Trying to reverse a string on Java. Why can't I get the reversed text as my only output?

Question

I can't get my function to return the reversed string. I keep getting the original string, plus the reversed sting attached together.

P.S this is obviously a question from someone new. Cut me some slack and save me from the horribly demoralizing down vote.

    int i;
    reverse = reverse.replaceAll("[^a-zA-Z]+", "").toLowerCase();

    for (i = reverse.length() - 1; i >= 0; i--) {
    reverse = reverse + reverse.charAt(i);
    }

    return reverse;
}

So, you want to delete all non letters from you string and then reverse it? — , Jul 13 '17 at 03:02

score 3 · Answer 1 · answered Jul 13 '17 at 03:03

3

You need another String (or some other method / memory) to build your return value (consider "a", starting from the end of the String add "a" - thus you get "aa"). Instead, I would use a StringBuilder - and the entire method might be written like

return new StringBuilder(reverse.replaceAll("[^a-zA-Z]+", "")
        .toLowerCase()).reverse().toString();

answered Jul 13 '17 at 03:03

Elliott Frisch

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Hi Elliot, I tried to look up some of the documentation, but I couldn't really understand why the "+" is there. What would happen if I left that out? – JWL1988 Jul 13 '17 at 23:23
@JWL1988 It matches one or more consecutive non-letters, since you replace them all anyway it wouldn't really change anything in this specific case. The '+' *specifically* adds "or more". Consider `"a b"` ("a", 3 spaces and "b"), `"a b".split("\\s+")` yields a very different result to `"a b".split("\\s")` – Elliott Frisch Jul 14 '17 at 02:16

score 2 · Accepted Answer · answered Jul 13 '17 at 03:03

2

Change the snippet to,

String reverse_string="";
for (i = reverse.length() - 1; i >= 0; i--) {
    reverse_string += reverse.charAt(i);
    }
return reverse_string;

You will need a separate String variable to contsruct the newly reversed string.

answered Jul 13 '17 at 03:03

Joey Pinto

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It does not work properly when `reverse` includes surrogate pairs. – Jul 13 '17 at 03:08
Thank you Joey, this is what I was looking for. – JWL1988 Jul 13 '17 at 03:14

score 2 · Answer 3 · answered Jul 13 '17 at 03:05

2

Why not just let the existing Java api do this for you?

 String someString = "somestring123";
    System.out.println(new StringBuilder(someString).reverse().toString());

output: StringBuilder

answered Jul 13 '17 at 03:05

Perrin

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This is for a CS class and this was the code snippet the professor gave to us to use. – JWL1988 Jul 13 '17 at 03:15

Trying to reverse a string on Java. Why can't I get the reversed text as my only output?

3 Answers3