# Mysterious behavior with loops

**URL:** <https://www.ruby-forum.com/t/mysterious-behavior-with-loops/138984>\
**Category:** Ruby\
**Created:** [May 29, 2008, 7:01pm UTC](https://www.ruby-forum.com/t/mysterious-behavior-with-loops/138984 "2008-05-29T19:01:49Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [May 29, 2008, 7:01pm UTC](https://www.ruby-forum.com/t/mysterious-behavior-with-loops/138984/1 "2008-05-29T19:01:49Z")

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Enter these in irb:

a = [1,2,3]

a.each do |i|  
a.delete(i)  
end

Result?

a =\> [2]

Shouldn’t a be [] ?

Eric

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<div class="post-metadata">

**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [May 29, 2008, 7:19pm UTC](https://www.ruby-forum.com/t/mysterious-behavior-with-loops/138984/2 "2008-05-29T19:19:28Z")

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On Thursday 29 May 2008, removed\_email\_address@domain.invalid wrote:

> a =\> [2]
> 
> Shouldn’t a be [] ?
> 
> Eric

Iterating on an array while deleting elements from it is usually a bad  
idea,  
because items may be skipped, as it happens in your case. In particular,  
this  
is the C code for Array#each:

VALUE  
rb\_ary\_each(ary)  
VALUE ary;  
{  
long i;  
for (i=0; i\<RARRAY(ary)-\>len; i++) {  
rb\_yield(RARRAY(ary)-\>ptr[i]);  
}  
return ary;  
}

If you don’t understand C code, this is what it means: for all the  
numbers  
from 0 to the number of elements in the array (excluded), take the  
element of  
the array with that index and pass it to the block.

Now, look what happens for the first element of the array. The index is  
0,  
corresponding to the element 1. The element is passed to the block,  
which  
deletes it. Now, the array contains only two elements: 2 and 3, with 2  
corresponding to the index 0 and 3 to the index 1. But the index used by  
each  
to iterate on the array elements is increased to 1 (since Array#each  
can’t  
know that you deleted an item). This means that the next element which  
will be  
passed to the block will be the one corresponding to index 1, which is  
3. This  
means that one item won’t be passed to the block and won’t be deleted.

If you want to delete all items of the arryay, you can simply use the  
delete  
method. If you want to delete only some items, you can use  
Array#delete\_if,  
which takes a block and removes from the array all the elements for  
which the  
block returns true.

I hope this helps

Stefano

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<div class="post-metadata">

**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [May 29, 2008, 8:36pm UTC](https://www.ruby-forum.com/t/mysterious-behavior-with-loops/138984/3 "2008-05-29T20:36:38Z")

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Thanks Stefano, that definitely answered my question. It’s quite  
obvious when the index variable is made explicit.

Eric