Android Question How to check duplicates in array

Germán Arduino · Jan 16, 2018

Hi Guys:

I must calculate 10 random numbers between 00 and 99 and populate an array with such ten numbers, but I need to take care that the final array do not contains duplicates values. Each element of the array must be an unique value between 00 and 99.

Exist some function (as in other languages) to test duplicates in an array?

Thanks for your comments.

Cheers.

Lee Gillie CCP · Jan 16, 2018

Why not use a List rather than array? Then you can check to see if it already contains your prospective new addition with INDEXOF.

Germán Arduino · Jan 16, 2018

Thanks for your answer!

I not considered a List, simply because I'm new with B4 family of tools.

Can you point some example or documentation?

Thanks.

jahswant · Jan 16, 2018

Germán Arduino said:
I not considered a List, simply because I'm new with B4 family of tools.

Look at this.

Peter Simpson · Jan 16, 2018

NOT TESTED BUT SHOULD WORK.

Use RND in a loop to generate 10 unique numbers and add one number at a time to the list, use with IndexOf to make sure that the generated number is not already in the list. I've not added the loop but it will work. I'm not currently on my laptop, I'm typing on a tablet right now.

B4X:

Globals_Sub
    Private MyList As List

Activity_Sub 'Just for testing
    MyList.Initialize
    MyList.AddAll(Array As Int(0,1,2,4,5,6,7,8,9)) 'Adds 0,1,2,4,5,6,7,8,9 to list

    Dim ArrayItem As Int = 3 'Item to be added to list
    Log(MyList.IndexOf(ArrayItem)) 'See if 3 is already in the list (returns item index or -1)

    If MyList.IndexOf(ArrayItem) = -1 Then MyList.Add(ArrayItem) '-1 means not in list, Add 3 to list

    For i = 0 To MyList.Size - 1
        Log(MyList.Get(i)) 'Show the list in the logs (3 should be at the bottom of the list, hopefully)
    Next

Enjoy...

Germán Arduino · Jan 16, 2018

Thanks Peter, I understand the sense of the example, I will try with my application.

Cheers.

udg · Jan 16, 2018

Another option: use a map where the extracted number is the key. Once your map's size reaches 10 know you have 10 different values and you can fill the array.

This is based on the fact that a map doesn't allow duplicates.

Germán Arduino · Jan 16, 2018

Thanks udg, I stiil must read about map.

Cheers.

Jeffrey Cameron · Jan 16, 2018

See this thread as well, for more ideas.

Germán Arduino · Jan 16, 2018

Thanks you Jeffrey!

Erel · Jan 17, 2018

Map is the best collection for such problems. See this tutorial: https://www.b4x.com/android/forum/threads/map-collection-the-most-useful-collection.60304/

Germán Arduino · Jan 17, 2018

Thanks you Erel, I will check!

Germán Arduino · Jan 17, 2018

Erel said:
Map is the best collection for such problems. See this tutorial: https://www.b4x.com/android/forum/threads/map-collection-the-most-useful-collection.60304/

I did a quick look at the tutorial, and tried some code, but I do not find a way to test if some element is in the map, for example, I see

B4X:

If days.ContainsKey(8) Then ...

But do not exist some sort of "ContainsValue" or similar to check if some value exist in any key?

Germán Arduino · Jan 18, 2018

Peter Simpson said:
NOT TESTED BUT SHOULD WORK.

Use RND in a loop to generate 10 unique numbers and add one number at a time to the list, use with IndexOf to make sure that the generated number is not already in the list. I've not added the loop but it will work. I'm not currently on my laptop, I'm typing on a tablet right now.

B4X:

Globals_Sub Private MyList As List Activity_Sub 'Just for testing MyList.Initialize MyList.AddAll(Array As Int(0,1,2,4,5,6,7,8,9)) 'Adds 0,1,2,4,5,6,7,8,9 to list Dim ArrayItem As Int = 3 'Item to be added to list Log(MyList.IndexOf(ArrayItem)) 'See if 3 is already in the list (returns item index or -1) If MyList.IndexOf(ArrayItem) = -1 Then MyList.Add(ArrayItem) '-1 means not in list, Add 3 to list For i = 0 To MyList.Size - 1 Log(MyList.Get(i)) 'Show the list in the logs (3 should be at the bottom of the list, hopefully) Next

Enjoy...

I did it with the basics of your example! Thanks you very much!!!

udg · Jan 18, 2018

I did a quick look at the tutorial, and tried some code, but I do not find a way to test if some element is in the map

The idea was to avoid the test since a map doesn't allow duplicate keys.
So, when you extract again a random value already used, this one will take the place of itself in the map. You exit the extraction loop when your map has 10 items (i.e. 10 different rnd numbers).

Peter Simpson · Jan 18, 2018

udg said:
The idea was to avoid the test since a map doesn't allow duplicate keys.
So, when you extract again a random value already used, this one will take the place of itself in the map. You exit the extraction loop when your map has 10 items (i.e. 10 different rnd numbers).

UDG is saying to just do the following instead, personally it never actually crossed my mind

B4X:

'Declare your map and Initialize it
    Dim Mappy As Map
        Mappy.Initialize

'Populate the map with 10 unique random numbers
    Do Until Mappy.Size = 10
        Mappy.Put(Rnd(0, 99), "")
    Loop

'Read the 10 unique numbers out of the map into the logs
    For Each Key As Int In Mappy.Keys
        Log(Key)
    Next

Enjoy...

Mahares · Jan 18, 2018

Peter Simpson said:
Mappy.Put(Rnd(0, 99), "")

The only thing I would change is this line. It should be:
Mappy.Put(Rnd(0, 100), ""). Otherwise, you are excluding 99 as a key

Germán Arduino · Jan 18, 2018

Ahhh, you means using the keys (no the values) of the map as the random! never crossed my mind neither.

I finally did it at this way:

B4X:

MyList.Initialize
   
    ' I need 6 different numbers between 0 and 45
    MyList.AddAll(Array As Int(46,46,46,46,46,46))
   
    For i = 1 To 6
        Dim ArrayItem As Int = Rnd(00, 46)
        Do While MyList.IndexOf(ArrayItem) <> -1
             ArrayItem = Rnd(00, 46)
        Loop
        MyList.InsertAt(i, ArrayItem)
     Next

Seems to work!

Mahares · Jan 19, 2018

Germán Arduino said:
Seems to work!

Although using a map is the preferred way as Peter pointed it out with a clear example, I am afraid your code is not correct:. If you are intent in using a list and your code, you should add a second list. See example below:

B4X:

Dim MyList,MyList2 As List
    MyList.Initialize
    MyList2.Initialize
    MyList.AddAll(Array As Int(46,46,46,46,46,46)) 
    For i = 1 To 6
        Dim ArrayItem As Int = Rnd(00, 46)
        Do While MyList.IndexOf(ArrayItem) <> -1
            ArrayItem = Rnd(00, 46)
        Loop
        MyList.InsertAt(i, ArrayItem)
        MyList2.Add(MyList.Get(i))  'YOU NEED TO ADD THIS LINE
    Next
    Log(MyList2)  'displays 6 random non duplicate numbers

Germán Arduino · Jan 19, 2018

Thanks for your comment Mahares.

mmm, you are right (I must still understand how work the lists) but I do not noticed the problem because I extracted (in my code) the elements 1 to 6 of the resulting list.

Thanks for your clarification!

Android Question How to check duplicates in array

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