R - conditional incrementation

问题

This should be trivial to code but could not think of an elegant one-liner in R. I have a dataframe as below:

data <- data.frame( index= seq(1:20), event=rep(0,20)   )
data$event[10] <- 1
data$event[15] <- 1

I simply want to add start and stop counter columns that increment in 10's and reset right after an event=1 is observed. So the desired output with these two additional columns would be:

  index event start stop
1   1    0     0    10
2   2    0    10    20
3   3    0    20    30
4   4    0    30    40
5   5    0    40    50
6   6    0    50    60
7   7    0    60    70
8   8    0    70    80
9   9    0    80    90
10  10   1    90    100
11  11   0    0     10
12  12   0    10    20
13  13   0    20    30
14  14   0    30    40
15  15   1    40    50
16  16   0    0     10
17  17   0    10    20
18  18   0    20    30
19  19   0    30    40
20  20   0    40    50

Obviously, data$stop <- data$start + 10 but how can I apply() the start incrementation loigc as described?

回答1:

How about this:

Reduce(function(x,y) (1-y)*(x+10), data$event[-nrow(data)], accumulate=T, init=0)

回答2:

You could get your values with

data$start <- 10*(ave(
    rep(0,nrow(data)), 
    cumsum(c(0, head(data$event,-1))), 
    FUN=seq_along)-1
)
data$end <- data$start + 10

Here we use cumsum to track when events occur (but we need to shift them a step so the reset occurs after the event rather than at the event). And we use ave within the groups to generate sequences per group.

回答3:

So unfortunately, the apply family of functions only replaces a for loop when the iterations of that loop do not depend on previous iterations.

You could write a for loop like:

data <- data.frame( index= seq(1:20), event=rep(0,20)   )
data$event[10] <- 1
data$event[15] <- 1
print(data)
data$start = rep(0, 20)
for(i in 2:20){
  if(data$event[i] == 1){
    data$start[i] = 0
  } else  data$start[i] = data$start[i-1] + 10
}
data$stop = data$start+10
print(data)

来源：https://stackoverflow.com/questions/27156082/r-conditional-incrementation

标签

dataframe

apply