Welcome back to our "Binary Tree" week!
Today we will discuss the
enum function. It can be used to emulate arrays (as you might have noticed, PicoLisp doesn't have an array data type). But what do arrays have to do with binary trees...?!
enum function is only available in **pil21. If you get a
enum -- Undefined error when you executing the examples below, you probably haven't installed the latest version.
Let's see what we can learn from the documentation about the
(enum 'var 'cnt ['cnt ..]) -> lst
(enum 'var) -> lst
Enumerates cells by maintaining a binary tree in
var. The keys are implicit from the enumerated
cnts, and the resulting tree is balanced (independent from the insertion order). (...)
enumcan be used to emulate (possibly sparse) arrays.
enum function expects a
cnt - which means a (small) integer - which specifies the position of a cell in a balanced tree. Like for any cell, the corresponding value could be defined with the
The cell is returned if it already exists, otherwise it is created as a new node in the tree. A typical use case would be the conversion of a list, as in the following example:
: (for (I . S) '(a b c d e f g) (set (enum 'E I) S) ) -> o
for loop takes the index (
I) and value (
S) of each list item.
I is stored in the
enum tree, then its value is set to
Let's check the result:
: (view E T) g c e a f b d
We see a perfectly balanced tree. To read out some values, we use
val (enum ...) ):
: (val (enum 'E 1)) -> a : (val (enum 'E 3)) -> c
We can check if a cell already exists using the
enum? function. If it exists, the whole subtree is returned, otherwise
: (enum? E 3) -> (c (e) g) : (enum? E 10) -> NIL
enumdoesn't store any keys. The key is implicit due to its position in the tree.
On the other hand,
cacheis creating the index using hash function, which implies that any key can be stored. The backdraw is that the hashing step needs to be done everytime an item should be looked up.
cacheaccepts a program as argument. If the program has already been evaluated, the value is returned without re-evaluating.
enum?can only check if an index exists or not.
enumis creating balanced trees,
cacheis creating probably balanced trees (in real applications, this gap should be rather small)
Probably you can already guess in which regards
enum is similar to arrays: Arrays can usually be indexed directly by a syntax like
enum allows a very similar handling by
(val (enum 'E 1)).
But how does it look like in terms of efficiency? Let's take a list of 1 Mio. items and assume we want to look up the item at position 900.000.
Arrays. Usually arrays take up consecutive space in the memory. If we know the size per entry, the computer "knows" the required position by calculating "starting point + 900.000*size". --> approx. 1 calculation
Lists. In PicoLisp, lists are connected via pointers. Therefore we need to jump from pointer to pointer to get to our desired cell --> approx. 900.000 calculations.
enum/binary tree. The
enumfunction guarantees that the tree is balanced, so we should be able to find our item within 20 steps --> approx. 20 calculations.
We can even create multidimensional arrays, since
enum accepts more than one
cnt variable. Let's create a 4x4 Matrix:
: (off A) : (for I 4 (for J 4 (set (enum 'A I J) (pack I "-" J)) ))
To get the item in the "second line, second row":
: (val (enum 'A 2 2)) -> "2-2"
And now finally, let's consider our Fibonacci example again and compare it to the
cache version. We can expect a difference, because we can store and access the variables directly under the iteration number
However, it requires some further work from us. While
cache accepts a program as parameter and evaluates it only in case the value has not been stored yet, we need to do this manually for
- check the value of
enum '(NIL) Nand set it to
E. If it has already been calculated,
Ewill be non-
NIL, otherwise it will be
(val E)evaluates to non-
NIL(i. e. has already been calculated), this value is returned.
- Otherwise we set
Eto the result of the recursive calculation.
(de fiboEnum (N) (let E (enum '(NIL) N) (or (val E) (set E (if (>= 2 N) 1 (+ (fiboEnum (dec N)) (fiboEnum (- N 2))) ) ) ) ) )
In the previous post, we showed that the cached version can calculate 10.000 Fibonacci numbers in 0.1 s (of course depending on the respective hardware). How many can we do with
fiboEnum in the same time?
: (bench (fiboEnum 14500)) 0.098 sec
We get close to it at Fibonacci number 14500. This a speed increase of almost 50% compared to the
If you still haven't seen enough of Fibonacci, stay tuned for the next Rosetta-Code post which will include analytic and iterative approaches, and the second Euler Project challenge where we will sum up the even Fibonacci numbers.
After that we will start a new topic - Web Application Programming 😃