Swift: Test boundary of String.Index for substring function

Question

Wow. Swift makes it really fiddly to copy a substring from a simple String.

Most programming languages allow characters to be simply indexed by their intege

Answer 1

You can use

let ind1 = str.startIndex.advancedBy(c1, limit: str.endIndex)
let ind2 = str.startIndex.advancedBy(c1+c2, limit: str.endIndex)

to advance the start index by the given amounts, but not beyond the end index of the string. With that modification, your function gives

subStr ("Stack Overflow", c1: 6, c2: 12) // "Overflow"
subStr ("Stack Overflow", c1: 12, c2: 20) // ""