Aren’t there already methods that stick elements between others (and only
between, neither before the first nor after the last)?
//=====
fun toCSV2(values: [CustomStringConvertible]) -> String {
return values.map { $0.description }.joinWithSeparator(“, “)
}
//=====
—
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT mac DOT com
> On May 17, 2016, at 11:49 AM, Haravikk via swift-evolution
> <[email protected]> wrote:
>
> This only works with the example exactly as-is, I think a fairer re-write
> would be something like the following:
>
> func toCSV(values:[CustomStringConvertible]) -> String {
> var gen = values.generate()
>
> var result = gen.next()?.description ?? ""
> while let value = gen.next() { result += ", \(value)" }
> return result
> }
>
> toCSV([1, 2, 3, 4, 5])
>
> For brevity I opted to use nil-coalescing and a default value to handle the
> first (potentially nil) element cleanly.
>
> I’m undecided about whether I want the feature or not; as shown the example
> can actually be handled very neatly already, so it doesn’t really highlight a
> need for the feature. One thing I like about it though is the ability to
> possibly eliminate the need for a generator and use a for in loop instead,
> like so:
>
> func toCSV(values:[CustomStringConvertible]) -> String {
> var result = ""
> for value in values {
> once { result += "\(value)"; continue }
> result += ", \(value)"
> }
> return result
> }
>
> But this isn’t really much of a saving. I can’t think of a real-world example
> offhand that can better demonstrate a use for this; ideally you’d want to use
> several once {} blocks, or do something in the loop that requires other
> statements in the loop, i.e- you use the once to avoid duplicating statements
> in the loop while processing the first element.
>
>> On 17 May 2016, at 07:13, Robert Widmann via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>> Not to doubt the usefulness of this proposal in general, but your example is
>> subsumed by simply declaring
>>
>> var result = "\(gen.next()!)"
>>
>> and proceeding with the rest of the example sans `once`. I think you'll
>> also have to address how this is any different from dispatch_once or a DSL
>> over the same.
>>
>> Cheers,
>>
>> ~Robert Widmann
>>
>> 2016/05/17 0:03、Nicholas Maccharoli via swift-evolution
>> <[email protected] <mailto:[email protected]>> のメッセージ:
>>
>>> Hello Swift Evolution,
>>>
>>> Its not uncommon to have to do a piece of work only once or on the first
>>> iteration of
>>> a loop.
>>> Take for example producing a comma separated string from an Array:
>>>
>>> var result = ""
>>>
>>> let values = [1, 2, 3, 4, 5]
>>>
>>> var gen = values.generate()
>>>
>>>
>>>
>>> if let first = gen.next() {
>>>
>>> result += "\(first)"
>>>
>>> while let value = gen.next() {
>>>
>>>
>>> result += ", "
>>>
>>> result += "\(value)"
>>>
>>> }
>>>
>>> }
>>>
>>>
>>>
>>> Since on the first iteration we want to skip putting a comma in front we
>>> use an `if let` to grab the first element and then embed a `while let`
>>> inside the `if let` to handle the rest.
>>>
>>>
>>>
>>> Another way to do this could be using a bool to keep track of the first
>>> iteration:
>>>
>>>
>>>
>>> var first = true
>>>
>>> while let value = gen.next() {
>>>
>>> if first {
>>>
>>> result += "\(value)"
>>>
>>> first = false
>>>
>>> continue
>>>
>>> } else {
>>>
>>> result += ", "
>>>
>>> result += "\(value)"
>>>
>>> }
>>>
>>>
>>> }
>>>
>>>
>>>
>>> These approaches work, but I think there may be a way to do this with less
>>> noise.
>>>
>>> If there was a keyword to execute a block of code only on the first
>>> iteration of a loop I think that would make code like this more concise.
>>>
>>> If there was a keyword like `once` then the same thing could be achieved
>>> with something like:
>>>
>>>
>>>
>>> while let value = gen.next() {
>>>
>>> once {
>>>
>>> result += "\(value)"
>>>
>>> continue
>>>
>>> }
>>>
>>> result += ", "
>>>
>>> result += "\(value)"
>>>
>>>
>>> }
>>>
>>>
>>>
>>> How does it sound?
>>>
>>>
>>>
>>> - Nick
>>>
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