r - Create counter within consecutive runs of certain values

Question

I have an hourly value. I want to count how many consecutive hours the value has been zero since the last time it was not zero. This is an easy job for a spreadsheet or for loop, but I am hoping for a snappy vectorized one-liner to accomplish the task.

x <- c(1, 0, 1, 0, 0, 0, 1, 1, 0, 0)
df <- data.frame(x, zcount = NA)

df$zcount[1] <- ifelse(df$x[1] == 0, 1, 0)
for(i in 2:nrow(df)) 
  df$zcount[i] <- ifelse(df$x[i] == 0, df$zcount[i - 1] + 1, 0)

Desired output:

R> df
   x zcount
1  1      0
2  0      1
3  1      0
4  0      1
5  0      2
6  0      3
7  1      0
8  1      0
9  0      1
10 0      2

Answer 1

William Dunlap's posts on R-help are the place to look for all things related to run lengths. His f7 from this post is

f7 <- function(x){ tmp<-cumsum(x);tmp-cummax((!x)*tmp)}

and in the current situation f7(!x). In terms of performance there is

> x <- sample(0:1, 1000000, TRUE)
> system.time(res7 <- f7(!x))
   user  system elapsed 
  0.076   0.000   0.077 
> system.time(res0 <- cumul_zeros(x))
   user  system elapsed 
  0.345   0.003   0.349 
> identical(res7, res0)
[1] TRUE