indexing multiple values in two arrays with different size

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I have a simple problem that unfortunately I am failiing so solve (and to find solutions in internet).
Assuming I have 2 matrixes
a = [1 1 1 1 2 2 2 2 2 5 5 5; 0 0 0 0 0 0 0 0 0 0 0 0]';
b = [1 2 5; 11 12 15]';
I would like (without a for loop, that is my current, very slow implementation) to assign in a the corresponding values in the second column of b, when a(x, 1) == b(x,1). Basically the final result should be
a = [1 1 1 1 2 2 2 2 2 5 5 5; 11 11 11 11 12 12 12 12 12 15 15 15]
Thanks a lot for any help

Accepted Answer

madhan ravi
madhan ravi on 13 Feb 2019
A=sum(a(:,1)==b(:,1).');
a(:,2)=repelem(b(:,2),A).'
  1 Comment
madhan ravi
madhan ravi on 13 Feb 2019
For version prior to 2016b:
A=sum(bsxfun(@eq,a(:,1),b(:,1).'));
a(:,2)=repelem(b(:,2),A).'

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More Answers (1)

Pietro
Pietro on 14 Feb 2019
I received also another way to do it
a(:,2) = interp1(b(:,1), b(:,2), a(:,1));
I do not know which one is the more efficient, but both solutions work
I will use this one as it is only 1 line of code instead of two
Thanks for the support
  4 Comments
Pietro
Pietro on 15 Feb 2019
Unfortunately your method does not work because this operation
A=sum(a(:,1)==b(:,1).');
requires too much memory (in my case b(:,1) is bigger than 1M values), so even if more efficient in speed, it is not very efficient in memory management (the tipical trade off)
Thanks anyway
madhan ravi
madhan ravi on 15 Feb 2019
So instead of creating variable A in workspace why not directly implement it ?
a(:,2)=repelem(b(:,2),sum(a(:,1)==b(:,1).')).';

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