Loops and iterations - algorithm

Question

Is there a way that a function has 2 options of return and the option of return is chosen after a certain interval of iterations?
example:
a function that swaps from "a" to "b" and "b" to "a" after 4 iterations returns:
a
a
a
a
b
b
b
b
a
a
a
a
b
b
b
b
.
.
.
Edit I did something like this to solve the problem:
var counter = 0;
var state = true;
var changeState = function(){
var x = 0
while(x != 12){
if(counter == 4){
counter = 0;
state = !state;
}
if (state){
console.log("a");
} else {
console.log("b")
}
counter += 1;
x += 1
}
}
changeState();

Answer 1

You will need to have a stateful function, as it needs to somehow remember something from previous call(s) that were made to it. This is a so-called side effect, and means that the function is not pure.
For the example you have given, the function would need to know (i.e. have a state with) the number of previous calls (modulo 8), or the previous four returned values, or some mix of this information.
How such a function is implemented, depends on the programming language.
In object oriented programming languages you would create a class with the function as method, and a property reflecting that state. When the function is called, it also updates the state. For instance, in Java:
class MyClass {
int callCount = 0;
char myFunction() {
return "aaaabbbb".charAt(this.callCount++ % 8);
}
}
You would call the function repeatedly like so:
MyClass obj = new MyClass();
for (int i = 0; i < 10; i++) {
System.out.println(obj.myFunction());
}
In JavaScript you could do the same, or you could create a closure:
function createFunction() {
let callCount = 0;
return function myFunction() {
return "aaaabbbb"[callCount++ % 8];
}
}
let myFunction = createFunction();
for (let i = 0; i < 10; i++) {
console.log(myFunction());
}
In Python you can do the same, but it allows also to use default arguments (which are only initialised at function definition time), and so you could define an argument that is an object holding a counter:
def myFunction(counter=[0]):
counter[0] += 1
return "aaaabbbb"[counter[0] % 8]
for _ in range(10):
print(myFunction())
This is of course not an exhaustive list of possibilities, but the essence is that programming languages offer their own ways to construct such stateful functions.
Iterators
Some languages offer a different approach: iterators. This means the function produces a stream of return values. The function can run to produce the first value after which the running state is saved until a new value is requested from it. The function's execution context is then restored to run until it can produce the next value, ...etc.
Here is how that design would look in JavaScript:
function * myIterator() {
let callCount = 0;
while (true) {
yield "aaaabbbb"[callCount++ % 8];
// ^^^^^ a language construct for yielding back to the caller
}
}
let iter = myIterator();
for (let i = 0; i < 10; i++) {
console.log(iter.next().value);
}
When a programming language offers this possibility, it is often preferred over the other alternatives listed earlier.