What's a good way to iterate backwards through the Characters of a String? - swift

Question

What's the most Swiftian way to iterate backwards through the Characters in a String? i.e. like for ch in str, only in reverse?
I think I must be missing something obvious, because the best I could come up with just now was:
for var index = str.endIndex;
index != str.startIndex;
index = index.predecessor() {
let ch = str[index.predecessor()]
...
}
I realise "what's the best..." may be classed as subjective; I suppose what I'm really looking for is a terse yet readable way of doing this.
Edit: While reverse() works and is terse, it looks like this might be quite inefficient compared to the above, i.e. it seems like it's not actually iterating backwards, but creating a full reverse copy of the characters in the String. This would be much worse than my original if, say, you were looking for something that was usually a few characters from the end of a 10,000-character String. I'm therefore leaving this question open for a bit to attract other approaches.

Answer 1

The reversed function reverses a C: CollectionType and returns a ReversedCollection:
for char in "string".characters.reversed() {
// ...
}
If you find that reversed pre-reverses the string, try:
for char in "string".characters.lazy.reversed() {
// ...
}
lazy returns a lazily evaluated sequence (LazyBidirectionalCollection) then reversed() returns another LazyBidirectionalCollection that is visited in reverse.

Answer 2

As of December 2015 with Swift version 2.1, the proper way to do this is
for char in string.characters.reverse() {
//loop backwards
}
String no longer conforms to SequenceType<T> but its character set does.

Answer 3

Not sure about efficiency, but I will suggest
for ch in reverse(str) {
println(ch)
}

Answer 4

Here is a code for reversing a string that doesn't use reverse(str)
// Reverse String
func myReverse(str:String) -> String {
var buffer = ""
for character in str {
buffer.insert(character, atIndex: buffer.startIndex)
}
return buffer
}
myReverse("Paul") // gives “luaP”
Just a little experiment. For what its worth.
Ok, leant how to read the question....
Would this work Matt?
func ReverseIteration(str:String) {
func myReverse(str:String) -> String {
var buffer = ""
for character in str {
buffer.insert(character, atIndex: buffer.startIndex)
}
return buffer
}
// reverse string then iterate forward.
var newStr = myReverse(str)
for char in newStr {
println(char)
// do some code here
}

Answer 5

this?
extension String {
var reverse: String {
var reverseStr = ""
for character in self {
reverseStr = String(character) + reverseStr
}
return reverseStr
}
}