Printing sorted matrix elements in descending order with array indices in fastest way
This seems like a simple problem, but I'm having problems with it quickly.
Let's say I have a matrix and I want to sort this matrix and keep the element indices in descending order. Is there a quick way to do this? Right now, I'm fetching the maximum, storing the result, changing it to -2, and then fetching the next maximum in a for loop. This is most likely the most inefficient way to do it.
My problem actually requires me to work with a 20,000 x 20,000 matrix. Memory is not an issue. Any ideas on the fastest way to do this would be great.
For example, if I have a matrix
>m<-matrix(c(1,4,2,3),2,2)
>m
[,1] [,2]
[1,] 1 2
[2,] 4 3
I want the result to list the numbers in descending order:
row col val
2 1 4
2 2 3
1 2 2
1 1 1
+3
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1 answer
Here's a possible solution data.table
library(data.table)
rows <- nrow(m) ; cols <- ncol(m)
res <- data.table(
row = rep(seq_len(rows), cols),
col = rep(seq_len(cols), each = rows),
val = c(m)
)
setorder(res, -val)
res
# row col val
# 1: 2 1 4
# 2: 2 2 3
# 3: 1 2 2
# 4: 1 1 1
Edit : basic alternative to R
res <- cbind(
row = rep(seq_len(rows), cols),
col = rep(seq_len(cols), each = rows),
val = c(m)
)
res[order(-res[, 3]),]
# row col val
# [1,] 2 1 4
# [2,] 2 2 3
# [3,] 1 2 2
# [4,] 1 1 1
+4
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