Things I Learned Writing a Fibonacci Generator in JavaScript
medium.com
medium.com
a = fib(5);
for (i = 0; i < 7; ++i) console.log(a.next());
Object {value: 0, done: false}
Object {value: 1, done: false}
Object {value: 1, done: false}
Object {value: 2, done: false}
Object {value: 3, done: false}
Object {value: undefined, done: true}
Object {value: undefined, done: true}
b = fib(3)
for (i = 0; i < 5; ++i) console.log(b.next());
Object {value: 1, done: false}
Object {value: 0, done: false}
Object {value: 1, done: false}
Object {value: 1, done: false}
Object {value: undefined, done: true}https://mitpress.mit.edu/sicp/full-text/book/book-Z-H-11.htm...
Edit: using karatsuba like the page below is probably preferrable, though.
I think using 'iter' in the name is misleading. Recursion is a form of iteration. In that exercise fib-iter is recursive - it calls itself. That's the very definition of recursion. I think they use the name 'iter' because the overall process is linear in time the same way an iterative loop in other languages is. In this case it's logarithmic in time. The additional arguments and tail call optimisation (guaranteed by the Scheme standard) avoid a stack of intermediate values so it's constant space as well.
If you delve further it's not actually constant space OR logarithmic times since the numbers get large so fast that storage, addition and multiplication and evaluation are no longer atomic and will have an effect on the time and space characteristics.
Still it's one of my favourite exercise from the book and they use it and reference it in Racket's number theory module.[1]
[1] https://github.com/racket/math/blob/5cc1080d90d2603f790abd41...
Yet SICP addresses exactly this point!
The authors still mandate it's not truly recursive in that the required state is passed along to each iterative call.
From the parent's SICP link, but many paragraphs back :
"In contrasting iteration and recursion, we must be careful not to confuse the notion of a recursive process with the notion of a recursive procedure. When we describe a procedure as recursive, we are referring to the syntactic fact that the procedure definition refers (either directly or indirectly) to the procedure itself. But when we describe a process as following a pattern that is, say, linearly recursive, we are speaking about how the process evolves, not about the syntax of how a procedure is written. It may seem disturbing that we refer to a recursive procedure such as fact-iter as generating an iterative process. However, the process really is iterative: Its state is captured completely by its three state variables, and an interpreter need keep track of only three variables in order to execute the process."
They were clearly aware of that too ... "It may seem disturbing that we refer to a recursive procedure such as fact-iter as generating an iterative process. "
var fib = (function () {
let current = 0;
let next = 1;
return function (increment) {
for (var i = 0; i < increment; i++) {
[current, next] = [next, current + next]
}
return current
};
})();
Edit: Altogether, good show. I enjoyed the post.
o_O
Most people don't think of a function with an inherited scope as a closure since by the standard PLT people will tell you that Javascript does not provide "real" closures.
I'm sure you know what I meant. It's the "Javascript Way" TM of doing this.
My "little known" I mean that most programmers do not think of doing this out right. It took me seeing an amazing talk by the creator of JSON called "Javascript the Good Parts" to realize how powerful this construct was, and how Javascript made it so easy.
The point is that ES2015 defines clear interface for iterating over iterables. With help of Symbol and *.
If you would use this code in production, probably you would document it: ... returns iterator function.
next = fib()
next() // 0
next() // 1
next() // 1 ...
then you can do
for (f of fib())
console.log(f)
from a given for loop:
for (x in function())
If you instead did...
next = fib()
while (true) {
number = next();
}
Now don't get me wrong, I DO fully agree that a generator is the correct method for doing this, I'm just pointing out another construct to allow you to do this.