I may be missing something obvious, but I can’t find a library function that maps a Dictionary to another Dictionary, transforming the keys and/or values. The only method I’ve found is the regular map(), which results in an Array. Does this just not exist in the standard library?
On Mon, Aug 29, 2016 at 11:36 PM, Jens Alfke via swift-users <swift-users@swift.org> wrote:
I may be missing something obvious, but I can’t find a library function that maps a Dictionary to another Dictionary, transforming the keys and/or values. The only method I’ve found is the regular map(), which results in an Array. Does this just not exist in the standard library?
I don''t quite understand your question. In Swift, Dictionaries are
structs. You can always use `let dict2 = dict1`. That is enough. Or did you
mean below code?
var dict1 = ...
var dict2 = ...
for (key, value) in dic1 {
dic2.updateValue(value, forKey:key)
}
Zhaoxin
···
On Tue, Aug 30, 2016 at 4:36 AM, Jens Alfke via swift-users < swift-users@swift.org> wrote:
I may be missing something obvious, but I can’t find a library function
that maps a Dictionary to another Dictionary, transforming the keys and/or
values. The only method I’ve found is the regular map(), which results in
an Array. Does this just not exist in the standard library?
I’m talking about a function that does for Dictionaries what map() does for Arrays: it transforms every key and value in the input Dictionary (through a caller-provided function), producing a new Dictionary.
You could use this to take a dictionary [String:String] that maps user IDs to product IDs, and produce a dictionary [User:Product].
Or you could invert a dictionary (swapping keys and values.)
—Jens
···
On Aug 29, 2016, at 2:07 PM, Zhao Xin <owenzx@gmail.com> wrote:
I don''t quite understand your question. In Swift, Dictionaries are structs. You can always use `let dict2 = dict1`.
I’m talking about a function that does for Dictionaries what map() does
for Arrays: it transforms every key and value in the input Dictionary
(through a caller-provided function), producing a new Dictionary.
You could use this to take a dictionary [String:String] that maps user IDs
to product IDs, and produce a dictionary [User:Product].
Or you could invert a dictionary (swapping keys and values.)
Jens, this is untested outside of the repl, but maybe it will help:
extension Dictionary {
func dictMap<U, V>(closure: @noescape (k: Key, v: Value) -> (U, V)) ->
[U:V] {
var ret = [U:V]()
for (k0, v0) in self {
let (k1, v1) = closure(k: k0, v: v0)
ret[k1] = v1
}
return ret
}
}
let mapped = [1: 1, 2: 2].dictMap() { (k,v) in return (String(k),
String(v)) }
print(mapped)
// Prints:
// ["2": "2", "1": "1"]
Jens, I see now. I believe this is talked months ago. There is no directly
function at the moment. You can do:
var dict1:[String:String] = ...
var dict2: [User:Product] = [:]
_ = dict1.map {
....
dict2.updateValue(value, forKey:key)
return nil
}
// use dict2
It is ugly, but it works. However, some people thought this was not a good
use of map. And they make an extension of Dictionary themselves.
You can search it in [swift-user], [swift-evolution] if you can't find it
in [swift-user].
Zhaoxin
···
On Tue, Aug 30, 2016 at 5:52 AM, Jens Alfke <jens@mooseyard.com> wrote:
> On Aug 29, 2016, at 2:07 PM, Zhao Xin <owenzx@gmail.com> wrote:
>
> I don''t quite understand your question. In Swift, Dictionaries are
structs. You can always use `let dict2 = dict1`.
I’m talking about a function that does for Dictionaries what map() does
for Arrays: it transforms every key and value in the input Dictionary
(through a caller-provided function), producing a new Dictionary.
You could use this to take a dictionary [String:String] that maps user IDs
to product IDs, and produce a dictionary [User:Product].
Or you could invert a dictionary (swapping keys and values.)
I just use a `for` loop to iterate dict1, and inside it populate dict2. Using `map` here seems wasteful since it’s building an array that’s thrown away.
dict2.updateValue(value, forKey:key)
This is a tangent, but: Why do you use this syntax instead of the more readable `dict2[key] = value`? (I assume they’re equivalent internally.)
—Jens
···
On Aug 29, 2016, at 4:38 PM, Zhao Xin <owenzx@gmail.com> wrote:
I just thought updateValue(:,:) is more natural than subscript function in
Dictionary.
For example, if there is no key in the dictionary, both `dict2[key] =
value` and `dict2.updateValue(value, forKey:key)` will add a new key-value
pair. However, the subscript function does it by adding something on both
side of the equation mark, which seems not natural to me. So I prefer to
use updateValue(:,:) function instead.
On Tue, Aug 30, 2016 at 8:13 AM, Jens Alfke <jens@mooseyard.com> wrote:
On Aug 29, 2016, at 4:38 PM, Zhao Xin <owenzx@gmail.com> wrote:
...
_ = dict1.map {
I just use a `for` loop to iterate dict1, and inside it populate dict2.
Using `map` here seems wasteful since it’s building an array that’s thrown
away.
dict2.updateValue(value, forKey:key)
This is a tangent, but: Why do you use this syntax instead of the more
readable `dict2[key] = value`? (I assume they’re equivalent internally.)