How can I use the "dplyr" chain operator %>% get the left side itself in R? [duplicate]

Question

package "dplyr" has a chain operator. But I have a problem with how to get the right side term itself.

For example:

c(5,7,8,1) %>% sum(`[`(1:3)) # get result 27 (This is wrong)

c(5,7,8,1) %>% sum(.[1:3]) # get result 41 (This is also wrong)

c(5,7,8,1) %>% `[`(1:3) %>% sum()   # get result 20 (This is right)

Why the first line and second line codes are wrong? What has happened in them?

Answer 1

The dot . is correct. However, %>% also inserts it as the first argument:

x = c(5,7,8,1)
x %>% sum(.[1 : 3])

Is the same as:

sum(x, x[1 : 3])

You can explicitly prevent this behaviour by wrapping the expression in braces:

x %>% {sum(.[1 : 3])}

However, at this point it may be better to split the pipeline up a bit more (as you’ve done yourself):

x %>% `[`(1 : 3) %>% sum()

Or, using magrittr helper functions (requires library(magrittr)):

x %>% extract(1 : 3) %>% sum()