Find distribution of consecutive zeros

Question

I have a vector, say x which contains only the integer numbers 0,1 and 2. For example;

x <- c(0,1,0,2,0,0,1,0,0,1,0,0,0,1,0)

From this I would like to extract how many times zero occurs in each "pattern". In this simple example it occurs three times on it own, twice as 00 and exactly once as 000, so I would like to output something like:

0      3
00     2
000    1

My actual dataset is quite large (1000-2000 elements in the vector) and at least in theory the maximum number of consecutive zeros is length(x)

Answer 1

1) rle Use rle and table like this. No packages are needed.

tab <- with(rle(x), table(lengths[values == 0]))

giving:

> tab
1 2 3 
3 2 1 

or

> as.data.frame(tab)
  Var1 Freq
1    1    3
2    2    2
3    3    1

That is, there are 3 runs of one zero, 2 runs of two zeros and 1 run of three zeros.

The output format in the question is not really feasible if there are very long runs but just for fun here it is:

data.frame(Sequence = strrep(0, names(tab)), Freq = as.numeric(tab))

giving:

  Sequence Freq
1        0    3
2       00    2
3      000    1

2) gregexpr Another possibility is to use a regular expression:

tab2 <- table(attr(gregexpr("0+", paste(x, collapse = ""))[[1]], "match.length"))

giving:

> tab2
1 2 3 
3 2 1 

Other output formats could be derived as in (1).

Note

I checked the speed with a length(x) of 2000 and (1) took about 1.6 ms on my laptop and (2) took about 9 ms.