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    <p>EDIT: Fixed. I originally assumed that I would have to modify most of rle(), but it turns out only a few tweaks were needed.</p> <p>This isn't an answer about an *apply method, but I wonder if this might not be a faster approach to the process overall. As Shane says, loops aren't so bad. And... I rarely get to show my code to anyone, so I'd be happy to hear some critique of this.</p> <pre><code>#Shane, I told you this was awesome dat &lt;- getSOTable("http://stackoverflow.com/questions/1504832/help-me-replace-a-for-loop-with-an-apply-function", 1) colnames(dat) &lt;- c("day", "user_id") #Convert to dates so that arithmetic works properly on them dat$day &lt;- as.Date(dat$day) #Custom rle for dates rle.date &lt;- function (x) { #Accept only dates if (class(x) != "Date") stop("'x' must be an object of class \"Date\"") n &lt;- length(x) if (n == 0L) return(list(lengths = integer(0L), values = x)) #Dates need to be sorted x.sort &lt;- sort(x) #y is a vector indicating at which indices the date is not consecutive with its predecessor y &lt;- x.sort[-1L] != (x.sort + 1)[-n] #i returns the indices of y that are TRUE, and appends the index of the last value i &lt;- c(which(y | is.na(y)), n) #diff tells you the distances in between TRUE/non-consecutive dates. max gets the largest of these. max(diff(c(0L, i))) } #Loop max.consec.use &lt;- matrix(nrow = length(unique(dat$user_id)), ncol = 1) rownames(max.consec.use) &lt;- unique(dat$user_id) for(i in 1:length(unique(dat$user_id))){ user &lt;- unique(dat$user_id)[i] uses &lt;- subset(dat, user_id %in% user) max.consec.use[paste(user), 1] &lt;- rle.date(uses$day) } max.consec.use </code></pre>
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    1. POHelp me replace a for loop with an "apply" function
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    1. COForgot to add: the getSOTable function is from Shane's answer here: http://stackoverflow.com/questions/1434897/how-do-i-load-example-datasets-in-r/1434927#1434927
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    2. COoh that's Sweet. And thanks to Shane.
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