ismember - Find set members of data - MATLAB (original) (raw)

Syntax

Description

[Lia](#btcnv43-1-Lia) = ismember([A](#btcnv43-1-A),[B](#btcnv43-1-B)) returns an array containing logical 1 (true) where the data in A is found inB. Elsewhere, the array contains logical0 (false).

If A and B are tables or timetables, then ismember returns a logical value for each row. For timetables, ismember takes row times into account to determine equality. The output,Lia, is a column vector.

example

[Lia](#btcnv43-1-Lia) = ismember([A](#btcnv43-1-A),[B](#btcnv43-1-B),"rows") treats each row of A and each row of B as single entities and returns a column vector containing logical1 (true) where the rows ofA are also rows of B. Elsewhere, the array contains logical 0 (false).

The "rows" option does not support cell arrays, unless one of the inputs is either a categorical array or a datetime array.

[[Lia](#btcnv43-1-Lia),[Locb](#btcnv43-1-Locb)] = ismember(___) also returns an array, Locb, using any of the previous syntaxes.

Generally, Locb contains the lowest index inB for each value in A that is a member of B. Values of 0 indicate where A is not a member ofB.
If the "rows" option is specified, thenLocb contains the lowest index inB for each row in A that is also a row in B. Values of0 indicate where A is not a row of B.
If A and B are tables or timetables, then Locb contains the lowest index in B for each row in A that is also a row in B. Values of 0 indicate where A is not a row ofB.

example

[[Lia](#btcnv43-1-Lia),[Locb](#btcnv43-1-Locb)] = ismember(___,"legacy") preserves the behavior of theismember function from R2012b and prior releases using any of the input arguments in previous syntaxes.

The 'legacy' option does not support categorical arrays, datetime arrays, duration arrays, tables, or timetables.

example

Examples

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Create two vectors with values in common.

A = [5 3 4 2]; B = [2 4 4 4 6 8];

Determine which elements of A are also in B.

Lia = 1×4 logical array

0 0 1 1

A(3) and A(4) are found in B.

Create two tables with rows in common.

A = table([1:5]',["red";"green";"blue";"cyan";"magenta"],logical([0;1;0;1;0]))

A=5×3 table Var1 Var2 Var3 ____ _________ _____

 1      "red"        false
 2      "green"      true 
 3      "blue"       false
 4      "cyan"       true 
 5      "magenta"    false

B = table([1:2:10]',["red";"blue";"magenta";"yellow";"black"],logical(zeros(5,1)))

B=5×3 table Var1 Var2 Var3 ____ _________ _____

 1      "red"        false
 3      "blue"       false
 5      "magenta"    false
 7      "yellow"     false
 9      "black"      false

Determine which rows of A are also in B.

Lia = 5×1 logical array

1 0 1 0 1

The rows A(1,:), A(3,:), and A(5,:) are found in B.

Create two vectors with values in common.

A = [5 3 4 2]; B = [2 4 4 4 6 8];

Determine which elements of A are also in B as well as their corresponding locations in B.

[Lia,Locb] = ismember(A,B)

Lia = 1×4 logical array

0 0 1 1

The lowest index to A(3) is B(2), and A(4) is found in B(1).

Create a vector x. Obtain a second vector y by transforming and untransforming x. This transformation introduces round-off differences in y.

x = (1:6)'*pi; y = 10.^log10(x);

Verify that x and y are not identical by taking the difference.

ans = 6×1 10-14 ×

-0.3553

Use ismember to find the elements of x that are in y. The ismember function performs exact comparisons and determines that some of the matrix elements in x are not members of y.

lia = 6×1 logical array

0 1 1 1 1 0

Use ismembertol to perform the comparison using a small tolerance. ismembertol treats elements that are within tolerance as equal and determines that all of the elements in x are members of y.

LIA = 6×1 logical array

1 1 1 1 1 1

Create a table, A, of smoker status, age, and height for five people.

A = table([true;true;false;true;false],[27;52;31;46;35],[74;68;64;61;64],... VariableNames=["Smoker" "Age" "Height"],... RowNames=["Sanchez" "Johnson" "Li" "Diaz" "Brown"])

A=5×3 table Smoker Age Height ______ ___ ______

Sanchez    true      27       74  
Johnson    true      52       68  
Li         false     31       64  
Diaz       true      46       61  
Brown      false     35       64

Create another table, B, with rows in common with A.

B = table([false;false;false;false],[47;31;35;23],[68;64;62;58],... VariableNames=["Smoker" "Age" "Height"],... RowNames=["Nguyen" "Cohen" "Jones" "Garcia"])

B=4×3 table Smoker Age Height ______ ___ ______

Nguyen    false     47       68  
Cohen     false     31       64  
Jones     false     35       62  
Garcia    false     23       58

Determine which rows of A are also in B, as well as their corresponding locations in B.

[Lia,Locb] = ismember(A,B)

Lia = 5×1 logical array

0 0 1 0 0

Two table rows that have the same values, but different row names, are considered equal. The same data for Li is found in B(2,:), which corresponds to Cohen.

Create two matrices with a row in common.

A = [1 3 5 6; 2 4 6 8]; B = [2 4 6 8; 1 3 5 7; 2 4 6 8];

Determine which rows of A are also in B as well as their corresponding locations in B.

[Lia, Locb] = ismember(A,B,"rows")

Lia = 2×1 logical array

0 1

The lowest index to A(2,:) is B(1,:).

Create two vectors containing NaN.

A = [5 NaN NaN]; B = [5 NaN NaN];

Determine which elements of A are also in B, as well as their corresponding locations in B.

[Lia,Locb] = ismember(A,B)

Lia = 1×3 logical array

1 0 0

ismember treats NaN values as distinct.

Create a string array, A.

A = ["dog","cat","fish","horse"];

Create another string array, B, where some elements have trailing white space.

B = ["dog ","cat","fish ","horse"];

Determine which strings in A are also in B.

[Lia,Locb] = ismember(A,B)

Lia = 1×4 logical array

0 1 0 1

ismember treats trailing white space in string arrays, and also in cell arrays of character vectors, as distinct characters.

Use the "legacy" flag to preserve the behavior of ismember from R2012b and prior releases in your code.

Find the members of B with the current behavior.

A = [5 3 4 2]; B = [2 4 4 4 6 8]; [Lia1,Locb1] = ismember(A,B)

Lia1 = 1×4 logical array

0 0 1 1

Find the members of B, and preserve the legacy behavior.

[Lia2,Locb2] = ismember(A,B,"legacy")

Lia2 = 1×4 logical array

0 0 1 1

Input Arguments

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Query array, specified as an array, table, or timetable. If you specify the "rows" option, A andB must have the same number of columns.

A must belong to the same class as B with the following exceptions:

logical, char, and all numeric classes can combine with double arrays.
String arrays can combine with character vectors and cell arrays of character vectors.
Categorical arrays can combine with string arrays, character vectors, and cell arrays of character vectors.
Datetime and duration arrays can combine with string scalars and character vectors that are formatted to represent dates and times.

There are additional requirements for A andB based on data type:

If A is a categorical array, thenB can be a categorical array, a string array, a character vector, or a cell array of character vectors. In this case, the second input B is a set of category names.
If A and B are both ordinal categorical arrays, they must have the same sets of categories, including their order.
If neither A nor B are ordinal, they need not have the same sets of categories, and the comparison is performed using the category names.
If A is a table or timetable, it must have the same variable names as B (except for order).
- For tables, row names are ignored, so that two rows that have the same values, but different names, are considered equal.
- For timetables, row times are taken into account, so that two rows that have the same values, but different times, are not considered equal.
If A and B are datetime arrays, then both arrays must either specify time zones or be unzoned.

A also can be an object with the following class methods:

sort (or sortrows for the "rows" option)
eq
ne

The object class methods must be consistent with each other. These objects include heterogeneous arrays derived from the same root class. For example, A can be an array of handles to graphics objects.

Set array, specified as an array, table, or timetable. If you specify the"rows" option, A andB must have the same number of columns.

B must belong to the same class as A with the following exceptions:

logical, char, and all numeric classes can combine with double arrays.
String arrays can combine with character vectors and cell arrays of character vectors.
Categorical arrays can combine with string arrays, character vectors, and cell arrays of character vectors.
Datetime and duration arrays can combine with string scalars and character vectors that are formatted to represent dates and times.

There are additional requirements for A andB based on data type:

If A is a categorical array, thenB can be a categorical array, a string array, a character vector, or a cell array of character vectors. In this case, the second input B is a set of category names.
If A and B are both ordinal categorical arrays, they must have the same sets of categories, including their order.
If neither A nor B are ordinal, they need not have the same sets of categories, and the comparison is performed using the category names.
If A is a table or timetable, it must have the same variable names as B (except for order).
- For tables, row names are ignored, so that two rows that have the same values, but different names, are considered equal.
- For timetables, row times are taken into account, so that two rows that have the same values, but different times, are not considered equal.
If A and B are datetime arrays, then both arrays must either specify time zones or be unzoned.

B also can be an object with the following class methods:

sort (or sortrows for the "rows" option)
eq
ne

The object class methods must be consistent with each other. These objects include heterogeneous arrays derived from the same root class. For example, B can be an array of handles to graphics objects.

Output Arguments

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Logical index to A, returned as a vector, matrix or N-D array containing logical 1 (true) wherever the values (or rows) in A are members ofB. Elsewhere, it contains logical0 (false).

Lia is an array of the same size asA, unless you specify the "rows" flag.

If the "rows" flag is specified or ifA is a table or timetable, Lia is a column vector with the same number of rows as A.

Locations in B, returned as a vector, matrix, or N-D array. If the "legacy" flag is not specified,Locb contains the lowest indices to the values (or rows) in B that are found in A. Values of 0 indicate where A is not a member of B.

Locb is an array of the same size asA unless you specify the "rows" flag.

If the "rows" flag is specified or ifA is a table or timetable, Locb is a column vector with the same number of rows as A.

Tips

Use ismembertol to perform comparisons between floating-point numbers using a tolerance.
To find the rows from table or timetable A that are found in B with respect to a subset of variables, you can use column subscripting. For example, you can useismember(A(:,_`vars`_),B(:,_`vars`_)), where vars is a positive integer, a vector of positive integers, a variable name, a string array or cell array of variable names, or a logical vector. Alternatively, you can use vartype to create a subscript that selects variables of a specified type.

Extended Capabilities

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Theismember function supports tall arrays with the following usage notes and limitations:

If both A and B are tall arrays, then one must be the result of a reduction operation applied in the first dimension (such as sum, prod,max, and so on).

For more information, see Tall Arrays.

The ismember function supports GPU array input with these usage notes and limitations:

The "legacy" flag is not supported.
64-bit integers are not supported.

For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).

Version History

Introduced before R2006a