dput(jj)
structure(list(month = structure(c(1, 2, 3, 1, 2, 3, 1, 2, 3,
1, 2, 3), class = "Date"), student = structure(c(1L, 1L, 1L,
1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("Amy", "Bob"), class = "factor"),
    A = c(9, 7, 6, 8, 6, 9, 3, 2, 1, 5, 6, 5), B = c(6, 7, 8,
    5, 6, 7, 5, 4, 6, 3, 1, 5)), .Names = c("month", "student",
"A", "B"), row.names = c(NA, -12L), class = "data.frame")

IDs<-unique(jj$student)
uniq <- unique(unlist(jj$month))
king<-list() #necessary
for (i in 1:length(IDs)){
  for (j in 1:length(uniq)){
  tmp <- jj[jj$student==IDs[i]& jj$month==uniq[j],]
  tmp1 <- `dimnames<-`(as.matrix(tmp), NULL) #necessary
  #storing column sums as list element
  king[j]<- sum(tmp1$1)
  king[j]<- sum(tmp1$2)
  kong <- `dimnames<-`(do.call(cbind, lapply(king, as.numeric)), NULL) #necessary
  #dividing column sums
  bong<- kong$1/kong$2
  SUMM[i&j,]<- bong #problem here
}}

Error: unexpected numeric constant in:

    "  #storing column sums as list element
      king[]<- sum(tmp1$1"