Why does Array.delete inside Array.each not delete all items?

Question

I am new to Ruby and I wrote a very simple application to print the days of week and then delete one day in a loop:

def print_days(days)
    days.each do |day|
        print "The day of the week is: #{day}\n"
        days.delete(day)
        print "\n*****************************************************\n"
        print days
        print "\n*****************************************************\n"
    end
end

wd = %w[Monday Tuesday Wednesday Thursday Friday Saturday Sunday]

print print_days(wd

This gives the following output when run. Can anyone explain me why Tuesday, Thursday and Saturday are skipped when I am deleting each element sequentially and the array shows them being there? You can run this simple code at your setup:

The day of the week is: Monday

*****************************************************
["Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
*****************************************************
The day of the week is: Wednesday

*****************************************************
["Tuesday", "Thursday", "Friday", "Saturday", "Sunday"]
*****************************************************
The day of the week is: Friday

*****************************************************
["Tuesday", "Thursday", "Saturday", "Sunday"]
*****************************************************
The day of the week is: Sunday

*****************************************************
["Tuesday", "Thursday", "Saturday"]
*****************************************************
["Tuesday", "Thursday", "Saturday"]

why you want to call delete inside each ? what you are trying to achieve? — zed_0xff
– zed_0xff, Commented Jan 18, 2013 at 20:58

AShelly · Accepted Answer · 2013-01-18 21:04:30Z

4

You are deleting elements from the array while you are iterating through it, invalidating the iterator.

You could try

   until (days.empty?) 
       day = days.shift
       print "The day of the week is: #{day}\n"
   end

or

   days.each{|day| print "The day of the week is: #{day}\n"}
   days.clear

edited Jan 18, 2013 at 21:04

answered Jan 18, 2013 at 20:58

AShelly

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3 Comments

Aaron Kurtzhals Over a year ago

Should shift be used instead of unshift?

Qufr kaifi Over a year ago

AShelly, that helped. Thanks a lot! I have a question when I iterating using array.each is not the same as iterating using 'until'? Does until invoke an iterator?

digidigo Over a year ago

Until does not invoke the iterator and doesn't work the way .each works either. Technically .each isn't an iterator either , but it is useful to think of it that way especially if you are coming from java.

digidigo · Accepted Answer · 2013-01-18 21:00:15Z

3

You are modifying the array during the iteration of all the elements. Internally the each method is keeping the index of the last item it yielded to your block. This is basically invalidating the iterator. Other languages would throw an exception for you.

Ruby does not.

So on the first time through it yields the element at index 0

Then you delete the element at index 0

Then the next time through the it yields the element at index 1, which basically skips the Tuesday since it is now at index 0.

answered Jan 18, 2013 at 21:00

digidigo

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Comments

Alex Wayne · Accepted Answer · 2013-01-18 21:02:25Z

2

You broke the cardinal rule, do not mutate the object you are iterating over, while you are iterating over it!

Here what's happeneing:

Iterate over the array
Start with the first item
Delete the first item
The second item in the array is now the first.
Grab the second item, and since the previous second item is now first, it grabs what used to be the third item.
Delete the second item (which used to be third) in the mutated array

So it sort of skips deleting every other item. These types of bizarre bugs are why it's very frowned upon to change the object that you are iterating over, while you are iterating over it. Don't do that.

But given how contrived your example is, it's hard to suggest a better way. Depending on your actual goal, there is better ways to do this.

answered Jan 18, 2013 at 21:02

Alex Wayne

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2 Comments

the Tin Man Over a year ago

I thought the cardinal rule was "never get involved in a land war in Asia", followed by "Never go against a Sicilian when death is on the line"

Alex Wayne Over a year ago

Mutating an array while iterating over it? Inconceivable!

Leif · Accepted Answer · 2013-01-18 23:46:43Z

0

Given most of the above answers explain why what you're doing is failing, you can get what you seem to want using variations of

Array#drop_while
Array#delete_if
Array#select!

or

Array#keep_if

e.g.

a=[1,2,3,4]
a.drop_while{|e| puts e; true}

Gives

1
2
3
4
 => []

edited Jan 18, 2013 at 23:46

answered Jan 18, 2013 at 22:53

Leif

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