How do I push unique copies of array into another array?

Question

I'm trying to push subsets into an array:

def subsets(arr)
    subsets = [[]]
    temp_arr = []
    i = 0
    while i < arr.length
        temp_arr << arr[i]
        subsets << temp_arr
        p subsets
        i+=1
    end
    return subsets
end

My results look like this:

[[], ["a"]]
[[], ["a", "b"], ["a", "b"]]
[[], ["a", "b", "c"], ["a", "b", "c"], ["a", "b", "c"]]

Why is it that every time I push the temp_array to the subset array, the previous result of pushing temp_array also changes?

Is there a way I can push the unique instances of temp_array and keep it in that state?

Also, can anyone give me a hint as to how I can get all subsets from an array?

Yu Hao · Accepted Answer · 2015-08-21 04:58:42Z

2

subsets << temp_arr

temp_arr references to the actual Array object. After this, both the element appended to subsets and temp_arr reference to the same object.

If this is not what you expected, use dup to gets its copy:

subsets << temp_arr.dup

answered Aug 21, 2015 at 4:58

Yu Hao

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Comments

Schwern · Accepted Answer · 2015-08-21 05:10:13Z

The problem is you're pushing the same object, temp_arr, onto subsets.

Ruby generally does not copy data, it passes around references to objects. subsets << temp_arr puts a reference to temp_arr onto subsets, not whatever happens to be in temp_arr at the moment.

subsets[0] -> temp_arr
subsets[1] -> temp_arr
subsets[2] -> temp_arr

To avoid this, you want to make a copy of temp_arr and push that onto subsets.

while i < arr.length
    temp_arr << arr[i]
    subsets << temp_arr.clone
    p subsets
    i+=1
end

As an aside, your loop can be written better using each.

arr.each { |element|
    temp_arr << element
    subsets << temp_arr.clone
    p subsets
}

Or avoid the temp variable entirely by using an array slice.

arr.each_index { |idx|
    subsets << arr[0..idx]
    p subsets
}

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