find value of matrix by position

Hi everyone

Can you help me to optimize code?

this is my code:

 aa =

     1     4     5     7
     9     2     3     8
bb =

     1     3
     2     4

I want to get value of each row aa by position of row bb. Like this

cc = [1 5; 2 8]

Thank

Answer 1

[Jan]

I guess, that the elements of bb are the row indices related to aa, and the column index is the corresponding column of bb:

aa = [1     4     5     7; ...
      9     2     3     8; ...
      13   21    33    44];
bb = [1     3; ... 
      2     4; ...
      2     3];

s = size(bb);
c = aa(sub2ind(size(aa), repmat((1:s(1)).', 1, s(2)), bb))

Or faster with a direct call, but without exhaustive error checking:

sa = size(aa, 1);
sb = size(bb, 1);
index = (1:sb).' + (bb - 1) .* sa;
aa(index)

[Tiki Tiki]

Thanks so much.

[Jan]

You are welcome.

[Tiki Tiki]

Hi. a long time to see you.

can i ask you one thing?

i want to learn some tricks like your answer.

"index = (1:sb).' + (bb - 1) .* sa;"

how can i find some resources to learn this.

because i think that, this is really fast to solve some problems without using function in matlab.

Hope you can help me.

Thank. Have a good day.

God blesses you.

[Jan]

I've learned it by using sub2ind at first, and then reading the code of sub2ind.m. I've removed all parts for the error checking and simplified the code such that it works for 2 dimensions only. During implementing this as code, I understood the underlying logic: With linear indexing, the formula is almost trivial. :-)

[Tiki Tiki]

Thank

But I find many code of sub2ind.m

How can i know which one? If I try to learn another function like function repmat as you mentioned above.

Have a nice day.

God blesses you.