Clarifying the plot command in R

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?

Answers


Generally, R knows recycling, i.e. in many situations R recycles vectors if they have not the required length, e.g.

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.


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