Why put + and %>% at the end of lines?

[cricketbird]

The general practice is to put + at the end of the line when creating ggplots, and %>% at the end of the line when using the pipe.

ggplot(data = mpg, aes(x = displ, y = hwy)) +
    geom_point() +
    stat_smooth()

delays <- flights %>% 
    group_by(dest) %>% 
    summarise(distance = mean(distance))

However, this makes commenting out particular lines harder. If I comment out the last line in either example, I'll get an error because the + or %>% is just "hanging out there".

I'd rather do it the way below. This lets me comment out lines as I wish, and to me seems aesthetically more pleasing (like a bulleted list).

ggplot(data = mpg, aes(x = displ, y = hwy))
    + geom_point()
    + stat_smooth()

delays <- flights
     %>% group_by(dest) 
     %>% summarise(distance = mean(distance))

Yet these examples throw errors.
Why won't R let me do it either way?
And, if it must be one way, what are the reasons for the trailing + instead of the leading +?

[nick]

Because R doesn't require an explicit statement terminator (like the semi-colon in C/C++/many other languages), the R interpreter/compiler will determine that a statement is complete whenever there is a line that completes the statement. So, the following is a valid R statement:

delays <- flights

During execution, after reaching the end of that line, R will stop and execute that complete statement. It then reaches the next line

    %>% group_by(dest)

That is not a valid statement (and can't be turned into such by additional code), since %>% is a binary operator (along with +) and requires code on both sides of it.

Changing that so that allowing leading binary operators to continue statements would require changing R to look ahead to subsequent lines to see if the statement is being continued. While that would allow your preferred style, I'm sure it would have some problems. Even knowing little-to-nothing about the interpreter inner workings, the following would be highly ambiguous:

x <- 1
-1

Both lines are valid statements, but the negation operator - could also be subtraction if you looked ahead.

Edit: If it's a problem on a regular basis while you develop code, you could end your %>% chains with {.}:

delays <- flights %>%
    group_by(dest) %>%
    #summarise(distance = mean(distance)) %>%
    {.}

That last statement is basically an identity operator for pipes and will not change the output. I'm not sure what the ggplot2 equivalent would be offhand.

[tjmahr]

@nick

That last statement is basically an identity operator for pipes and will not change the output. I’m not sure what the ggplot2 equivalent would be offhand.

I'm planning to write a thing about identity elements for ggplot2, actually. geom_blank() or NULL do the trick.

[alistaire]

@nick

Edit: If it’s a problem on a regular basis while you develop code, you could end your %>% chains with {.}:

delays <- flights %>%
    group_by(dest) %>%
    #summarise(distance = mean(distance)) %>%
    {.}

That last statement is basically an identity operator for pipes and will not change the output.

You can also pipe into I or identity—which similarly terminate the pipeline without changing the output—to debug:

library(dplyr)

nycflights13::flights %>%
    group_by(dest) %>%
    #summarise(distance = mean(distance)) %>%
    I
#> # A tibble: 336,776 x 19
#> # Groups:   dest [105]
#>     year month   day dep_time sched_dep_time dep_delay arr_time
#>  * <int> <int> <int>    <int>          <int>     <dbl>    <int>
#>  1  2013     1     1      517            515         2      830
#>  2  2013     1     1      533            529         4      850
#>  3  2013     1     1      542            540         2      923
#>  4  2013     1     1      544            545        -1     1004
#>  5  2013     1     1      554            600        -6      812
#>  6  2013     1     1      554            558        -4      740
#>  7  2013     1     1      555            600        -5      913
#>  8  2013     1     1      557            600        -3      709
#>  9  2013     1     1      557            600        -3      838
#> 10  2013     1     1      558            600        -2      753
#> # ... with 336,766 more rows, and 12 more variables: sched_arr_time <int>,
#> #   arr_delay <dbl>, carrier <chr>, flight <int>, tailnum <chr>,
#> #   origin <chr>, dest <chr>, air_time <dbl>, distance <dbl>, hour <dbl>,
#> #   minute <dbl>, time_hour <dttm>

[nick]

Technically, I adds the class AsIs to the object. That probably won't be a problem in any well-written code, but identity seems marginally safer (though more verbose).

[cricketbird]

Thanks! The x <- -1 example really makes it clear what the issue is. The workarounds with {.} and geom_blank() are really helpful - I will definitely incorporate those.

[Roger]

Yes the {.} is a super handy to let you comment out any of your lines, including the last ‘proper’ one, without having to delete the pipe symbols off the end of a line.

[mara]

@tjmahr

I’m planning to write a thing about identity elements for ggplot2, actually.

@tjmahr has written this post now, and I read it and it's pretty darn great! @cricketbird, even though your question's been answered (@nick's such a champ on here, ), you might find it interesting nonetheless!

Simplifying ggplot2 code by doing nothing

[Arne]

Thanks for the blogpost (and thanks to @mara to give us a heads-up).

FTR, I've been using and empty theme() call at the end of a ggplot2 statement (and had been using opts() before for the purpose of being able to outcomment the last (productive) step in a statement without breaking my code. Worked for me, haven't had problems so far. If someone has any objections to that, please pray tell.

I only recently found out that identity() at the end of a pipeline does the same thing for me, and was delighted!