View difference between Paste ID: KjdkSVZq and

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+# create big data frame:
 n <- 1000
 x <- data.frame(group = sample(letters[1:4], n, replace=TRUE), condition = sample(LETTERS[1:10], n, replace = TRUE), data = rnorm(n))
 
 # reasonable operations:
 marginal.means.1 <- aggregate(data ~ group + condition, data = x, FUN=mean)
 
 # unreasonable operations:
 marginal.means.2 <- marginal.means.1[NULL,]
 
 row.counter <- 1
 for (condition in levels(x$condition)) {
   for (group in levels(x$group)) {
     tmp.value <- 0
     tmp.length <- 0
     for (c in 1:nrow(x)) {
       if ((x[c,"group"] == group) & (x[c,"condition"] == condition)) {
         tmp.value <- tmp.value + x[c,"data"]
         tmp.length <- tmp.length + 1
       }
     }
     marginal.means.2[row.counter,"group"] <- group
     marginal.means.2[row.counter,"condition"] <- condition
     marginal.means.2[row.counter,"data"] <- tmp.value / tmp.length
     row.counter <- row.counter + 1
   }
 }
 
 # does it produce the same results?
 all.equal(marginal.means.1, marginal.means.2)