> On Apr 12, 2016, at 4:15 AM, Dmitri Gribenko via swift-evolution
> <[email protected]> wrote:
>
> On Mon, Apr 11, 2016 at 9:56 PM, Brent Royal-Gordon via
> swift-evolution <[email protected]> wrote:
>> (On the other hand, it might be that I'm conceiving of the purpose of
>> `limitedBy` differently from you—I think of it as a safety measure, but you
>> may be thinking of it specifically as an automatic truncation mechanism.)
>
> Hi Brent,
>
> Could you explain what kind of safety do you have in mind? Swift will
> guarantee memory safety even if you attempt to advance an index past
> endIndex using the non-limiting overload.
One challenge that I've run into is that the `limitedBy` methods throw away one
bit of information—namely, did I move as far as I requested or not? Example:
let j = c.index(10, stepsFrom: i, limitedBy: c.endIndex)
There's no way to interpolate the answer to that question efficiently in a
non-random-access collection. If `j` is equal to `c.endIndex`, that could be
because `c.endIndex` is ten steps after `i` *or* because the limit kicked in,
and without checking `c.distance(from: i, to: j)` there's no way to know for
sure.
If the `limitedBy` methods returned an optional index, we'd get all the
information that the index-moving algorithm finds (let's hear it for the Law of
Useful Return!). With that API, we could decide whether to use the returned
index or not:
// Use the resulting index no matter what:
let i = c.index(10, stepsFrom: c.startIndex, limitedBy: c.endIndex) ??
c.endIndex
let prefix = c.prefix(upTo: i)
// Only use the result if it wasn't limited:
if let j = c.index(10, stepsFrom: i, limitedBy: c.endIndex) {
let sub = c[i..<j] // sub.count == 10
} else {
// not enough elements...
}
// "Safe" successor:
if let j = c.index(1, stepsFrom: i, limitedBy: c.endIndex) {
// ...
}
Nate
> Dmitri
>
> --
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <[email protected]>*/
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