Do something every nth iteration of loop in Objective-c or Swift - objective-c

Question

Within a loop, it is possible to do something every other time by using odd and even and testing for even. I would like to do something a fixed number of times eg four no matter how many iterations there are in the loop (value of i). Is there a way to do something every nth time?
I think there must be a way by incrementing a second counter, but having trouble getting it.
A Loop
int i = 0;
int movinglimit = 0;
int incr = [myItems count]/4;;// there might be 10, 20 or 30 items. I only want to do something four times
while (i < [myItems count]) {
if ((i % 2) == 0) {
//do something if even
}
if (i>movinglimit) {
//do something a total of four times
}
if (i==moving limit+1) {
movinglimit = movinglimit + incr;
}
i++;
}

Answer 1

Assuming myItems is an NSArray or NSMutableArray or NSSet you can apply methods on all objects at once which is smarter than implementing an iteration looping thru object by object yourself. That way you take full advantage of objC.
[myItems makeObjectsPerformSelector:#selector(yourObjectMethodToApply)];
This has the side effect that nothing is done/applied if myItems is nil
A short comment on your pseudocode.
Be careful with while loops without any increment inside.
The one you show here is only looping once.
//iteration backward
NSUInteger i = myItems.count-1;
for ( ; i>0; ) {
if ((i % 2) == 0) {
// do stuff if 4th is even
}
// do stuff on every 4th one
i-=4; //increment does not have to be in for () declaration
}
identify the similarity (also backward iteration) to
NSUInteger i = myItems.count-1;
while (i>0) {
// do stuff on each 4th item
//check for odd/even
myItems[i] = i % 2 ? /* even stuff */ : /* odd stuff */;
i-=4; //decrement i
}

Answer 2

You tagged Swift so I assume you want Ob-C or Swift answers. You can enumerate the loop and use that enumeration (which I call n below). The first enumeration will always be 0, of course, so to simplify the even/odd parity, just add 1 to make the first iteration 1.
let data = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"]
for (n, i) in data.enumerated() {
if (n+1).isMultiple(of: 2) {
print(n+1, i) // every even iteration
}
}
// output: 2 b, 4 d, 6 f, 8 h, 10 j
for (n, i) in data.enumerated() {
if (n+1) <= (4*2), (n+1).isMultiple(of: 2) {
print(n+1, i) // every even iteration but only 4 times
}
}
// output: 2 b, 4 d, 6 f, 8 h