Unique

Unique(a, `I`)

Without the index parameter «I», returns a list containing all the unique atoms in «a». The resulting list has any duplicates removed. «a» can be a multidimensional array, and the result contains the unique atoms (cells) across all dimensions.

When an index «I» is specified, it returns a maximal subset of index «I» such that each indicated slice of array «a» along «I» is unique. This usage can be used to remove duplicate slices from an array, or to identify a single member of each equivalence class.

Example

Let:

Variable DataSet :=

	Field ▶
PersonNum ▼	LastName	FirstName	Company
1	Smith	Bob	Acme
2	Jones	John	Acme
3	Johnson	Bob	Floorworks
4	Smith	Bob	Acme

Then:

Unique(DataSet) → ['Smith', 'Jones', 'Johnson', 'Bob', 'John', 'Acme', 'Floorworks' ]

Unique(DataSet, PersonNum) → [1, 2, 3]

Unique(DataSet[Field = 'Company'], PersonNum) → [1, 3]

Optional parameters

Position

By default, Unique returns the elements of the index. Setting the optional parameter «position» equal true (position: true) will return the positions of the elements in «I» , rather than the elements themselves (see Associative vs. Positional Indexing).

This parameter is not used when the «I» index parameter is omitted.

CaseInsensitive

When applying Unique to text values, values are considered by default in a case-sensitive fashion, for example, "Apple" and "apple" are considered distinct elements.

Specifying caseInsensitive: true ignores differences in upper and lower case in text values when determining if values are unique.

ResultIndex

When an index is provided for this parameter, the result is indexed by the given index instead of being an unindexed list. If the index supplied is shorter than the number of unique items, then only the first n items are returned, where n is the size of the index. When the result index is too long, the result is null-padded.

«ResultIndex» can be useful when you want to array abstract. For example, in a 2-D array a, you may want to identify the unique items along I separately for each item in index J, as follows.

For jj := J Do Unique(a[J = jj], I, resultIndex: I)

Without the «resultIndex» parameter, each iteration would return a list, and the For loop would then need to combine multiple list (i.e., implicit dimensions), which would be disallowed and result in an error. By ensuring that each result has an explicit index -- I in this example -- the results can be successfully combined.

It is also worth noting that the For loop example just given is not equivalent to

Unique(a, I, resultIndex: I)

The reason is that Unique(a, I) compares entire slices -- it isn't operating over each slice of the exogenous dimensions separately as most other array functions do.

Notes

The Set Functions such as SetDifference or SetUnion ensure that no duplicates exist in the final result, and hence can also be used to find the unique elements. For example, when L is a 1-D array or list

#SetDifference(\L)

and

Unique(L)

do the same thing. Since no set is being differences, SetDifference returns the set \L after duplicates are removed. In some instances, SetDifference has advantages over Unique. For example, if you also want to ignore certain values, including Null or others, you could compute the unique elements and then follow that with a call to SetDifference to remove the other values, but when so doing, you might as well skip the call to Unique entirely since SetDifference already does that for you.

The ordering of elements in the result follows the ordering of the elements in «a», which often feels arbitrary. Hence, it is common to wrap the call to Unique is a call to SortIndex such as

SortIndex(Unique(a))

History

The «I» parameter was first made optional in Analytica 5.0. Prior to that, the Unique(a) usage was not available. Without to omitted index, use a[I = Unique(a, I)] or #SetDifference(\a) to define an index.

The «resultIndex» parameter is present in Analytica 4.3 and later, but hidden (doesn't show up in Expression Assist, etc.) until Analytica 5.0. You can still use it in those earlier releases even though it is hidden.