[R] change values in data frames

[poul]

Hello every body,
I am quite a new user of R so I beg your pardon for this naive question and
the lake of syntax with wich I ask it:

I have a data frames like this:

cola	colb
1	c
1	i
1	i
1	c
2	i
2	c
2	c
2	i
2	i
...
10000

I would like ,for each level of cola and for x in colb:

-if colb[x]=="i" and colb[x-1] does not exist (first row in dataframes),
then replace colb[x] by "l"
-if colb[x]=="i" and colb[x-1]=="c" (previous row in data frmes), then
replace colb[x] by "l"
-if colb[x]=="i" and colb[x-1]=="i" (previous row in data frmes), then
replace colb[x] by "h"

Thanks in advance

Paul

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