indexing multiple values in two arrays with different size

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Pietro 2019-2-13

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https://ww2.mathworks.cn/matlabcentral/answers/444791-indexing-multiple-values-in-two-arrays-with-different-size

评论： madhan ravi 2019-2-15

采纳的回答： madhan ravi

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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

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Answer 1

madhan ravi 2019-2-13

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

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madhan ravi 2019-2-13

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For version prior to 2016b:

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

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Answer 2

Pietro 2019-2-14

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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

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Pietro 2019-2-15

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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