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How should I scramble a table?
Posted: Wed Dec 03, 2008 11:50 pm
by Mr. Strange
I have a table of 52 elements - a deck of cards. I want to shuffle them - that is, assign each of the 52 elements to a new, unique, random location.
My first idea was to use table.sort along with some random number generator. I could assign each element a random value, and then sort by those, or maybe I could do the whole thing in one step? Someone must have figured out a slick way to do this. My potentially clumsy attempt:
(assume the random number generator has been seeded previously.)
Code: Select all
table = {}
for i=1,100 do
table[i] = i
end
mixer = {}
for i=1, #table do
mixer[i] = math.random()
end
... now I want to do table.sort on mixer, and apply the same transposition of elements to table. Maybe I can't do that? Could I sort table by providing a comparison function which references values in mixer?
--Mr. Strange
Re: How should I scramble a table?
Posted: Wed Dec 03, 2008 11:59 pm
by surtic
I think the "standard" way to shuffle an array is this:
Code: Select all
function shuffle(tab)
local len = #tab
local r, tmp
for i = 1, len do
r = math.random(i, len)
tmp = tab[i]
tab[i] = tab[r]
tab[r] = tmp
end
end
edit: you can also read a bit more at
http://en.wikipedia.org/wiki/Shuffle#Sh ... algorithms which describes your method as well (which is slightly less efficient because sorting an array is slower than just traversing it).
Re: How should I scramble a table?
Posted: Thu Dec 04, 2008 12:14 am
by Kaze
surtic wrote:Code: Select all
tmp = tab[i]
tab[i] = tab[r]
tab[r] = tmp
Re: How should I scramble a table?
Posted: Thu Dec 04, 2008 12:53 am
by Mr. Strange
Perfect. Thank you.
--Mr. Strange
Re: How should I scramble a table?
Posted: Thu Dec 04, 2008 9:29 pm
by surtic
Kaze, you're right... too many years of bad habits...
Sometimes I do catch myself and use the idom "a, b = b, a" but in many other cases the code just looks right to me...
Re: How should I scramble a table?
Posted: Thu Sep 13, 2012 7:58 pm
by Codex
So the earlier post was a way to shuffle a single array. (ex. a deck of cards)
Kaze wrote:
-snip-
surtic wrote:
-snip-
Code: Select all
function shuffle(tab)
local len = #tab
local r
for i = 1, len do
r = math.random(i, len)
tab[i], tab[r] = tab[r], tab[i]
end
end
For a project I'm working on, I needed to shuffle tables inside one another. (ex. table[x][y]) So after playing with the above formula a bit, I came up with this.
Code: Select all
function shuffle(tab)
local x_len = #tab
local r_x, r_y
for x=1, x_len do
local y_len = #tab[x]
for y=1, y_len do
r_x = math.random(x, x_len)
r_y = math.random(y, y_len)
tab[x][y], tab[r_x][r_y] = tab[r_x][r_y], tab[x][y]
end
end
end
This lets me generate an image that is stored in an [x][y] format randomly.
I'm very pleased with the result!
(edit: Also I feel this thread belongs in the support and development more than general section...?)
Re: How should I scramble a table?
Posted: Fri Sep 14, 2012 4:27 pm
by qaisjp
Code: Select all
function shuffle(tab)
local len = #tab
local r
for i = 1, len do
r = math.random(i, len)
tab[i], tab[r] = type(tab[r])=="table" and shuffle(tab[r]) or tab[r], type(tab[i])=="table" and shuffle(tab[i]) or tab[i]
end
end
Re: How should I scramble a table?
Posted: Fri Sep 14, 2012 4:47 pm
by Roland_Yonaba
Err... Why always testing if tab[r] is a table ?
Re: How should I scramble a table?
Posted: Fri Sep 14, 2012 5:08 pm
by qaisjp
Roland_Yonaba wrote:Err... Why always testing if tab[r] is a table ?
deep scrambling - if this was the table:
Code: Select all
{
{1, 2, 3},
{4, {5, 6}, 7, {8, 9, {10, 11} }}
}
I'd get something like this:
Code: Select all
{
{7, {9, {11, 10}, 8}, {6, 5}, 4},
{2, 1, 3}
}
Re: How should I scramble a table?
Posted: Sun Sep 16, 2012 4:05 pm
by tv_user
qaisjp wrote:Code: Select all
tab[i], tab[r] = type(tab[r])=="table" and shuffle(tab[r]) or tab[r], type(tab[i])=="table" and shuffle(tab[i]) or tab[i]
I just love these mind-twisting assignment operations! Nice and simple solution qaisjp!
