When should we add aes() ? The first code is giving error while the other isn't

Question

Code giving error: ggplot(mtcars, aes(wt, mpg)) + geom_point(aes(shape = 1, size = 4))

Code Not giving error: ggplot(mtcars, aes(wt, mpg)) + geom_point(shape = 1, size = 4)

Answer 1

It's not a question of timing. It's a question of default behaviour.

The actual error message is

Error: A continuous variable can not be mapped to shape

So in the first example, because shape is inside the call to aes(), it is defining a scale. 1 is regarded not as a constant but as a continuous variable.

In the second example, because shape is outside the call to aes(), it does not define a scale and therefore the error does not occur.

A variation of the first example which does not produce an error is

gplot(mtcars, aes(wt, mpg)) + geom_point(aes(shape = as.factor(1), size = 4))

because the call to as.factor() means that 1 is no longer regarded as continuous.