Finding ranges in runs of numbers

Question

I'm trying to find runs of years in a data frame (ideally using plyr) I'd like to get from this: ``` require(plyr) dat<-data.frame( name=c(rep("A", 11), rep("B", 11)), year=c(2000:2010, 2000:2005, 2007:2011) ) ``` To this: ``` out<-data.frame( name=c("A", "B", "B"), range=c("2000-2010", "2000-2005", "2007-2011")) ``` It's easy enough to identify whether each group has a continuous run of years: ``` ddply(dat, .(name), summarise, continuous=(max(year)-min(year))+1==length(year)) ``` How do I go about breaking down group "B" into two ranges? Any ideas or strategies would be really appreciated. Thanks

Accepted Answer

Whether you use a function from "plyr" or from base R, you need to first establish some groups. One way to detect the change in groups since your years are sequential is to look for where `diff` is not equal to 1. `diff` creates a vector of length one less than the input vector, so we'll initialize that with "1" and take the `cumsum` of the result.

Putting that mouthful of an explanation into practice, you can try something like this:

```
dat$id2 <- cumsum(c(1, diff(dat$year) != 1))
```

From here, you can use `aggregate` or your favorite grouping function to get the output you're looking for.

```
aggregate(year ~ name + id2, dat, function(x) paste(min(x), max(x), sep = "-"))
#   name id2      year
# 1    A   1 2000-2010
# 2    B   2 2000-2005
# 3    B   3 2007-2011
```

To use `range` with `aggregate`, you need to change `sep` to `collapse`, as below:

```
aggregate(year ~ name + id2, dat, function(x) paste(range(x), collapse = "-"))
```

Finding ranges in runs of numbers

Solution

Problem