posted 10 February 2010 05:07            

array := array reverse.

But is there a nice simple way to shuffle the indices?

posted 10 February 2010 11:13            

To shuffle the elements in the array, think of the array as a deck of cards being shuffled by a dealer. On each shuffle, the dealer is permuting the order of the cards. We want to pick one of these permutations at random. Here's an example (if you copy/paste this into a text file in Kyma, you can select it and use ctrl+Y to evaluate it. Each time you evaluate it, you'll get a different random permutation of the array):

code:

---

| ps r shuffled |
ps := PermutationStream on: #(1 2 3 4 5).
r := Random new.
(((r next * ps size) truncated + 1) min: ps size) timesRepeat: [shuffled := ps next].
shuffled

---

Here is the same code but with comments and an extra variable for clarity:

code:

---

| ps r shuffled nbrRepetitions |
"Create a PermutationStream on the original Array that
will give us a new permutation each time we send it the
message next. And create a Random number stream that
will give us a random number between 0 and 1 each time
we send it the message next."
ps := PermutationStream on: #(1 2 3 4 5).
r := Random new.
"Generate a random number of permutations by sending the
next message a random number of times.  To get the random
number of repetitions send next to the Random stream (giving
us a value between 0 and 1).  Multiply that by the size of the
PermutationStream (the number of permutations).  We want an
integer number of repetitions so truncate the result (giving us
a value from 0 to ps size).  Add 1 because Smalltalk arrays are
indexed starting with 1, not 0.  To be extra careful, take the min
of this and size of the PermutationStream."
nbrRepetitions := ((r next * ps size) truncated + 1) min: ps size.
"Permute the order of the array nbrRepetitions times."
nbrRepetitions timesRepeat: [shuffled := ps next].
shuffled

---

posted 10 February 2010 11:59            

thanks!

is this an example of the use of the new varTimesRepeat: ?

Also, what is PermutationStream on: ?

Will this be documented somewhere? It's a very useful type of stream.

posted 10 February 2010 13:34            

try executing this with 10 elements - it hangs! - and around 9 elements goes real slow

posted 10 February 2010 13:35            

code:

---

| a |
a := 'helloworld'.
a shuffle

---

posted 10 February 2010 13:41            

What I was intrigued by about this in Kyma, was the idea of shuffling an array of files, of kyma objects, perhaps like an array of running generators like ramps and bpms

will this be possible, or will it be only strings, or only elements at compile time?

edit: actually, a few seconds thinking about that - the best wasy to shuffle through arrays of objects , especially capytalk or pasted objects, is with my old favourite 'AnalogSequencer' right?

posted 10 February 2010 13:42            

The number of permutations of an n-long array is n! To get an idea of why it goes 'real slow', try evaluating:

10 factorial

in Kyma by selecting it and using ctrl+Y. (Hint the answer is 3,628,800).

posted 10 February 2010 13:44            

I was running it on a collection of filenames from a directory, so yes I can understand why Kyma went awry!

posted 10 February 2010 13:48            

quote:

---

What I was intrigued by about this in Kyma, was the idea of shuffling an array of files, of kyma objects, perhaps like an array of running generators like ramps and bpms

will this be possible, or will it be only strings, or only elements at compile time?

---

It works on Arrays or OrderedCollections of any elements (so yes, file names, CapyTalk code, etc). The shuffling is in Smalltalk so it takes place at compile time. But the elements could be CapyTalk expressions.

For example, in the Samples field of a Multisample module, you could use:

{'pottery break 1.aif' sampleFileNamesInSameFolder shuffle}