Understanding sort method and in general blocks in ruby

Question

I am just trying to understand the Ruby sort function and the blocks, and I came up with the following code:

a = [1,2,3]
a.sort do |x,y|
  x
end

Won't the returning x be taken as the factor to sort the two elements? I expect the following behaviour:

1,2 get passed as block parameters, 1 is returned.
2,3 get passed as block parameters, 2 is returned.
1,3 get passed as block parameters, 3 is returned.

So considering the returned values, won't the sorted array still be [1,2,3]? Where am I getting it wrong?

There is another method which uses the return value from the block as a sorting factor, it's called sort_by. — Mladen Jablanović
– Mladen Jablanović, Commented Aug 6, 2012 at 7:42

Darshan Rivka Whittle · Accepted Answer · 2012-08-06 06:12:08Z

7

The block needs to return -1, 0, or 1. I don't believe there are any guarantees about the order the values get passed in. Since you are not honoring the contract with your return value, the result is undefined.

In reality, what I believe is happening is that you are always returning a positive value, so the second (later in the array) value is always moved forward. But again, that's not guaranteed according to the documentation.

This behaves more or less like your description:

a = [1,2,3]
a.sort do |x,y|
  x <=> y
end

answered Aug 6, 2012 at 6:12

Darshan Rivka Whittle

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3 Comments

nik7 Over a year ago

is there any strict path in what order x,y values gets passed to the block ? when i use a bigger array like [1,2,3,4,5], the x,y order is (1,3),(3,5),(2,3),(4,3),(2,4),(4,1)

Darshan Rivka Whittle Over a year ago

@nlingutla The documentation doesn't specify an order, so I don't consider it safe to count on any specific order. Different implementations (and future versions of the same implementation) may well use a different algorithm.

Alex D Over a year ago

Interesting... I checked the order which compares are done on my system (running Ruby 1.9.3), and they are a bit different from @nlingutla. It appears that up to 4 elements, a special-purpose sort function which uses a fixed sequence of compares is used. After that, it seems to be using quicksort. I haven't checked the interpreter source -- I'm just guessing from observing the order which compares are done.

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