How to get hourly diff from cumulative daily data R

I have data in two columns:

DateTime    Profit
20130319T01  5
20130319T02  135
20130319T03  245
20130320T01  10
20130320T02  115

and I want to create a column that has the diff for each hour BUT the problem is that Profit resets to zero each day. I want to get the following:

DateTime     Diff
20130319T01  5
20130319T02  130
20130319T03  110
20130320T01  10
20130320T02  105


Assuming the format of your DateTime character vector is always "YYYYMMDD" then you can use the ddply function from plyr to get what you want:

df$Date <- substr( df$DateTime , 1 , 8 )
ddply( df ,  .(Date)  , summarise , Diff = diff(c(0,Profit)) )
#     Date Diff
#1 20130319    5
#2 20130319  130
#3 20130319  110
#4 20130320   10
#5 20130320  105

Another way using base's ave:

within(df, { Profit_diff <- ave(Profit, list(gsub("T.*$", "", DateTime)), 
             FUN=function(x) c(x[1], diff(x)))})

#      DateTime Profit Profit_diff
# 1 20130319T01      5           5
# 2 20130319T02    135         130
# 3 20130319T03    245         110
# 4 20130320T01     10          10
# 5 20130320T02    115         105

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