Swift, the for loop index value does not change as expectation - arrays

Question

Here is my code, inside of my for loop, there is a while loop changes
the variable i's value. I printed out the value of i before while loop
and and after loop, the new "before index value" is expected to start
at "after index value +1". However, the "new before index value" just
increase as normal, which increase one each time. Appreciate your time.
class Solution
{
func summaryRange(nums: [Int]) -> [String]
{
var result = [String]()
if nums.count == 1
{
result.append(String(nums[0])+"")
return result
}
for var i in (0...nums.count-1)
{
let a = nums[i]
print("before: \(i)")
while i+1<nums.count && (nums[i+1] - nums[i] == 1)
{
i += 1
}
if a != nums[i]
{
result.append(String(a) + "->" + String(nums[i]))
}
else
{
result.append(String(a)+"")
}
print("after: \(i)")
}
return result
}
}
var test = Solution()
var input = [0,1,2,4,5,7]
var result = test.summaryRange(input)
print(result)
This code tries to get result like ["0->2","4->5","7"]

Answer 1

This is my own solution. I figure out the problem. Basically, I switch
the "for loop" to a "while loop". Then, it works.
class Solution
{
func summaryRanges(nums: [Int]) -> [String]
{
var result = [String]()
var i = 0
while(i<nums.count)
{
let a = nums[i]
while i+1<nums.count && (nums[i+1] - nums[i] == 1)
{
i += 1
}
if a != nums[i]
{
result.append(String(a) + "->" + String(nums[i]))
}
else
{
result.append(String(a)+"")
}
i += 1
}
return result
}
}

Answer 2

In swift, for in loop is a wrapper around the Sequence Type. In this particular case, 0...numCount-1 is a Sequence of Ints. When you call for in this is what happens internally.
let a = 0..<5
var generator = a.generate()
while var i = generator.next() {
print(i + 1)
i+= 1
}
So even if you increment the value of i inside the loop, it wont affect the loop itself. This is because loop is controller only by calling genertator.next().
This helps understand the basics.

Answer 3

This was an interesting one. I think this does the trick (with some code cleanup as well :-]
class Solution
{
func summaryRange(nums: [Int]) -> [String] {
var result = [String]()
var currentRange = [Int]()
for num in nums {
if currentRange.isEmpty { //if range is empty, start a new range
currentRange.append(num)
continue
}
if num - 1 != currentRange.last! { //if not next number in sequence, close range, start new
result.append(closeRange(currentRange))
currentRange = []
currentRange.append(num)
} else if num - 1 == currentRange.last {
currentRange.append(num) //if next number, add to range
}
}
if !currentRange.isEmpty {
result.append(closeRange(currentRange)) //check for unclosed ranges
}
return result
}
func closeRange(range: [Int]) -> String {
guard range.count > 0 else { return "" }
if range.count == 1 { return String(range.first!) }
return "\(range.first!)->\(range.last!)"
}
}
var test = Solution()
var input = [0,1,2,4,5,7]
var result = test.summaryRange(input)
print(result)
Let me know how this works for you!