secons max number in a vector

Hi I need to find maximum and second maximum number in a vector. Can you please help?

For max, its max(X), but for second max I am unable to figure out.

Your help is appreciated

P

Answer 1

[Rick Rosson]

   a = max(X);
   Y = X(X ~= a);
   b = max(Y);

[bym]

wouldn't that be

Y = (X(X ~= a)); % ??

[Jan]

Yes, proecsm, now it is revealed that Rick thinks in C.

@Rick: This is a nice example for the limitations of Matlab. MAX(X) searchs the complete vector for the largest element. X~=a creates a LOGICAL vector of the same size as X and X(X~=a) processes the complete vector again. The temporary Y occupies almost as much memory as X. Finally the complete vector Y is searched.

If the input vector does not match in the processor cache, waiting for the slow RAM will be a brute bottleneck for this program. Usign SORT and taking the last two elements is not a good alternative for large inputs also, because it takes a lot of time and demands for storing another temporary array also.

A C-Mex function will be much faster - nearly as fast as a single MAX command: If the current element is larger than the old maximum, store the old maximum as 2nd max value and the current element as new maximum. This can even be multithreaded.

[Walter Roberson]

There are apparently efficient linear-time routines to find the N largest or N smallest numbers in a list, and supposedly they do not need to do a mini-sort of N elements to find the proper place in the list for each new value being tested. I have not yet been able to understand the logic in the paper that describes the internal comparison scheme.

[Rick Rosson]

Everyone makes mistakes. Glass houses...