indexing multiple values in two arrays with different size

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

Answer 1

[madhan ravi]

A=sum(a(:,1)==b(:,1).');
a(:,2)=repelem(b(:,2),A).'

[madhan ravi]

For version prior to 2016b:

A=sum(bsxfun(@eq,a(:,1),b(:,1).'));
a(:,2)=repelem(b(:,2),A).'

Answer 2

[Pietro]

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

[madhan ravi]

Typing short lines of code doesn't make the code efficient:

tic
for k=1:1e5
a(:,2) = interp1(b(:,1), b(:,2), a(:,1));
end
toc
tic
for k=1:1e5
    A=sum(a(:,1)==b(:,1).');
a(:,2)=repelem(b(:,2),A).';
end
toc

Gives:

Elapsed time is 10.439265 seconds. % interp1() method
Elapsed time is 1.281436 seconds. % other method

[Pietro]

Thanks a lot, I wanted to do the same but just did not have the time

Thanks again for the support

[Pietro]

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]

So instead of creating variable A in workspace why not directly implement it ?

a(:,2)=repelem(b(:,2),sum(a(:,1)==b(:,1).')).';