This post was deleted by me.

A funny thing that I stumbled into just yesterday, which I suspect is directly related to the scoping rules in Julia. Working on my Algorithms assignment I was asked to calculate the time and space complexity of this code: int main(int n) { int r = 0; for (int i = 1; i < n; ++i) { i--; …

This has nothing to do with scoping but how a C-loop is defined. In Julia, you would write the equivalent of the C-code as function main(n::Int) r = 0 i = 1 while i < n i-=1 r+=1 i+=1 end return r end What you have in your original Julia code …

I think that has nothing to do with scope, but with the way that for works in Julia, just iterating over a well-defined collection (which is of finite length). At each step you assign i to the next element in the collection. This is way off topic, but if you want to do something like the c code you …

Oops, @kristoffer.carlsson beat me to it! He seems to be everywhere all the time. Thanks for all your help in discourse Kristoffer!

We can of course create a macro that simulates the C for loop style macro cfor(loop_preamble, body) init, cond, post = loop_preamble.args return quote let $init while $cond $body $post end end end end j…

It is not the same. Using “for i in 1:n” in Julia, you update i using an iterator, it does not use the previous value, so if you change it inside the block, it does not matter. Using a while the result should be the same than in C. It has not relationship with the scope of variable i.

Yes, exactly, good point with the iterator. I tried using all sorts of global and local declarations to access the i but yes, inside the loop I was always dealing with a different variable and I didn’t know how Julia implements that. @kristoffer.carlsson @pablosanjose thanks for the rewrite, very h…

This has nothing to do with scoping but how a C-loop is defined. Pretty interesting, this means that for loops in Julia are primitive recursive, no?

Could you elaborate what is interesting? Remember that for i in iter; #body; end is just syntax for next = iterate(iter) while next !== nothing (i, state) = next # body next = iterate(iter, state) end and that it is trivial to write an iterator that works just like the C-loop.

Its just interesting from a theoretical point of view since it means for loops are really less powerful in Julia than in C. Note that this is a good thing. If one can avoid while loops and recursion it should be possible showing that a given function halts.

Stating my opininon

Community

stevengj Split this topic October 9, 2018, 4:37pm 26

2 posts were split to a new topic: Allocations in function timing

Topic		Replies	Views
Another possible solution to the global scope debacle Internals & Design	231	19813	November 6, 2018
Global scope confusion again General Usage question , scope	52	6908	May 17, 2025
Local vs global scope again General Usage	30	2476	August 9, 2020
Tips to cope with scoping rules of top level `for` loops etc General Usage scope	30	6933	February 12, 2021
Explain scoping confusion to a programming beginner New to Julia question	26	3347	August 11, 2020

Stating my opininon

Related topics