find the index of top n elements in a vector in order [R]

Question

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