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Thread: Compare the last 4 strings in String s1 and String s2

  1. #1
    Junior Member javabro's Avatar
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    Default Compare the last 4 strings in String s1 and String s2

    Hi!

    I'm quite new to Java and I've got this question that requires me to solve a basic algorithm.

    "Implement a method public static boolean compareLast4(String s1, String s2) that returns true if the final four-character substrings of s1 and s2 are equal. If the length of either string is less than 4, the method should return false.

    You may use the methods.substring(n,m) that yields the substring of s beginning at index and ending at index m-1."

    Here's my code:

    public class compareLast4
    {
        public static void main(String[] args){
            String s1 = "Elephant";
            String s2 = "Tiger";
            System.out.println(compareLast4(s1,s2));
        }
     
        public static boolean compareLast4(String s1, String s2){
            int p1 = s1.length() - 4;
            int p2 = s2.length() - 4;
            int t1 = 0;
            int t2 = 0;
     
            for (int i = p1; i < s1.length(); i++) {
                t1++;   
            }
            t1+=p1;
     
            for (int j = p2; j < s2.length(); j++) {
                t2++;
            }
            t2+=p2;
     
            if(s1.substring(p1, t1) == s2.substring(p2, t2)){
                return true;
            }
            else{
                return false;
            } 
        }
    }

    No matter what words I put in there, it's always false. The pointer is there to show the starting index of the string and it will count to the end. For example, "Elephant" starts at index 5, which is "h" until it reaches "t" and "tiger" starts at index 1 which is "i" until it reaches "r".

    Am I missing something?

    --- Update ---

    I've actually solved the problem, funnily enough.

    The following line has been added:
            if(s1.substring(p1, t1).length() == s2.substring(p2, t2).length()){
                return true; 
            }
    Last edited by javabro; August 26th, 2019 at 03:09 PM.

  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Compare the last 4 strings in String s1 and String s2

      if(s1.substring(p1, t1) == s2.substring(p2, t2)){

    You should use the equals method for comparing the contents of String objects, not the == operator.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
    Junior Member javabro's Avatar
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    Default Re: Compare the last 4 strings in String s1 and String s2

    I've tried the equals method and it doesn't work.

    The one I've created works because I'm comparing numbers using the .length().

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: Compare the last 4 strings in String s1 and String s2

    I've tried the equals method and it doesn't work.
    Sorry, the equals method is the only reliable way to compare the contents of String objects.
    Can you make a small, simple program that shows the problem and paste it here?
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
    Junior Member javabro's Avatar
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    Default Re: Compare the last 4 strings in String s1 and String s2

    If you're referring to compare two strings with the same last 4 letters, then yes, equals method does work.

    public class compareLast4
    {
        public static void main(String[] args){
            String s1 = "bathroom";
            String s2 = "bedroom";
            System.out.println(compareLast4(s1,s2));
        }
     
        public static boolean compareLast4(String s1, String s2){
            int p1 = s1.length() - 4; 
            int p2 = s2.length() - 4; 
            int t1 = 0;
            int t2 = 0;
     
            if(s1.length() >=4 && s2.length() >= 4){
     
                for (int i = p1; i < s1.length(); i++) {
                    t1++;   
                }
     
                t1+=p1;
     
                for (int j = p2; j < s2.length(); j++) {
                    t2++;
                }
     
                t2+=p2;
     
                if(s1.substring(p1, t1).equals(s2.substring(p2, t2))){
                    return true; 
                }
                else{
                    return false;
                } 
            }
            else{
                return false;
            }
        }
    }

    But the question says something like this:
    "returns true if the final four-character substrings of s1 and s2 are equal"

    And I think it means comparing to the number of substrings?

  6. #6
    Super Moderator Norm's Avatar
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    Default Re: Compare the last 4 strings in String s1 and String s2

    What is the purpose of the two for loops?
    If you don't understand my answer, don't ignore it, ask a question.

  7. #7
    Junior Member javabro's Avatar
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    Default Re: Compare the last 4 strings in String s1 and String s2

    The first for loop is to get the total length of s1 and the second for loop is to get the total length of s2. I'm doing these to get the last index of the strings.

    I realised I can actually do something like this as well:
          if(s1.substring(p1, s1.length()).equals(s2.substring(p2, s2.length()))){
                    return true; 
                }

    Which means, there's no need to create two for loops.

  8. #8
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    Default Re: Compare the last 4 strings in String s1 and String s2

    get the last index of the strings.
    It is not needed.
    Look at the API doc for the String class's substring method. There is a better way.
    If you don't understand my answer, don't ignore it, ask a question.

  9. #9
    Junior Member javabro's Avatar
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    Default Re: Compare the last 4 strings in String s1 and String s2

    Do you mean something like this?
                    if(s1.substring(p1).equals(s2.substring(p2))){
                        return true; 
                    }

    Originally, I want to do something like this but I wasn't sure about my interpretation with the question. When I read the API doc, it says that it can only compare if the characters are the same. So I guess, what it meant by equal, is that, if they're the same characters then it's true.

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    Default Re: Compare the last 4 strings in String s1 and String s2

    Yes that looks like what I was talking about. There is no need for the ending index if you want all of the String starting from the first index.
    If you don't understand my answer, don't ignore it, ask a question.

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