CodingBat string_bits problem solved using swit for loop - swift

Question

Question:
Given a string, return a new string made of every other char starting with the first, so "Hello" yields "Hlo".
string_bits('Hello') → 'Hlo'
string_bits('Hi') → 'H'
string_bits('Heeololeo') → 'Hello'
Solution:
func string_bits(userString: String) ->String{
var myString = ""
for(i, v) in userString.enumerated(){
if i % 2 == 0{
myString.append(v)
}
}
return myString
}
Output: Hello
Now my question:
Is there any I can iterate my index any way in swift like object-c, c, or other programming languages does. For instance:
result = ""
# On each iteration, add the substring of the chars 0..i
for i in range(len(str)):
result = result + str[:i+1]
return result
str[:i+1]
Here, I am adding +1 with the current index and getting the index value. How can I do this in swift.

Answer 1

extension Collection {
func everyNthIndex(n: Int) -> UnfoldSequence<Index,Index> {
sequence(state: startIndex) { index in
guard index < endIndex else { return nil }
defer { index = self.index(index, offsetBy: n, limitedBy: endIndex) ?? endIndex }
return index
}
}
}
let alphabet = "abcdefghijklmnopqrstuvwxyz"
for evenIndex in alphabet.everyNthIndex(n: 2) {
print("evenIndex", evenIndex, "char:", alphabet[evenIndex])
}
for oddIndex in alphabet.dropFirst().everyNthIndex(n: 2) {
print("oddIndex", oddIndex, "char:", alphabet[oddIndex])
}
regular approach using while loop:
var index = alphabet.startIndex
while index < alphabet.endIndex {
defer { index = alphabet.index(index, offsetBy: 1) }
print(alphabet[index])
print(index)
}
or enumerating the string indices:
func string_bits(userString: String) -> String {
var myString = ""
for (offset,index) in userString.indices.enumerated() {
if offset.isMultiple(of: 2) {
myString.append(userString[index])
}
}
return myString
}