[ find the index of top n elements in a vector in order [R] ]
I have a matrix of type numeric, with dim 10000 * 50. Now I want to find the index of top 5 elements in every row in the order of their values. e.g. a sample might look like :
set.seed(2)
v1 <- matrix(runif(20 , 0 ,20 ) , 2 ,10)
v1
# [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
#[1,] 3.697645 11.466527 18.87679 2.58318 9.36037 11.053481 15.210266 8.105644 19.527970 8.896185
#[2,] 14.047481 3.361038 18.86950 16.66898 10.99967 4.777895 3.616402 17.070969 4.516509 1.499588
Then I want the output to look like :
#[1,] 9 3 7 2 6
#[2,] 3 8 4 1 5
I could find only this question, which explains how to find top n elements, but not in the order of values.
Answer 1
apply()
is perfect for row-wise operations on matrices. You could do
t(apply(v1, 1, function(x) order(-x)[1:5]))
# [,1] [,2] [,3] [,4] [,5]
# [1,] 9 3 7 2 6
# [2,] 3 8 4 1 5
This runs the order()
function row-wise down the matrix v1
then takes the first five values for each one, transposing the result since you specify rows not columns.
Answer 2
This can also be done with data.table
after melting
into 'long' format, grouped by 'Var1', we get the order
of 'value'
library(reshape2)
library(data.table)
setDT(melt(v1))[, head(order(-value),5), Var1]
# Var1 V1
#1: 1 9
#2: 1 3
#3: 1 7
#4: 1 2
#5: 1 6
#6: 2 3
#7: 2 8
#8: 2 4
#9: 2 1
#10: 2 5
Or using base R
ave(-v1, row(v1), FUN = order)[,1:5]
# [,1] [,2] [,3] [,4] [,5]
#[1,] 9 3 7 2 6
#[2,] 3 8 4 1 5