Finding the index of the minimum value which is larger than a threshold in R

Question

This is probably very simple, but I'm missing the correct syntax in order to simplify it.

Given a matrix, find the entry in one column which is the lowest value, greater than some input parameter. Then, return an entry in a different column on that corresponding row. Not very complicated... and I've found something that works but, a more efficient solution would be greatly appreciated.

I found this link:Better way to find a minimum value that fits a condition?

which is great.. but that method of finding the least entry loses the index information required to find a corresponding value in a corresponding row.

Let's say column 2 is the condition column, and column 1 is the one I want to return.... currently I've made this: (note that this only works because row two is full of numbers which are less than 1).

matrix[which.max((matrix[,2]>threshhold)/matrix[,2]),1]

Any thoughts? I'm expecting that there is probably some quick and easy function which has this effect... it's just never been introduced to me haha.

Answer 1

You could try the following. Say,

df <- matrix(sample(1:35,35),7,5)
> df
     [,1] [,2] [,3] [,4] [,5]
[1,]   18   16   27   19   31
[2,]   24    1    7   12    5
[3,]   28   35   23    4    6
[4,]   33    3   25   26   15
[5,]   14   10   11   21   20
[6,]    9    2   32   17   13
[7,]   30    8   29   22   34

Say your threshold is 5:

apply(df,2,function(x){ x[x<5] <- max(x);which.min(x)})
[1] 6 7 2 2 2

Corresponding to the values:

[1]  9  8  7 12  5

This should give you the index of the smallest entry in each column greater than threshold according to the original column indexing.

Answer 2

rmk's answer shows the basic way to get a lot of info out of your matrix. But if you know which column you're testing for the minimum value (above your threshold), and then want to return a different value in that row, maybe something like

incol<- df[,4] # select the column to search
outcol <- 2 # select the element of the found row you want to get
threshold <- 5
df[ rev(order(incol>threshold))[1] ,outcol]