# Select the largest two numbers with their indices

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Saed am 3 Jan. 2012
Hi,
I have the following random variable:
h=randn(4,4)+1i.*randn(4,4);
suppose that
|hi|^2=abs(h(i,1))^2+abs(h(i,2))^2+abs(h(i,3))^2+abs(h(i,4))^2
I need to select the largest two, i.e.: |hi|^2+||hj||^2 is the maximum, and the indices i and j, in the most efficient way. How?
Thanks
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### Akzeptierte Antwort

Andrew Newell am 3 Jan. 2012
Here is one approach that is efficient enough:
n = 4;
h=randn(n)+1i.*randn(n);
h2 = sum(h.*conj(h),2); %sum of squares for each row
[h2sort,isort] = sort(h2,'descend');
iLargest = isort(1:2);
h2Largest = h2(iLargest);
disp(['Sum of two largest values = ',num2str(h2Largest'*h2Largest)])
(Edited in view of the discussion below.)
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Andrew Newell am 4 Jan. 2012
Oh, is that what he means by "the largest two"?
Saed am 4 Jan. 2012
yes right, the largest two numbers will have the largest sum. Thanks

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### Weitere Antworten (1)

Walter Roberson am 3 Jan. 2012
Does "the most efficient way" have to do with time to program the solution, best-case execution time, worst-case execution time, average execution time, memory consumption, algorithmic complexity, some other factor?
If you want the best execution time, then the solution could involve sending the numbers to an FPGA (which you would have to program) and retrieving the answer from it. On the other hand, due to the overhead of communicating with an FPGA, perhaps a Mex routine would be faster.
Is strict IEEE754 compliance required in the calculations?
You will be writing the code in Assembly Language, right?
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Saed am 3 Jan. 2012
What I meant in terms MATLAB syntax without involving too much loops.

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