Why does ggplot not plot when storing to variable?

Question

I thought this was obvious, but recently I was using the package openair and noticed that when I run the following:

library(openair)
myplot <- windRose(mydata)

the plot myplot is still plotted in the viewer. After looking at the windRose function it is obvious the plot function is being called.

However, why does myggplot <- ggplot(mtcars, aes(cyl, mpg)) + geom_point() not have the same outcome of plotting to the viewer. I am guessing the difference is in how these functions are programmed but I cannot easily identify how ggplot handles the plotting part.

Answer 1

这不是 ggplot 特定的行为，而是一个更通用的原则：R REPL 一般不会打印赋值语句，而对于表达式，它根据对象类型调用print()或对结果值进行处理（请参阅R 的自动打印部分）内部细节）。例如，¹show()

> 1 + 1       # expression
[1] 2
> x <- 1 + 1  # assignment
>

对于 ggplot 对象，调用print该对象会触发渲染。因此，如果您不分配，它就会被渲染。例如，

> ggplot(mtcars, aes(hp, mpg)) + geom_point()

> g <- ggplot(mtcars, aes(hp, mpg)) + geom_point()
>

plot另一方面，该函数包含渲染作为其一部分，这就是为什么您调用的另一个函数尽管进行了赋值仍会被渲染。

请注意，可以使用该invisible函数临时设置R_Visible为FALSE，这会关闭打印表达式的默认行为，但仍会将计算结果推送到.Last.value。

> invisible(1 + 1)
> .Last.value
[1] 2

但是，由于plot()调用图形设备的渲染作为其代码的一部分，invisible()因此不会阻止其渲染。

> invisible(plot(mtcars$hp, mtcars$mpg))

^{[1]图片来源@Gregor}