Bug #14641: Shouldn't Array#map not modify its array?

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 2.5.0 and 2.4.2
• Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN

[hartator (Julien Khaleghy)]

Hi,

Thanks for the awesome work you guys have been doing on Ruby for all this years.

I might be missing something obvious, but I thought Array#map doesn't modify the array it's called on?

Ie.

ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-linux]
    a = ['a','b'] 
..    
=> ["a", "b"]
    a.map{|e| e << 'c'} 
=> ["ac", "bc"]
    a.map{|e| e << 'c'} 
=> ["acc", "bcc"]
    a.map{|e| e << 'c'} 
=> ["accc", "bccc"]
    a.map{|e| e << 'c'} 
=> ["acccc", "bcccc"]
   a
=> ["acccc", "bcccc"]

Expected a to be still ["a", "b"].

Same in Ruby 2.4 ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-darwin16]

irb(main):003:0*  a = ['a','b']
=> ["a", "b"]
irb(main):004:0>  a.map{|e| e << 'c'} 
=> ["ac", "bc"]
irb(main):005:0>  a.map{|e| e << 'c'} 
=> ["acc", "bcc"]
irb(main):006:0>  a.map{|e| e << 'c'} 
=> ["accc", "bccc"]
irb(main):007:0>  a.map{|e| e << 'c'} 
=> ["acccc", "bcccc"]
irb(main):008:0>  a 
=> ["acccc", "bcccc"]

Thanks for you guys time.

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Rejected

The array itself is not modified, you are mutating elements of the current array during iteration (as String#<< mutates the receiver), then returning a new array with the mutated elements (since String#<< returns the receiver). If you don't want to mutate the elements, use String#+:

irb(main):001:0> a = ['a', 'b']
=> ["a", "b"]
irb(main):002:0> a.map{|e| e + 'c'}
=> ["ac", "bc"]
irb(main):003:0> a
=> ["a", "b"]
irb(main):004:0> a.object_id
=> 11790377160340
irb(main):005:0> a.map{|e| e << 'c'}.object_id
=> 11788696580840

[hartator (Julien Khaleghy)]

jeremyevans0 (Jeremy Evans) wrote:

The array itself is not modified, you are mutating elements of the current array during iteration (as String#<< mutates the receiver), then returning a new array with the mutated elements (since String#<< returns the receiver). If you don't want to mutate the elements, use String#+:

irb(main):001:0> a = ['a', 'b']
=> ["a", "b"]
irb(main):002:0> a.map{|e| e + 'c'}
=> ["ac", "bc"]
irb(main):003:0> a
=> ["a", "b"]
irb(main):004:0> a.object_id
=> 11790377160340
irb(main):005:0> a.map{|e| e << 'c'}.object_id
=> 11788696580840

It was my bad. Thanks for the explanation, Jeremy.