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searching row indices in a matrix

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Chaitanya Sanghavi
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.
  5 Comments
Chaitanya Sanghavi
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. :)

Answers (2)

Stephen23
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
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.
if you find this answer useful would you please be so kind to consider marking my answer as Accepted Answer?
To any other reader, if you find this answer useful please consider clicking on the thumbs-up vote link
thanks in advance for time and attention
John BG

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