How to write this code in swift 3.0. using For loop

Question

How to write this code in swift 3.0. using For loop?

for (int index = 0; index < 100; index += (1 + index)) {        
    printf("%d\t",index);      
}

Answer 1

You increment is increasing. Since the C-style for loop isn't available, you might use code like this:

var index = 0
while index < 100 {
  print("\(index)", terminator: "\t")
  index += 1 + index
}
print("")

You could also write this code in functional style:

for f in sequence(first: 0,
                  next: {
                    $0 + $0 + 1
})
  .prefix(while: {$0 < last}) {
    print(f, terminator: "\t")
}
print("")

(The prefix function requires Swift 3.1)

Answer 2

This kind of for loop came from C, and counter to the design and spirit of the Swift language. That’s why it was eliminated in Swift 3 (you can find out more about its removal here).

C-style for loops have three statements:

First, there’s the initialization statement, where you set up one or more variables. In this case, the initialization statement is:

int index = 0

which in Swift becomes

var index = 0

Then, there’s the loop invariant, which is a condition that must be true at the very start and end of each pass through the loop. In this case, the loop invariant is:

index < 100

The Swift code is pretty much the same as the C code.

And finally, there’s what I call the change statement, which makes a change to some condition in the loop, which needs to be evaluated to see if another pass needs to be made through the loop. In this case, it’s this statement:

index += 1 + index

Again, the Swift code is pretty much the same as the C code.

You should use a while loop, and the equivalent Swift code looks like this:

while index < 100 {
  print("\(index)\t")
  index += 1 + index
}

The index += 1 + index code, while valid, is unusual. Are you sure you wanted that, or did you want the more common index += 1?

Answer 3

If you must use a for loop, you could use a sequence but that would be more verbose and cumbersome than a while loop:

for index in  sequence(first:0, next:{ let index = $0 + $0 + 1; return index < 100 ? index : nil})
{
  print("\(index)")
}

You could also generalize this to any type and increments using a generic function:

func cFor<T>(_ start:T, _ condition:@escaping (T)->Bool, _ increment:@escaping (inout T)->()) -> UnfoldSequence<T, (T?, Bool)>
{
   return sequence(first:start, next:{ var next = $0; increment(&next); return condition(next) ? next : nil })
}

for index in cFor(0,{$0<100},{$0 += $0 + 1})
{
  print("\(index)")
}