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rloopsmatrixdeterminants

How to perform complex looping operation on matrix

I have a sample matrix like 

 5 4 3  
 2 6 8   
 1 9 7

and I want output like 

max(5*6,5*8,5*9,5*7)  // i!=j condition  
max(4*2,4*8,4*1,4*7)  
max(3*2,3*6,3*1,3*9)

And so on...

This maximum values obtained after computation should be in matrix form. I need to generalize it, therefore I need a generic code.

Solution

  • This gets the job done but is a pretty unimaginative solution, in that it just loops through the rows and columns performing the requested calculation instead of doing anything vectorized.

    sapply(1:ncol(m), function(j) sapply(1:nrow(m), function(i) max(m[i,j]*m[-i,-j])))
#      [,1] [,2] [,3]
# [1,]   45   32   27
# [2,]   18   42   72
# [3,]    8   72   42

    Data:

    (m <- matrix(c(5, 2, 1, 4, 6, 9, 3, 8, 7), nrow=3))
#      [,1] [,2] [,3]
# [1,]    5    4    3
# [2,]    2    6    8
# [3,]    1    9    7