Create a column that increments for each unique ID in R

I have a dataset that looks something like this:

ID     Minutes Read    Comprehension     
1      25              1      
1      30              1      
2      20              2      
2      25              2     
2      30              1        

I want to create a column called "day" that counts the days each person reported reading, such as below:

ID     Minutes Read    Comprehension     Day  
1      25              1                 1
1      30              1                 2
2      20              2                 1
2      25              2                 2
2      30              1                 3

How would I go about doing that? The end goal is to use the "day" column to reshape my data,

df2 <- reshape(df, idvar="ID", timevar = "day", direction="wide").

>Solution :

Since your aim is to reshape the data, try:

reshape(transform(df, time = ave(ID, ID, FUN = seq)), dir = 'wide', idvar = 'ID')

  ID Minutes.Read.1 Comprehension.1 Minutes.Read.2 Comprehension.2 Minutes.Read.3 Comprehension.3
1  1             25               1             30               1             NA              NA
3  2             20               2             25               2             30               1

If you are only interested in the day column, then

df <- transform(df, day = ave(ID, ID, FUN = seq))