CODEWITHSUNDEEP
rsubset
user191010
user191010

Reputation:

How do I find peak values/row numbers?

I have a large dataset (202k points). I know that there are 8 values over 0.5. I want to subset on those rows.

How do I find/return a list the row numbers where the values are > 0.5?

Upvotes: 1

Views: 585

Answers (4)

mattdevlin
mattdevlin

Reputation: 1095

Using the argument arr.ind=TRUE with which is a great way for finding the row (or column) numbers where a condition is TRUE,

df <- matrix(c(0.6,0.2,0.1,0.25,0.11,0.13,0.23,0.18,0.21,0.29,0.23,0.51), nrow=4)

#      [,1] [,2] [,3]
# [1,] 0.60 0.11 0.21
# [2,] 0.20 0.13 0.29
# [3,] 0.10 0.23 0.23
# [4,] 0.25 0.18 0.51

which with arr.ind=TRUE returns the array indices where the condition is TRUE

which(df > 0.5, arr.ind=TRUE)
     row col
[1,]   1   1
[2,]   4   3

so the subset becomes

df[-which(df > 0.5, arr.ind=TRUE)[, "row"], ]

#      [,1] [,2] [,3]
# [1,]  0.2 0.13 0.29
# [2,]  0.1 0.23 0.23

Upvotes: 0

Jonathan Chang
Jonathan Chang

Reputation: 25367

which(x > 0.5)

Upvotes: 2

Jacob
Jacob

Reputation: 34621

Here's some dummy data:

D<-matrix(c(0.6,0.1,0.1,0.2,0.1,0.1,0.23,0.1,0.8,0.2,0.2,0.2),nrow=3)

Which looks like:

> D
     [,1] [,2] [,3] [,4]
[1,]  0.6  0.2 0.23  0.2
[2,]  0.1  0.1 0.10  0.2
[3,]  0.1  0.1 0.80  0.2

And here's the logical row index,

index <- (rowSums(D>0.5))>=1

You can use it to extract the rows you want:

PeakRows <- D[index,]

Which looks like this:

> PeakRows
     [,1] [,2] [,3] [,4]
[1,]  0.6  0.2 0.23  0.2
[2,]  0.1  0.1 0.80  0.2

Upvotes: 0

Rob Hyndman
Rob Hyndman

Reputation: 31820

If the dataset is a vector named x:

(1:length(x))[x > 0.5]

If the dataset is a data.frame or matrix named x and the variable of interest is in column j:

(1:nrow(x))[x[,j] > 0.5]

But if you just want to find the subset and don't really need the row numbers, use

subset(x, x > 0.5)

for a vector and 

subset(x, x[,j] > 0.5)

for a matrix or data.frame.

Upvotes: 5

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