R code to assign a sequence based off of multiple variables [duplicate]

I have data structured as below:

ID   Day   Desired Output
1    1      1
1    1      1
1    1      1
1    2      2
1    2      2
1    3      3
2    4      1
2    4      1
2    5      2
3    6      1
3    6      1

Is it possible to create a sequence for the desired output without using a loop? The dataset is quite large so a loop won't work, is it possible to do this with the dplyr package or maybe a combination of cumsum/diff?