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ddply and categorization function

Submitted by user1069539 on February 4, 2012

i would like to categorize a column in a dataframe based on a comparison of subsequent rows.

for:

DF <- data.frame(respondent=rep(letters[1:2], each=5), response=c(1,1,2,2,1,3,1,1,1,1))
   respondent response
1           a        1
2           a        1
3           a        2
4           a        2
5           a        1
6           b        3
7           b        1
8           b        1
9           b        1
10          b        1

I would like to add a new column (eg: check) that turns to 1 if response in row1 = response in row2 and 0 in case they are not the same. This should be done separately for each respondent.

This would give me

   respondent response check
1           a        1  1
2           a        1  0
3           a        2  1
4           a        2  0
5           a        1  
6           b        3  0
7           b        1  1
8           b        1  1
9           b        1  1
10          b        1

I think i can figure this out with a foor loop but it seems to be a suitable problem for ddply ... I just do not see how to address comparisons over adjacent rows ...