The limitedBy: parameter sets the last valid value, not the first invalid value. The docs should probably clarify that. @natecook1000, @milseman?

Comment by Fon (JIRA)

It's really a documentation problem? Please see the code below.
-------------~~~~Documentation~~~~-------------
let s = "Swift"
if let i = s.index(s.startIndex, offsetBy: 4, limitedBy: s.endIndex) {
print(s[i])
}
// Prints "t"

let j = s.index(s.startIndex, offsetBy: 6, limitedBy: s.endIndex)
print(j)
// Prints "nil"
-------------~~~~Example~~~~-------------
let i = s.index(s.startIndex, offsetBy: 5,limitedBy: s.endIndex)
printℹ
//Crashed because String index is out of bounds. This should print "nil".

No, i == s.endIndex in your example, which is the expected behavior. That's a valid index to form, but not to use as a subscript.

Comment by Fon (JIRA)

let s = "Swift"
let startIndex=s.startIndex
var endIndex=s.endIndex

//These are valid.
if var i=s.index(s.startIndex, offsetBy: 5, limitedBy: s.endIndex){
   print(s.formIndex(&i, offsetBy: 0, limitedBy:s.endIndex))//prints true
}
print(s.formIndex(&endIndex, offsetBy: 0, limitedBy: s.endIndex))//prints true

print(s[startIndex])//prints "S"
print(s[..<endIndex])//prints "Swift"
print(s[s.endIndex])//Crashed error String index is out of bounds.

let name = "Marie Curie"
let firstSpace = name.firstIndex(of: " ") ?? name.endIndex
let firstName = name[..<firstSpace]
// firstName == "Marie"

I got it. It's a designed operation. I can use i only by s[..<endIndex]. But why don't allow to subscript a string by endIndex.

endIndex represents a boundary position for a Collection. It turns out this representation is more useful than, say, lastIndex, because it allows you to represent empty Collections as Collections where startIndex == endIndex. For a 5-element Array, for example, the endIndex would be 5, and the valid indexes are 0, 1, 2, 3, 4. For a 0-element Array, the endIndex is 0.

Comment by Alexander Ignition (JIRA)

@belkadan can we update the documentation so that it is not misleading ?

for example

let numbers = [10, 20, 30, 40, 50]
if let i = numbers.index(numbers.startIndex,
                         offsetBy: 4,
                         limitedBy: numbers.index(before: numbers.endIndex)) {
    print(numbers[i])
}

I'm not at Apple anymore, so I'm not sure who's handling documentation these days. @krilnon?

That’d be amartini (JIRA User).