dplyr mutate not applying to individual element of field

I'm trying to do a calculation using an home made function inside a mutate call and it's not doing what I'm expecting it to do.

mutate

Here is a reproducible example:

library(dplyr)
#the data
dd <- structure(list(fire_zone = c("ET", "EJB", "WJB"), base_med = c(1, 1, 2)), class = "data.frame", row.names = c(NA, -3L))

# my home made function
med2lambda <- function(med) polyroot(c(-0.02, 1/3 - med, 1)) %>% suppressWarnings(as.numeric(.)) %>% max


So, what my function does is to estimate the lambda associated to the median from a Poisson distribution by calculating the root of a quadratic function. Despite the long explanation, it's actually pretty basic:

med2lambda(1)
[1] 0.695426
med2lambda(2)
[1] 1.678581


Now, I want to use it in a mutate call to add a field giving the lambda associated to each median in the table:

mutate

dd %>% mutate(lambda = med2lambda(base_med), log = log(base_med))
fire_zone base_med lambda log
1 ET 1 2.128966 0.0000000
2 EJB 1 2.128966 0.0000000
3 WJB 2 2.128966 0.6931472


The result is wrong, mutate actually gives me the results of:

med2lambda(dd$base_med)
[1] 2.128966


I've added the log call in the mutate to give an idea of what it should do. log works great in the mutate as it is called element by element.

log

mutate

log

mutate

Any insight about this behavior would be appreciated.

group_by(base_med) %>%

mutate

log

log

med2lambda

2 Answers
2

You need to allow med2lambda to take vectors instead of scalars.

med2lambda

One possibility is to use Vectorize:

Vectorize

med2lambda.vectorised <- Vectorize(med2lambda)
dd %>% mutate(lambda = med2lambda.vectorised(base_med), log = log(base_med))
# fire_zone base_med lambda log
#1 ET 1 0.695426 0.0000000
#2 EJB 1 0.695426 0.0000000
#3 WJB 2 1.678581 0.6931472


Alternatively, you could rewrite med2lambda to take a vector as argument:

med2lambda

med2lambda2 <- function(med) sapply(med, function(x)
polyroot(c(-0.02, 1/3 - x, 1)) %>% suppressWarnings(as.numeric(.)) %>% max)
dd %>% mutate(lambda = med2lambda2(base_med), log = log(base_med))
# fire_zone base_med lambda log
#1 ET 1 0.695426 0.0000000
#2 EJB 1 0.695426 0.0000000
#3 WJB 2 1.678581 0.6931472


Vectorize

med2lambda

sapply

med2lamba2

We can use map

map

library(tidyverse)
dd %>%
mutate(lambda = map_dbl(base_med, med2lambda), log = log(base_med))


Thanks for contributing an answer to Stack Overflow!

But avoid …

To learn more, see our tips on writing great answers.

Required, but never shown

Required, but never shown

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.