R Errors Explained: Replacement has length zero

Error messages are an unfortunate part of programming. What makes them even more annoying is when the error message does not provide any useful information about what is going on. At first, the “replacement has length zero” error seems like a good example of this, but once you understand it, it makes sense. However, the reason this error message occurs when it does is not intuitive, based only on the message itself.

The circumstances of this error.

This error message occurs when you try to equate a numeric member of a data structure to “numeric(0)”. However, it does not occur with a simple numeric variable.

> x=rep(1, 10)
> x[1] = x[0]
Error in x[1] = x[0] : replacement has length zero

This example shows how this error message can occur. However, it is not a situation that is likely to show up actual programming. It is simple enough to make the error message easy to understand.

> x = rep(1, 10)
> for (i in 1:10)
+ {
+ x[i] = x[i]+x[i-1]
+ }
Error in x[i] = x[i] + x[i – 1] : replacement has length zero

This is a more realistic example where the problem occurs inside a for-loop where the data structure is being worked on incrementally. In this case, it is easy to accidentally include the zeroth component.

What is causing this error?

The cause of this error comes from the fact that the zeroth component of a data structure has a value of “numeric(0)” and a zero-length. The other components of the data structure cannot have this value.

> x=rep(1, 10)
> a = 1
> a = x[0]
> x[0] = 1
> a
numeric(0)
> x[0]
numeric(0)
> x[1] = a
Error in x[1] = a : replacement has length zero

This sample code shows it all. You can equate the simple variable “a” to “numeric(0)” and at the same time, “x[0]” cannot be set to an actual value. However, if you try to set “x[1]” to “numeric(0)” you will get the error message. The error message results from trying to set a variable that needs a length to a value that has none.

How to fix this error.

Fixing this error is quite simple, all you need to do is avoid that zeroth component and there will be no problem. The way to do this is to make sure that you never use zero when accessing the components of a data structure.

> x=rep(1, 10)
> x[2] = x[1]

Here is a simple example of how to avoid this problem. This of courses to the simplest case, but it serves as an excellent illustration, anyhow.

> x = rep(1, 10)
> for (i in 2:10)
+ {
+ x[i] = x[i]+x[i-1]
+ }

Here, we are fixing the earlier for-loop by starting with two. This way, the index used in the data structure “x” is never zero. The point is, do not use zero as an index when accessing data structures.