Apply With Multiple Arguments
append_week_counts_to_entries <- function(entries) {
  entries$week1 <- apply(entries,1,helpfunc,row=row,week=count_week1)
  entries$week2 <- apply(entries,1,helpfunc,row=row,week=count_week2)
# ... to all N weeks
  return(entries)
}

helpfunc <- function(entries,row,week) {
  if(as.character(row[1]) %in% week$id) {
    return(week[which(as.character(week$id) == as.character(row[1])),2])
  }
  else {
    return(0)
  }
}

entries: structure(list(`entries$id` = c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10
)), .Names = "entries$id", row.names = c(NA, -10L), class = "data.frame")

count_week_i: structure(list(Var1 = structure(1:3, .Label = c("1", "2", "3"
), class = "factor"), Freq = c(1L, 2L, 4L)), .Names = c("Var1",
"Freq"), row.names = c(NA, -3L), class = "data.frame")