Read null values from file

Question

I need to read a data frame from a file containing NULL values. Here's an example file: ``` charCol floatCol intCol a 1.5 10 b NULL 3 c 3.9 NULL d -3.4 4 ``` I read this file into a data frame: ``` > df <- read.table('example.dat', header=TRUE) ``` But "NULL" entries are not interpreted by R as NULL: ``` > is.null(df$floatCol[2]) [1] FALSE ``` How should I format my input file so that R properly treats such entries as NULL?

Accepted Answer

Always always always do summary(thing) if something is unexpected.

```
> summary(df)
 charCol floatCol  intCol 
 a:1     1.5 :1   10  :1  
 b:1     -3.4:1   3   :1  
 c:1     3.9 :1   4   :1  
 d:1     NULL:1   NULL:1  
```

that looks a bit weird. Drill down:

```
> summary(df$floatCol)
 1.5 -3.4  3.9 NULL 
   1    1    1    1 
```

what the heck is it?

```
> class(df$floatCol)
[1] "factor"
```

The presence of an invalid numeric format (the string 'NULL') has caused R to go "oh I guess these aren't numbers, I'll read them into character strings and make a factor (categorical variable) for you".

The solution has just been posted to use na.string="NULL", but remember that NA isn't the same as NULL in R. NA is a marker for missing data, NULL is a genuine non-value. Compare:

```
> c(1,2,3,NULL,4)
[1] 1 2 3 4
> c(1,2,3,NA,4)
[1]  1  2  3 NA  4
```

Once you've read it in correctly, the appropriate test is usually is.na(foo)

Read null values from file

Solution

Problem