Array.binarySearch returning negative numbers

Question

I am trying to find the index of a string in a string array using the Arrays.binarySearch() method however the method seems to be return the postion integer "-5" when looking for the string "Free". Any idea why this would be?

String[] names = {"Arken","Ben","Darklark", "Free","group"};

void changeFriends(String uname, boolean b)
        {   // change a friend's "online" status

    Arrays.sort(names);
    int index = Arrays.binarySearch(names, uname);

    System.out.println("NAME OF ONLINE USER IS AT INDEX:" + index + "Name:" + uname);
        if(index > -1)
        {

             if(b == true)
            {
                loggedOn[index] = true;
            }
            else
            {
                loggedOn[index] = false;
            }
        }
        // call method to update buttons
        changeNameButtons();
    }

I tested your code, it returns NAME OF ONLINE USER IS AT INDEX: 3 Name:Free — bhuang3
– bhuang3, Commented Dec 5, 2012 at 23:56

Community · Accepted Answer · 2020-06-20 09:12:55Z

14

If it's returning a negative value it's not found:

http://docs.oracle.com/javase/6/docs/api/java/util/Arrays.html

public static int binarySearch(Object[] a,
Object key)

Returns: index of the search key, if it is contained in the array; otherwise, (-(insertion point) - 1). The insertion point is defined as the point at which the key would be inserted into the array: the index of the first element greater than the key, or a.length if all elements in the array are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found.

Whatever you're passing in as uname is not "Free". I highly suspect you're thinking case doesn't matter (or have trailing characters; whitespace or newline) ;)

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 5, 2012 at 23:57

Brian Roach

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Comments

Siddharth Choudhary · Accepted Answer · 2018-12-18 18:19:11Z

9

Those who are still having this trouble like I was having earlier, and you are thinking that why can't anyone else see that it(Arrays.binarySearch) still have some problems because you have inserted the correct values and you still get the negative results. Well this answer is for those:

It is because your array is not sorted, my friend. Even if it's of characters or strings

For arrays.binarySearch the array should be sorted(I know, you must be thinking that How did I miss the most important thing (Well yeah, that happens)).

answered Dec 18, 2018 at 18:19

Siddharth Choudhary

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1 Comment

Pablo Adames Over a year ago

Incredible but true!

PermGenError · Accepted Answer · 2012-12-06 00:03:29Z

6

Hmm, i just ran your code and i got the index 3.

Arrays.sort(names);
int index = Arrays.binarySearch(names, "Free");
System.out.println(index);

you probably are searching free or Free(with a trailing white space) instead of Free, in which case it returns -5.

edited Dec 6, 2012 at 0:03

answered Dec 5, 2012 at 23:56

PermGenError

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6 Comments

PermGenError Over a year ago

@MadProgrammer haha, i was not being rude, anyhow chhanged it :)

MadProgrammer Over a year ago

I didn't think you where, but not everyone will see it that way ;)

user207421 Over a year ago

@MadProgrammer I see no 'harsh' comment in any of the edits. What are you talking about?

MadProgrammer Over a year ago

@EJP - It was simply my opinion, it just felt a "little" off hand. Remember, the meaning of communication is the meaning that the receiver makes of it ;)

Stephane Over a year ago

I have the Arrays.binarySearch returning -10 when searching for the underscore character _ Doing it in debbuger plain sight.

|

Indiver Piyush · Accepted Answer · 2020-08-25 01:29:26Z

1

If value is not found and value is less than one or more elements in array , the negative number returned is the bitwise complement of the index of the first element that is larger than value

answered Aug 25, 2020 at 1:29

Indiver Piyush

111 bronze badge

1 Comment

TongChen Over a year ago

You should post your code and problems not just post the question.

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