Enter these in irb:
a = [1,2,3]
a.each do |i|
a.delete(i)
end
Result?
a => [2]
Shouldn’t a be [] ?
Eric
On Thursday 29 May 2008, [email protected] 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
Thanks Stefano, that definitely answered my question. It’s quite
obvious when the index variable is made explicit.
Eric