Combining 6 sets of 2 elements in Java

Question

1I'm making a "Virtual Keyboard" for typing passwords in Java.

The password must have 6 numeric chars and each button has 2 possible values. So, there are 2 values possible for each pressed key.

For example: after pressing 6 buttons, I have an array of 6 objects, each one representing the possible values for each character of the password, respectively:

(1 , 5) for char1
(3 , 2) for char2
(7 , 4) for char3
(1 , 5) for char4
(9 , 0) for char5
(8 , 6) for char6

How can I make all combination of 6-digit-strings with these entries? (I guess that would be 64 combinations: 2ˆ6)

Example of valid combinations:

137198
537198
127198
537196

and so on...

You can use this [link](http://stackoverflow.com/questions/2920315/permutation-of-array) — donut, Apr 11 '14 at 19:42
Don't think so. I'm not permuting all elements. I'm combining two elements for each char. Witch will give me 2ˆ6 combinations... — André Perazzi, Apr 11 '14 at 19:46
This seems rather easy to solve with recursion, have you tried? — Bernhard Barker, Apr 11 '14 at 19:50
Actually, each time the user presses the button, I'm re-shuffling all the numbers. But I save a record of the two possible values. — André Perazzi, Apr 11 '14 at 19:52
arr[pos] = val[pos][1]; rec(pos+1); arr[pos] = val[pos][2]; rec(pos+1); — Ralor, Apr 11 '14 at 20:11

score 2 · Accepted Answer · answered Apr 11 '14 at 19:56

2

Given a sequence of n buttons, where each can assume 2 values, this problem reduces to counting from 0 to 2^n-1 since you can interpret each bit at position i as one of the two values that each button can assume. For n=3:

000 -> 137
001 -> 134
010 -> 127
011 -> 124
100 -> 537
...

So just count from 0 to 2^6 and you automatically get your solution.

This is the same way you would go about generating the 2^n subsets of a set of n elements.

answered Apr 11 '14 at 19:56

Giovanni Botta

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So after having all 2ˆ6 numbers I'll have to loop over every bit of every number, generating a string? – André Perazzi Apr 11 '14 at 20:05
1

Well I would increment the counter and generate the string. No need to keep 2^6 numbers in memory! – Giovanni Botta Apr 11 '14 at 20:07

score 1 · Answer 2 · answered Jun 28 '15 at 22:45

1

Just create every combination possible and check if each of them is the correct one

answered Jun 28 '15 at 22:45

Diogo Ebert

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Combining 6 sets of 2 elements in Java

2 Answers2