Nth smallest value for every column in data.frame in R

Question

I want to find the nth smallest number for every column in a data.frame.

In the below example I specify actually the second smallest value using the dcast nth function. Can someone help with the coding of the function?

library(vegclust)
library(dplyr)
data(wetland)
dfnorm = decostand(wetland,"normalize")
dfchord = dist(dfnorm, method = "euclidean")
dfchord = data.frame(as.matrix(dfchord)
number_function = function(x) nth(x,2) # can change 2 to any number..

answer_vector = apply(dfchord, 2, number) # here, 2 specifying apply on columns

The actual answer would be something like this..

ans = c(0.5689322,0.579568297,0.315017693,0.315017693,0.632246369, 0.868563003, 0.704638684, 0.35827587, 0.725220337, 0.516397779) # length of 1:38

Answer 1

Just a warning, if you don't specify the order for dplyr's nth(), it will not actually do the sorting:

For example,

> sapply(mtcars, dplyr::nth, 2)
    mpg     cyl    disp      hp    drat      wt    qsec      vs      am    gear    carb 
 21.000   6.000 160.000 110.000   3.900   2.875  17.020   0.000   1.000   4.000   4.000 

which is actually just the second row of the data:

> mtcars[2,]
              mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX4 Wag  21   6  160 110  3.9 2.875 17.02  0  1    4    4

The nth function in Rfast does sort by default:

> sapply(mtcars, Rfast::nth, 2)
   mpg    cyl   disp     hp   drat     wt   qsec     vs     am   gear   carb 
10.400  4.000 75.700 62.000  2.760  1.615 14.600  0.000  0.000  3.000  1.000 

If you are sensitive to performance, the Rfast version was written to scale well by using a partial sort, which isn't true for solutions based on sort, order or rank (including dplyr::nth).

Answer 2

Here is my example;

num_func <- function(x, n) nth(sort(x), n)
sapply(dfchord, num_func, n = 2)  # edited (thanks for @thelatemail's comment)