zoo column name for single column object

Question

I have a question on column names in zoo. I usually create zoo objects from a data frame, and I pick up the column(s) from the data frame to be the zoo column(s). What I found is that, if I only specify one column to the zoo object, then the column name will not be taken by zoo. Does that mean it is not considered a "column" in zoo? Here is an example how I usually do it, with one and two columns. ``` Lines.1 = "Index,dbt 2008-08-20 15:03:18,88.74 2008-08-20 15:08:18,88.74 2008-08-20 15:13:18,86.56 2008-08-20 15:18:18,85.82" Lines.2 = "Index,dbt,rh 2008-08-20 15:03:18,88.74,18.25 2008-08-20 15:08:18,88.74,17.25 2008-08-20 15:13:18,86.56,18.75 2008-08-20 15:18:18,85.82,19.75" x =read.table(text = Lines.1, header = TRUE, sep = ",") y =read.table(text = Lines.2, header = TRUE, sep = ",") colnames(x) colnames(y) library(zoo) zx = zoo(x[,2], as.POSIXct(x$Index, tz="GMT")) zy = zoo(y[,2:3], as.POSIXct(y$Index, tz="GMT")) colnames(zx) colnames(zy) ``` The result shows as follows: ``` > colna…

Accepted Answer

This is the default behaviour of `[` when used with arrays or data frames; empty dimensions are dropped. Consider

```
> x[, 2]
[1] 88.74 88.74 86.56 85.82
> class(x[,2])
[1] "numeric"
> is.data.frame(x[,2])
[1] FALSE
```

In this case the 1-column data frame doesn't need information about which column it is and hence R drops that information and returns the contents of the column as a numeric (in this case) vector, as can be seen above. That vector doesn't have a `colname` attribute and hence zoo has nothing to work with.

A solution is to use `drop = FALSE` in the index `x[, 2, drop = FALSE]` as in

```
> zx <- zoo(x[, 2, drop = FALSE], as.POSIXct(x$Index, tz="GMT"))
> zx
                      dbt
2008-08-20 15:03:18 88.74
2008-08-20 15:08:18 88.74
2008-08-20 15:13:18 86.56
2008-08-20 15:18:18 85.82
```

To see why/how this works, look at

```
> x[, 2, drop = FALSE]
    dbt
1 88.74
2 88.74
3 86.56
4 85.82
> is.data.frame(x[, 2, drop = FALSE])
[1] TRUE
```

And note the lack of `colnames` when the default (`TRUE`) is used in the `[` index:

```
> colnames(x[, 2, drop = FALSE])
[1] "dbt"
> colnames(x[, 2, drop = TRUE])
NULL
```

Now read `?'['` for more details.

zoo column name for single column object

Solution

Problem