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function construction - Picking random items out of a list only once


I'm trying to create a function that randomly returns a value from a list but remembers the values that have been given before. At the end when the list is empty it should return an empty list. Basically like emptying a bucket full of eggs one at a time.


Suppose I have two lists:


data1 = Range[10];
data2 = Range[20];


Assume a function


getRandomItem[l_List]

I tried playing with down-values but that doesn't work.


Calling getRandomItem[data1] two times would give (e.g) {1} and {3}. Calling getRandomItem[data2] two times would give (e.g) {15} and {20}


At the end as stated before when all items are chosen both getRandomItem[data1] and getRandomItem[data2] should return {}.


I would like to do that without declaring data1 and data2 as global variables nor do I which to change/alter them. So, basically I presume the function itself should remember which data has been given to it and where it had left the previous time.



Answer



How about making a closure? A closure is a function with an internal state.


makeDrippingBucket[list_] := 

Module[{bucket = list},
If[bucket === {}, {},
With[{item = RandomChoice[bucket]},
bucket = DeleteCases[bucket, item]; {item}]] &]

Then use this to make a "bucket", like this:


bucket = makeDrippingBucket[{1,2,3,4,5}]

This object has an internal state that changes every time you call it. Every time you call bucket[], it will give you a new number, until it gets empty.


bucket[]


(* ==> {3} *)



EDIT


The same thing, using @Eli's solution of pre-randomizing the list:


makeDrippingBucket[list_] := 
Module[{bucket = RandomSample[list]},
If[bucket === {}, {},
With[{item = Last[bucket]}, bucket = Most[bucket]; {item}]] &]

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