Swift 4 changed how strings worked. However, seems to have got more complicated and less readable. Can anyone simplify this example (simply gets the third letter of a String as a String)? (Other than splitting out the lines.)
let myString="abc"
let thirdLetter = String(myString[myString.index(myString.startIndex, offsetBy: 2)])
In Swift 4 you can convert a String myString to [Character] with Array(myString). Then you can index that array with Int and then convert that [Character] to String.
let myString = "abc"
let thirdLetter = String(Array(myString)[2]) // "c"
let firstTwo = String(Array(myString)[0..<2]) // "ab"
If you are going to do a lot of operations on a String, it is frequently better to just convert and keep it as [Character].
Note: I have reworked this section to try to avoid any caching optimizations the compiler might do. Each method is now measured just once and a running total is kept for each method.
Converting to Array and indexing with Int is easy to write and read, but how does it perform? To answer this, I tested the following in a release build:
func time1(str: String, n: Int) -> (Double, String) {
// Method 1: Index string with String.Index, convert to String
let start = Date()
let a = String(str[str.index(str.startIndex, offsetBy: n)])
let interval = Date().timeIntervalSince(start)
return (interval, a)
}
func time2(str: String, n: Int) -> (Double, String) {
// Method 2: Convert string to array, index with Int, convert to String
let start = Date()
let a = String(Array(str)[n])
let interval = Date().timeIntervalSince(start)
return (interval, a)
}
func time3(str: String, n: Int) -> (Double, String) {
// Method 3: Use prefix() and last(), convert to String
let start = Date()
let a = String(str.prefix(n + 1).last!)
let interval = Date().timeIntervalSince(start)
return (interval, a)
}
func time4(str: String, n: Int) -> (Double, String) {
// Method 4: Use Leo Dabus' extensions
// https://stackoverflow.com/q/24092884/1630618
let start = Date()
let a = str[n]
let interval = Date().timeIntervalSince(start)
return (interval, a)
}
func time5(str: String, n: Int) -> (Double, String) {
// Method 5: Same as 2 but don't measure Array conversion time
let arr = Array(str)
let start = Date()
let a = String(arr[n])
let interval = Date().timeIntervalSince(start)
return (interval, a)
}
func test() {
for repetitions in [1, 10, 100, 1000] {
var input = ""
for _ in 0 ..< repetitions {
input.append("abcdefghijklmnopqrstuvwxyz")
}
var t = [0.0, 0.0, 0.0, 0.0, 0.0]
let funcs = [time1, time2, time3, time4, time5]
for i in 0 ..< input.count {
for f in funcs.indices {
let (interval, _) = funcs[f](input, i)
t[f] += interval
}
}
print("For string length \(input.count):")
for i in 0 ..< 5 {
print(String(format: "Method %d time: %.8f", i + 1, t[i]))
}
print("")
}
}
Results:
For string length 26: Method 1 time: 0.00108612 Method 2 time: 0.00085294 Method 3 time: 0.00005889 Method 4 time: 0.00002104 Method 5 time: 0.00000405 For string length 260: Method 1 time: 0.00117570 Method 2 time: 0.00670648 Method 3 time: 0.00115579 Method 4 time: 0.00110406 Method 5 time: 0.00007111 For string length 2600: Method 1 time: 0.09964919 Method 2 time: 0.57621503 Method 3 time: 0.09244329 Method 4 time: 0.09166771 Method 5 time: 0.00087011 For string length 26000: Method 1 time: 9.78054154 Method 2 time: 56.92994779 Method 3 time: 9.02372885 Method 4 time: 9.01480001 Method 5 time: 0.03442019
Analysis:
[Character] around, the indexing operations on it are quite quick. (see Method 5)
It appears you are trying to extract the third character. I would say
let c = myString.prefix(3).last
Granted, it's an Optional Character, but you can unwrap at leisure and coerce to String when needed.
The prefix method is extremely valuable because it takes an Int rather than forcing you into the wild and wacky world of String.Index.
