How to use which.min function for a list

Question

I have 3 sets of list called minDist. Each list has three columns hence it has three groups (1, 2 & 3).

minDist:

 [[1]]
          [,1]     [,2]     [,3]
 [1,]  1.000000 9.055385 9.000000
 [2,]  0.000000 9.000000 9.055385
 [3,]  1.414214 8.062258 8.000000
 [4,]  1.000000 8.000000 8.062258
 [5,]  9.055385 1.000000 0.000000
 [6,]  9.000000 0.000000 1.000000
 [7,] 10.049876 1.414214 1.000000
 [8,] 10.000000 1.000000 1.414214
 [9,]  5.000000 5.830952 6.403124
[10,]  5.656854 6.403124 7.071068


[[2]]
        [,1]      [,2]     [,3]
[1,] 10.000000 10.049876 9.055385
[2,] 10.049876 10.000000 9.000000
[3,]  9.000000  9.055385 8.062258
[4,]  9.055385  9.000000 8.000000
[5,]  1.000000  1.414214 1.000000
[6,]  1.414214  1.000000 0.000000
[7,]  0.000000  1.000000 1.414214
[8,]  1.000000  0.000000 1.000000
[9,]  7.211103  6.708204 5.830952
[10,]  7.810250  7.211103 6.403124


 [[3]]
         [,1]     [,2]     [,3]
 [1,]  0.000000 1.000000 9.055385
 [2,]  1.000000 1.414214 9.000000
 [3,]  1.000000 0.000000 8.062258
 [4,]  1.414214 1.000000 8.000000
 [5,]  9.000000 8.000000 1.000000
 [6,]  9.055385 8.062258 0.000000
 [7,] 10.000000 9.000000 1.414214
 [8,] 10.049876 9.055385 1.000000
 [9,]  5.656854 5.000000 5.830952
 [10,]  6.403124 5.830952 6.403124

I want to find out which row belongs to the same groups. For example, List 1:

 [1] 1 1 1 1 3 2 3 2 1 1

Meaning row 1,2,3,4,9,and 10 belongs to group 1, row 6 and 8 belongs to group 2 row 5 and 7 belongs to group 3.

If I only have one list, this is how I try to do the grouping:

 grouping <- apply(minDist, 1, which.min)

How can I get the answers for all the three list using loops?

nicola nicola · Accepted Answer · 2018-01-14T09:03:27

Try:

lapply(minDist, function(x) max.col(-x))

Don't use the highly inefficient apply(x,1,which.min); use max.col instead, which returns for each row of a matrix the index of the maximum value (if you want the minimum, you just find the maximum of the opposite of x).

How to use which.min function for a list

2 Answers