r - Increment by 1 for every change in column

Question

Lets say I have the following data frame

set.seed(123)
df <- data.frame(var1=(runif(10)>0.5)*1)

var1 could have any type / number of levels not specifically 0 and 1s

I would like to create a var2 which increments by 1 every time var1 changes without using a for loop

Expected result in this case is:

data.frame(var1=(runif(10)>0.5)*1, var2=c(1, 2, 3, 4, 4, 5, 6, 6, 6, 7))

var1 var2
   0    1
   1    2
   0    3
   1    4
   1    4
   0    5
   1    6
   1    6
   1    6
   0    7

Another option for the data frame could be:

df <- data.frame(var1=c("a", "a", "1", "0", "b", "b", "b", "c", "1", "1"))

in this case the result should be:

var1 var2
   a    1
   a    1
   1    2
   0    3
   b    4
   b    4
   b    4
   c    5
   1    6
   1    6

Answer 1

Building on Mr Flick answer:

df$var2 <- cumsum(c(0,as.numeric(diff(df$var1))!=0))

But if you don't want to use diff you can still use:

df$var2 <- c(0,cumsum(as.numeric(with(df,var1[1:(length(var1)-1)] != var1[2:length(var1)]))))

It starts at 0, not at 1 but I'm sure you see how to change it if you want to.