[[harlowe:Array|Array]] are useful for dealing with a sequence of related data values, especially if
they have a particular order. There are occasions, however, where you don't really
care about the order, and instead would simply use the [[harlowe:Array|Array]] as a storage place for
values - using ''%%contains%%'' and ''%%is in%%'' to check which values are inside.

Think of datasets as being like arrays, but with specific restrictions:

  * You can't access any positions within the dataset (so, for instance, the ''%%1st%%'', ''%%2ndlast%%'' and ''%%last%%'' aren't available, although the ''%%length%%'' still is) and can only use ''%%contains%%'' and ''%%is in%%'' to see whether a value is inside (or, by using ''%%any%%'' and ''%%all%%'', many values).
  * Datasets only contain unique values: adding the [[harlowe:string|string]] "Go" to a dataset already containing "Go" will do nothing.
  * Datasets are considered equal (by the ''%%is%%'' operator) if they have the same items, regardless of order (as they have no order).

These restrictions can be helpful in that they can stop programming mistakes from
occurring - you might accidentally try to modify a position in an array, but type the name of
a different array that should not be modified as such. Using a dataset for the second
array, if that is what best suits it, will cause an error to occur instead of allowing
this unintended operation to continue.

^Operator        ^ Meaning ^ Example ^
| ''is'' | Evaluates to boolean ''true'' if both sides contain equal items, otherwise ''false''. | ''(ds:1,2) is (ds 2,1)'' (is true)|
| ''is not'' | Evaluates to ''true'' if both sides differ in items. | ''(ds:5,4) is not (ds:5)'' (is true)|
| ''contains'' | Evaluates to ''true'' if the left side contains the right side. | ''(ds:"Ape") contains "Ape"'', ''(ds:(ds:99)) contains (ds:99)'', ''(ds: 1,2,3) contains all of (a:2,3)'', ''(ds: 1,2,3) contains any of (a:3,4)''|
| ''is in'' | Evaluates to ''true'' if the right side contains the left side. | ''"Ape" is in (ds:"Ape")'', ''(a:3,4) is in (ds:1,2,3)''|
| ''+'' | Joins datasets. | ''(ds:1,2,3) + (ds:1,2,4)'' (is ''(ds:1,2,3,4)'')|
| ''-'' | Subtracts datasets. | ''(ds:1,2,3) - (ds:1,3)'' (is ''(ds:2)'')|
| ''...'' | When used in a macro call, it separates each value in the right side., The dataset's values are sorted before they are spread out.| ''(a: 0, ...(ds:1,2,3,4), 5)'' (is ''(a:0,1,2,3,4,5)'')|