In the above example, ismember function first checks whether the values present in X are also a part of Y or not. To determine the corresponding location of the values that are present in Y array. 6 is not present in Y, so the resultant value is 0. Similarly, the values at the 3 rd and 4 th positions are also present in the Y, so the resultant values are 1. 4 is present in Y, so the first value of the resultant LX is 1. In the above example, ismember function checks whether the elements present in X are also present in Y and returns the logical values in the form of 1 and 0. To check whether the elements of X are present in Y. Legacy option is not supported if the types of arrays are categorical, datetime, duration, timetables or tables.īelow examples explain the concept of ismember function in Matlab: Example #1 =ismember (_,” legacy option”): If the legacy option is mentioned in the syntax, then the properties and behaviour of ismember function is considered from R2012b versions and previous releases.If the value is 0, then X is not in the row of Y. If X and Y are in the form of tables or timetables, then LocationY contains the lowest index of Y, provided that rows in X should be a part of rows in Y. If there are rows option specified in the syntax, then LocationY contains the lowest index of Y, provided that rows in X should be a part of rows in Y. If the value is 0, then it indicates that the elements present in X are not a part of Y. =ismember (_): Here LocationY is used to find the lowest index values present in Y if the values present in X are a member of Y.The rows option is not valid if cell arrays are used, provided that the input array is categorical or datetime array. If the values are present, then it returns logical 1 (True) else it returns logical 0 (False). Lx=ismember (X, Y, rows): This syntax considers the rows of X and Y as single entities and determines the logical values which are in the form of 1 and 0.The resultant value Lx is a column vector. If X and Y are in the form of tables or timetables, then it returns the logical value for every row present and if X and Y are in the form of timetables, then row times are considered. If the elements are present, then it returns 1(True) else it returns 0(False). Lx=ismember (X, Y): This checks whether the elements in X is present in Y.Please find the below syntaxes which are used in Matlab considering ismember function: The result is in the form of logical 1 (True) or logical 0 (False). Try and see what works for you.In Matlab, we can check if a particular element belongs to an array or not by using ismember () function. If you have a sorted B, it's even faster since the two built-in functions assume the second argument ( Bs) is sorted and waste no time with checks. Is it worth doing it this way, with a penalty for sort and two effective ismember calls? Maybe not, but I think it's an interesting solution. Testing operation with unsorted B: B(4:5) = B()įirstInds = builtin('_ismemberfirst',A,Bs) Looking closer at the third element: > allInds The first two cells are empty arrays, as expected. Įach cell has the indexes in B (if any) of each element of A. We can quickly do this lookup and package each range of indexes with arrayfun, keeping in mind that the computationally intensive task of actually finding the indexes is already done: allInds = We are able to ignore sortInds in the above example since B is already sorted, but an unsorted B is handled by simply looking up the locations in the unsorted array. The value 4 ( A(3)) occurs at locations 2:4 (i.e. There is no occurrence of A(1) or A(2) (5 or 3) in B, so those indexes are 0. The heavy lifting is now done - We have the first and last indexes in B for each element in A without having to do any looping. The thinking goes as follows: In a sorted B, what if you had the first and last indexes of each matching element? It turns out there are two helper functions used by ismember (if you have R2012b+, I think) that will give you both of these indexes: _ismemberfirst (a builtin) and ismembc2.įor the example data A = B = in the question, here is the implementation: = sort(B) % nop for this B, but required in generalįirstInds = builtin('_ismemberfirst',A,Bs) % newish version required for each element of A, find the indexes of all corresponding elements in B). However, for those interested in a solution with undocumented functionality, and an admittedly hackish approach, here is another way to do it (i.e. those without using iterations of find) involve swapping the inputs to ismember and grouping like indexes with accumarray, as in Eitan's answer, or vectorizing the find with bsxfun as in Luis Mendo's answer, IMHO.
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