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27 Sets

Octave has a number of functions for managing sets of data. A set is defined as a collection of unique elements and is typically represented by a vector of numbers sorted in ascending order. Any vector or matrix can be converted to a set by removing duplicates through the use of the unique function. However, it isn’t necessary to explicitly create a set as all of the functions which operate on sets will convert their input to a set before proceeding.

: unique (x)
: unique (x, "rows")
: unique (…, "sorted")
: unique (…, "stable")
: [y, i, j] = unique (…)
: [y, i, j] = unique (…, "first")
: [y, i, j] = unique (…, "last")
: [y, i, j] = unique (…, "legacy")

Return the unique elements of x.

If the input x is a column vector then return a column vector; Otherwise, return a row vector. x may also be a cell array of strings.

If the optional argument "rows" is given then return the unique rows of x. The input must be a 2-D numeric matrix to use this option.

The optional argument "sorted"/"stable" controls the order in which unique values appear in the output. The default is "sorted" and values in the output are placed in ascending order. The alternative "stable" preserves the order found in the input x.

If requested, return column index vectors i and j such that y = x(i) and x = y(j).

Additionally, if i is a requested output then one of the flags "first" or "last" may be given. If "last" is specified, return the highest possible indices in i, otherwise, if "first" is specified, return the lowest. The default is "first".

Example 1 : sort order

unique ([3, 1, 1, 2])
⇒ [1, 2, 3]
unique ([3, 1, 1, 2], "stable")
⇒ [3, 1, 2]

Example 2 : index selection

[~, i] = unique ([3, 1, 1, 2], "first")
⇒ i = [2; 4; 1]
[~, i] = unique ([3, 1, 1, 2], "last")
⇒ i = [3; 4; 1]

Programming Notes: The input flag "legacy" changes the algorithm to be compatible with MATLAB releases prior to R2012b. Specifically, The index ordering flag is changed to "last", and the shape of the outputs i, j will follow the shape of the input x rather than always being column vectors.

The third output, j, has not been implemented yet when the sort order is "stable".

See also: union, intersect, setdiff, setxor, ismember.

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