Clarifying the plot command in R

Question

I'm following this tutorial to implement a support vector machine in R.

I'm having trouble clarifying the working of the plot command

plot(x,col=ifelse(y>0,1,2))

x is of size 300 as length(x) confirms, but length(ifelse(y>0,1,2) shows 150.

How is the plot working then, if there are 300 x values but only 150 y values?

When I tried

plot(x,ifelse(y>0,1,2)) it shows the expected error ie,

Error in xy.coords(x, y, xlabel, ylabel, log) : 
  'x' and 'y' lengths differ

What is it about col=ifelse(y>0,1,2) that makes it work?

Answer 1

Generally, R knows recycling, ie in many situations R recycles vectors if they have not the required length, eg

1:4 + 1:2

The vector 1:2 is recycled, so effectively it becomes c(1,2,1,2) to fit the length of the other vector. Now, we can sum the vectors

c(1,2,3,4)  + c(1,2,1,2)

The same happens in your example for the col argument. The vector col is recycled so it fits the length of x . But as you see, not all argument values are always recycled automatically. In the case of the x and y argument in plot this is not the case. This makes sense as it is probably an error, if the length of these two vectors do not match.