"+" Continue Option in R

Question

I am a beginner and have recently started using R instead of Stata because of its open-source nature.

I am a little confused about "+" continue symbol.

Here's how my data looks like:

head(mtcars)
                   mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

Let's consider four scenarios:

Scenario 1: I have noticed that RStudio automatically appends "+" if I write the following block of code:

attach(mtcars)  
  cyl.f <- factor(cyl, levels= c(4,6,8),
                labels = c("4 cylinder", "6 cylinder",
                           "8 cylinder")) 

gets changed to :

cyl.f <- factor(cyl, levels= c(4,6,8),
+                 labels = c("4 cylinder", "6 cylinder",
+                            "8 cylinder"))

As you can see above, RStudio automatically appended "+"

Scenario 2: If I add "+" at the end of the code, it doesn't work for some strange reasons:

cyl.f <- factor(cyl, levels= c(4,6,8),    +
                 labels = c("4 cylinder", "6 cylinder",       +
                            "8 cylinder"))

This throws an error.

Scenario 3: If I add "+" at the beginning of the code, it doesn't work for some strange reasons:

cyl.f <- factor(cyl, levels= c(4,6,8),    
          +       labels = c("4 cylinder", "6 cylinder",       
           +                 "8 cylinder"))

This throws an error. This is understandable because RStudio automatically adds another "+" continue operator.

Scenario 4:

windows()
ggplot(data=mtcars, aes(x=wt, y=mpg))   +
  geom_point()  +
  labs(title="Automobile Data", x="Weight", y="Miles Per Gallon")

If I don't add "+", this doesn't work. Apparently, it seems "+" is a "piping" operator above.

Now, the challenge is that I can't google this or search on SO because both the search engines think that "+" is an OR operator.

Can someone please explain when "+" is an operator for "continue" and when it is an operator for "piping."

I'd appreciate any thoughts and detailed explanation. This is extremely confusing for me.

Answer 1

You are confusing code with the prompts the R console shows. The + appearing automatically in the console after the linebreaks is a console prompt indicating that further input is expected. It is not part of the code.

To illustrate this, I check the option for the continue prompt and change it for an example:

options("continue")
options(continue = "% ")
data.frame(a = 1,
           b = 2)

This is displayed in the console like this:

> options("continue")
$continue
[1] "+ "

> options(continue = "% ")
> data.frame(a = 1,
%            b = 2)
  a b
1 1 2

As you see, the continue prompt has been changed from + to %. Note that the prompts are always at the same position in the console. They are always the first character of any input line and by default they are always followed by a space character.

When part of the code the + operator is actually a function, which usually does arithmetic addition, but methods can be defined for different behavior. That's what ggplot2 does.

PS: If you are using RStudio I hope that you have created a new file and are writing a script from which you send the code to the console (e.g., by pressing Ctrl+Enter). Some beginners don't know how to use RStudio and simply write in the console.

Answer 2

Usually in R, the + just means addition. Ggplot2 is a special case, though, as it has overridden the default behaviour, so that + means that you can add objects together, to build up layers of a plot. Behind the scenes, ggplot is taking each individual function call, putting them into a list, and executing them together.

http://docs.ggplot2.org/current/gg-add.html

So, now to explain your code issues.

The + operator generally means addition, as in 1 + 1 = 2. Secondly, single function calls can wrap over lines without needing any continuation operator. So with those to points in mind, it makes no sense whatsoever to put a plus sign in the middle of a function call.

R would take this code:

cyl.f <- factor(cyl, levels= c(4,6,8),    +
             labels = c("4 cylinder", "6 cylinder",       +
                        "8 cylinder"))

and turn it into this:

cyl.f <- factor(cyl, levels= c(4,6,8), + labels = c("4 cylinder", "6 cylinder", + "8 cylinder"))

Secondly, your guess about it about a continuation code is incorrect. It's more of a continuation indicator, and is just a way for R to show that the code is all part of the same function call.