are for parameters, but I am just guessing how to do it, tried .append as well.

public class Task1 {

    public static void main(String[] args) {

        String str = "Reverse me";
        StringBuilder printStr = new StringBuilder();
        int j = 0;

        for (int i = str.length()-1; i > 0; i--) {
            printstr(j) = str[i];

        }
    }

}

You are using parenthesis to access indexes, WRONG ! Use charAt method and append. — whatamidoingwithmylife
– whatamidoingwithmylife, Commented Dec 10, 2017 at 21:54
Please note, that just reversing character order is not a correct solution. You will break all surrogate pairs, if there is any. StringBuilder specially handle such case: http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/07011844584f/src/share/classes/java/lang/AbstractStringBuilder.java — user882813
– user882813, Commented Dec 10, 2017 at 23:11

Luke Garrigan · Accepted Answer · 2017-12-11 09:16:53Z

I think you are on the right lines, just a few things I'd change: Firstly we have the ability to specify the length of the StringBuilder because we know it is going to be the same length as the String we are reversing.

It is not necessary to loop through the entire array, just half the array will suffice performing operations which are essentially just switching the characters. So the right-most character is now the left-most character, this saves a little computation.

public static void main(String[] args) throws IOException {
    String str = "Reverse me";
    StringBuilder printStr = new StringBuilder();
    printStr.setLength(str.length());

    for(int i =0; i<str.length()/2;i++){
        char left = str.charAt(i);
        final int fromRight = str.length() - i - 1;
        printStr.insert(i, str.charAt(fromRight));
        printStr.insert(fromRight, left);
    }
}

Here is a visual representation of how to character switching essentially works: More information can be found here

whatamidoingwithmylife · Accepted Answer · 2017-12-10 22:01:16Z

1

OK, so you made multiple mistakes, you are trying to access indexes by using parenthesis, you are not iterating over whole string as your loop condition is i > 0 and it should be i >= 0, and your loop is just wrong (disregard parenthesis), you can combine append() and charAt() methods to do what you need:

    for (int i = str.length()-1; i >= 0; i--) {
        printStr.append(str.charAt(i));
    }

We go from last index of your string up to index 0, so we start from the end of the string, and we append each character to our StringBuilder. In the end you have your reversed string.

answered Dec 10, 2017 at 22:01

whatamidoingwithmylife

1,1862 gold badges19 silver badges39 bronze badges

Comments

Mikalai Lushchytski · Accepted Answer · 2020-09-17 14:19:45Z

0

    public class HelloWorld {
    public static void main(String[] args) {

        System.out.println(reverseString("Example"));
    }

    public static String reverseString(String str) {
        StringBuilder builder = new StringBuilder();

        for (int i = str.length() - 1; i >= 0; i--) {
            builder.append(str.charAt(i));
        }
        return builder.toString();
    }
}

edited Sep 17, 2020 at 14:19

Mikalai Lushchytski

1,6711 gold badge11 silver badges18 bronze badges

answered Sep 17, 2020 at 12:20

Roland Tömösközi

10711 bronze badges

Collectives™ on Stack Overflow

How to reverse a String with Stringbuilder and for loop

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related