Suprised by destructive map (not map!)

Occasionally I run into bits of ruby that are surprising to me (although
might not be to matz). Here is one of them. Should I file this as a bug
(I thought .map is supposed to be non-destructive), or just keep in mind
which classes are passed by value and which are passed by reference.

Basically: map (and naturally also map!) is destructive when your array
includes elements which are normally not passed by value but by
reference.

ruby 1.8.6 (2007-09-24 patchlevel 111) [x86_64-linux]
boris:falcon:~:irb

“unexpected (for me)”
irb(main):001:0> array = [[1],[2]]
=> [[1], [2]]
irb(main):002:0> array.map {|i| i << 3}
=> [[1, 3], [2, 3]]
irb(main):003:0> array
=> [[1, 3], [2, 3]]
irb(main):004:0>

“expected behaviour”
irb(main):005:0* array = [1,2]
=> [1, 2]
irb(main):006:0> array.map {|i| i + 3}
=> [4, 5]
irb(main):007:0> array
=> [1, 2]
irb(main):008:0>

Boris S. wrote:

boris:falcon:~:irb

“unexpected (for me)”
irb(main):001:0> array = [[1],[2]]
=> [[1], [2]]
irb(main):002:0> array.map {|i| i << 3}
=> [[1, 3], [2, 3]]
irb(main):003:0> array
=> [[1, 3], [2, 3]]
irb(main):004:0>

It’s not map that’s destructive here, it’s the #<< method. Map iterates
over the original array, and creates a new one with the results of the
block - it doesn’t copy the original array first.

Use it:
irb(main):001:0> array = [[1], [2]]
=> [[1], [2]]
irb(main):002:0> array.map{|a| a += [3]}
=> [[1, 3], [2, 3]]
irb(main):003:0> array
=> [[1], [2]]

Bernardo Monteiro R. wrote:

Use it:
irb(main):001:0> array = [[1], [2]]
=> [[1], [2]]
irb(main):002:0> array.map{|a| a += [3]}

That expands to:

array.map{|a| a = a + [3] }

The Array#+ method isn’t destructive, and a variable assignment in a
block isn’t either.

Exactly

Thanks for clarifying that. I’ll compile a list of destructive methods
for myself, and be aware of them when I map.

Also learned not to trust blindly in the ! from another post.

2007/12/13, Boris S. [email protected]:

Thanks for clarifying that. I’ll compile a list of destructive methods
for myself, and be aware of them when I map.

Also learned not to trust blindly in the ! from another post.

You should also remember that all parameters are passed by reference*.
Whether hazard occurs depends on what you do with them, especially
the mutable ones. In your case elements in your Array were Arrays
which are mutable. The issue does not happen if you used integers
instead - even with <<:

irb(main):001:0> a1 = [1,2]
=> [1, 2]
irb(main):002:0> a2 = a1.map {|x| x << 1}
=> [2, 4]
irb(main):003:0> a1
=> [1, 2]

  • Note: this is not 100% correct implementation wise but describes the
    situation properly from a Ruby user’s point of view.

Kind regards

robert

irb(main):002:0> array.map {|i| i << 3}
=> [[1, 3], [2, 3]]

Well, Array#<< means: append an element to an array and return
the
array. It does not mean make a copy of the array plus the new
element.
What you expected would be one of

array.map {|i| i.dup << 3}
array.map {|i| i + [3]}

It’s not necessary to use += here since the value of “i += [3]” is i,
which you really don’t care about since you only want “i + [3]”.

Fixnum#<< means shift bits. Same symbol but totally different story.
Also, numerics are no container classes but have immediate values,
i.e.
they are not passed by reference (according to Programming Ruby 2,
p463), which is also why you can’t dup/clone them.

array.map{|a| a += [3]}

That expands to:

array.map{|a| a = a + [3] }

You might want to skip the useless assignment:

array.map{|a| a + [3]}

gegroet,
Erik V. - http://www.erikveen.dds.nl/

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