I would be ok with loop variables always being local to the loop so that
this would happen:
julia> i = 0
0
julia> for i = 1:5
println(i)
end
1
2
3
4
5
julia> i
0
That would simplify the semantics of for loops at the cost of making it
harder to access the final value of a loop variable – but that's a somewhat
rare thing to do and explicitly assigning the loop variable to an variable
from outside the loop is arguably clearer.
On Fri, Feb 6, 2015 at 7:05 AM, David Higgins <[email protected]>
wrote:
> So all of this seems to be a result of the for loop basically using a
> range/array type variable on the right hand side of the loop variable. So
> even if I change the value of the iterator variable within the loop I
> cannot change the array over which I am looping (anyone used to Python will
> find this natural, I'm more used to C).
>
> I guess everything is consistent now.
>
> David
>
> On Friday, 6 February 2015 12:25:34 UTC+1, David Higgins wrote:
>
>> I have a use case I found interesting. I guess it's well known to some of
>> you, but I never noticed it in the documentation and I find it
>> counterintuitive, both as an experienced programmer and with respect to the
>> typical scope rules of variables in Julia.
>>
>> I accidentally nested a loop within another loop using the same index
>> variable for both loops. First of all, there are reasons sometimes you'd
>> want to do this but I did it by accident as a typo. But the scope rules of
>> variables for Julia, as applied to functions, state that you can read a
>> variable value at a lower level in the hierarchy but you cannot write to it
>> so I actually assumed that this also applied to loops (*first mistake*).
>>
>> Secondly, as you'll see if you execute the simplified example of what I
>> actually did here:
>> for i = 1:5
>> for j = 1:5
>> for i = 1:3
>> print("tick ");
>> end
>> print(" $i\n");
>> end
>> print(" $j $i\n");
>>
>> Julia still knew to only run the outer loop 5 times, despite the fact
>> that I was setting the variable bound to $i at level 2 to the value 3 at
>> level 3 in the nested hierarchy of loops. I suspect that I can see what's
>> happening at the compiler/interpretter level, but I do think that this is
>> confusing.
>>
>> We seem to have the worst of both worlds here in that we can access and
>> change $i at an inner loop level and have the change be persistent, but yet
>> we cannot change the behaviour of the outer loop (as would be the case in
>> C).
>>
>> What do you guys think? Can this be clarified in the documentation, or
>> should the actual behaviour of julia be modified?
>>
>> David.
>>
>
