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# searching row indices in a matrix

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Chaitanya Sanghavi on 22 Dec 2017
Closed: MATLAB Answer Bot on 20 Aug 2021
I have a vector a = [] ; of size (n,1) I have another matrix B which is made is of size (m,2).
Eg. a(1) = [2 3 4 5]; a(2) = [5 6 7 8]; a(3) = [8 9 10 11] B(:,1) = [a(1) a(2) a(3)];
a(4) = [23 24 25 26]; a(5) = [26 27 28 29] ;a(6) = [29 30 31 32] B(:,2) = [a(4) a(5) a(6)];
Now,I want the row indices in B which correspond to a vectors.
For eg. if I check if a(4) is a member of B, I want output as [1 2 3 4] a(5) is a member of B, I want output as [5 6 7 8] and not [4 6 7 8];
Also the same when I check a(2) for B. The output should be [5 6 7 8] and not [4 6 7 8].
I can of course do this using loops and if conditions, but I want to use smart indexing commands.
Chaitanya Sanghavi on 22 Dec 2017
You are right with the assumption that everything is a vector in "RHS". Well, This works syntax works in Matlab. :)

Stephen23 on 22 Dec 2017
Use strfind if you want to preserve order. It will return the first index of each location where the substring is found in a longer string, and yes it works with numeric vectors too!

John BG on 4 Jan 2018
Edited: John BG on 4 Jan 2018
Hi Chaitanya
to meet the requirement
I want the row indices in B which correspond to a vectors.
you may want to consider using cells instead.
Start defining your data like this:
a{1} = [2 3 4 5]
a{2} = [5 6 7 8]
a{3} = [8 9 10 11]
and
B = [a(1) a(2) a(3)];
B is now a cell with 3 components, each a vector of your choice
to add rows to B, for instance
B=[B;B]
B =
2×3 cell array
[1×4 double] [1×4 double] [1×4 double]
[1×4 double] [1×4 double] [1×4 double]
the thing is, if you try to use ismember now, MATLAB returns
Error using cell/ismember (line 34)
Input A of class cell and input B of class cell must be cell arrays of character
vectors, unless one is a character vector.
so you may need a custom function to perform the checks mentioned.