Question

Sometimes I want to perform a function (eg difference calculation) on a dataset and store the results directly in the data frame

df <- data.frame(a$C, diff(a$C))

But I cannot do that because the number of rows is different.

Is there some syntax that will allow me to to that, perhaps having NA when the function (diff()) gives no results?

Answer 1

There isn’t a general solution to this without making vast assumptions about the whole panoply of function one may wish to use.

For the example you show, we can easily work out that the first value from diff() would be an NA if it returned it:

set.seed(5)
d <- rpois(10, 5)

> d
 [1] 3 6 8 4 2 6 5 7 9 2
> diff(d)
[1]  3  2 -4 -2  4 -1  2  2 -7

So if you are using diff() then you can always just do:

> dd <- data.frame(d, Diff = c(NA, diff(d)))
> dd
   d Diff
1  3   NA
2  6    3
3  8    2
4  4   -4
5  2   -2
6  6    4
7  5   -1
8  7    2
9  9    2
10 2   -7

But now consider what you would do with any other function that you might wish to use that doesn’t always return NA in the correct place.

For this example, we can use the zoo package which has an na.pad argument:

require(zoo)
d2 <- as.zoo(d)
ddd <- data.frame(d, Diff = diff(d2, na.pad = TRUE))

> ddd
   d Diff
1  3   NA
2  6    3
3  8    2
4  4   -4
5  2   -2
6  6    4
7  5   -1
8  7    2
9  9    2
10 2   -7

If you are using a modelling function with a formula interface (e.g. lm()) and that function has an na.action argument, then you can set na.action = na.exclude in the function call and extractor functions such as fitted(), resid() etc will add back in to their output NA in the correct places so that the output is of the same length as the data passed to the modelling function.

If you have other more specific cases you want to explore, please edit your Answer. In specific cases there will usually be a simple Answer to your Q. In the general case the Answer is no, it is not possible to do what you ask.