> Others have already replied to use std::views::iota and std::views::reverse (which I also mentioned a while ago) instead. I still think that this is a lot to type and so, people will most likely not use it. It would be a lot more useful if those lived directly in the std namespace.

Agreed+ the name makes it a bit opaque to realize that it is even there.
But perhaps a solution could be to make that easier to type? So that you would get the iota but give it a more teachable name/syntax.

________________________________
From: Std-Proposals <std-proposals-bounces_at_[hidden]> on behalf of Simon Schröder via Std-Proposals <std-proposals_at_[hidden]>
Sent: Saturday, February 15, 2025 9:54:26 AM
To: std-proposals_at_[hidden] <std-proposals_at_[hidden]>
Cc: Simon Schröder <dr.simon.schroeder_at_[hidden]>
Subject: Re: [std-proposals] for-loops revision (simplification and new syntax)


On Feb 14, 2025, at 7:41 AM, Sebastian Wittmeier via Std-Proposals <std-proposals_at_[hidden]> wrote:



Hi Simon,

could you shortly explain the possible bug in the example given?

For unsigned integers the condition “i>=0” is always true, so you got yourself an infinite loop. I didn’t even think about “size-1” being “negative” (and thus overflow). The loop would have been perfectly fine with a signed integer (but compilers at least warn because of an implicit cast from unsigned to signed, so I tend to not use signed integers to avoid warnings; at least ssize() instead of size() comes to the rescue).

Others have already replied to use std::views::iota and std::views::reverse (which I also mentioned a while ago) instead. I still think that this is a lot to type and so, people will most likely not use it. It would be a lot more useful if those lived directly in the std namespace.

I guess the main question to answer would be what something like 0..size would actually mean in C++. So far, C++ usually does not have any syntax elements that create a library type (with the exception of std::initializer_list which is not a full type like std::vector). Would we really want 0..size (or some similar syntax) to translate into a library type? Would I have to include a header in order to be able to write this kind of loop? Or would “for(std::size_t i : 0..vec.size())” just internally rewrite this to a regular loop counter “for(std::size_t i=0; i<vec.size(); ++i)”? In the latter case rewriting something like “for(std::size_t i : vec.size()..0)” (0 is inclusive, vec.size() is exclusive) would be more involved.

Currently, we could experiment with syntax by implementing an operator" to be able to write "0..42"_rng.



To get the boundaries of size-1 and the >=0 correct or to avoid unsigned integer underflow?



auto leads to unsigned and size is 0?


-----Ursprüngliche Nachricht-----
Von: Simon Schröder via Std-Proposals <std-proposals_at_[hidden]>
Gesendet: Fr 14.02.2025 06:52
Betreff: Re: [std-proposals] for-loops revision (simplification and new syntax)
An: std-proposals_at_[hidden];
CC: Simon Schröder <dr.simon.schroeder_at_[hidden]>;

Some of the discussion now steered towards: “Is a new loop syntax useful?”

I would prefer a syntax that leverages the existing range syntax (something similar to Rust or Swift). There is probably one case where such a new syntax would help avoid bugs:
for(std::size_t i = vec.size()-1; i >= 0; --i) …
i.e. counting down a loop with an unsigned integer. This might be obfuscated a little more if one uses ‘auto’ to declare the loop counter. With a new syntax (that also allows to specify the step size and the step size can be negative) the implementation would hopefully fix this edge case.

On Feb 13, 2025, at 10:15 PM, Sebastian Wittmeier via Std-Proposals <std-proposals_at_[hidden]> wrote:



One should add that the else branch is not run in Python, when the loop has been left with a corresponding break instruction.



-----Ursprüngliche Nachricht-----
Von: Magnus Fromreide via Std-Proposals <std-proposals_at_[hidden]>

If we are talking of loop Pythonisms then a loop else branch does add some
value as that case often is a bit contrieved to test for. (The else branch
of a loop is executed when the loop condition fails to run the loop more
times)
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