CODEWITHSUNDEEP
r
umair durrani
umair durrani

Reputation: 6155

How to determine the first maximum value in a variable in R?

Following is a data frame x:

> x <- data.frame(a = c(0.1,0.1,0.1,1,1.1,1.2,2.1,3.1,3.3,3.2,3.1,2.1,2.0,0.1,0.1,1.1,2.1,3.1,4))

The first maximum value of a is 3.3 before the values start decreasing. How can I determine this value using code without looking at the plot?

Upvotes: 1

Views: 157

Answers (3)

mra68
mra68

Reputation: 2960

"which.max" is a little bit faster:

> system.time(
+   for ( i in 1:1000000 ) { a <- x$a[which.max(diff(x$a)<0)] }
+ )
       User      System verstrichen 
      62.42        0.01       62.65 

> system.time(
+   for ( i in 1:1000000 ) { b <- x[which(diff(x$a) < 0L)[1L],] }
+ )
       User      System verstrichen 
     111.43        0.00      112.14 
>

The result is the same:

> a
[1] 3.3
> b
[1] 3.3

Upvotes: 2

David Arenburg
David Arenburg

Reputation: 92282

With base R I would go with 

x[which(diff(x$a) < 0L)[1L],]
## [1] 3.3

Upvotes: 4

Shenglin Chen
Shenglin Chen

Reputation: 4554

library(dplyr)
filter(x,lag(a)<a & a>lead(a))

Upvotes: 0

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