R help - Feb 2010 - First Appearance of value in splitted matrix

Hello,

I have a large matrix (foo), I have to split by weeks and in which measures
start at different points of time.

Now I am looking for the number of the week, in which a specific value
appears. I do not need the date or any other information from a row, but
(due to structure of result table) the indicator i.

One example:
foo has two columns: Date, ID


foo.all<- split(foo, cut(foo$Date, "weeks")
l_foo<-length(foo.all)
for (i in 1:l_foo){
    foo.i<-foo.all[[i]]
}

Question:
Given I search for ID=4. How do I find the i, in which 4 appears?


Thanks in advance!
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