replacing a for loop with map?

Question

I'd like to replace the following for-loop ...

data <- data.frame(x = c(2, 3, 3, 4, 5, 6, 5, 5, 6, 8, 9, 7, 6, 9, 10),
                   y = rep(0, 15))
for (i in 2:length(data$x)) {
  data$y[i] <- ifelse(data$x[i] > data$y[i-1], data$y[i-1] + 2, data$y[i-1] - 1)
}
data$y # 0  2  4  3  5  7  6  5  7  9  8  7  6  8 10

... and I want to use dplyr and tried the following:

data %>%
  mutate(y = if_else(x > lag(y), lag(y) + 2, lag(y) -1 ))

But obviously I didn't succeed. Any help would be much appreciated.

I might be wrong, but I think that ifelse or if_else or case_when don't help here. The for loop is going step-by-step through the data and calculates each y on the basis of the previous y. My guess was to use map. — Rumpl
– Rumpl, Commented Feb 11, 2020 at 10:34
What you want to look into is Reduce with the argument accumulate set to TRUE — Sotos
– Sotos, Commented Feb 11, 2020 at 10:36

tspano · Accepted Answer · 2020-02-11 11:04:31Z

4

You can use accumulate from purrr package for sequential operations:

data %>% 
  mutate( y = accumulate( .x = x[-1],
                          .init = 0,
                          .f = function(y,x) if_else(x > y, y + 2, y - 1) ) 
  )

answered Feb 11, 2020 at 11:04

tspano

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GKi · Accepted Answer · 2020-02-12 07:44:27Z

2

In case you are also interested in a base solution using Reduce as already suggested by @Sotos in the comments:

data$y <- Reduce(function(y,x) ifelse(x > y, y+2, y-1)
                 , data$x[-1], data$y[1], accumulate = TRUE)
data$y
# [1]  0  2  4  3  5  7  6  5  7  9  8  7  6  8 10

or

data$y <- Reduce(function(y,x) if(x > y) y+2 else y-1
                 , data$x[-1], data$y[1], accumulate = TRUE)

edited Feb 12, 2020 at 7:44

answered Feb 11, 2020 at 13:20

GKi

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1 Comment

Rumpl Over a year ago

Thanks. Nice to have both!

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