Simple iterator for the Cartesian product

gistfile1.r

cart_iter = function(li) {
  nextState = function(state, pos = 1L) {
    if (state[pos] < state.last[pos])
      replace(state, pos, state[pos] + 1L)
    else
      nextState(replace(state, pos, 1L), pos + 1L)
  }

  nextElem = function() {
    state <<- nextState(state)
    counter <<- counter + 1L
    mapply(function(i, j) li[[i]][j], i=names(state), j=state)
  }

  hasNext = function() {
    counter < counter.max
  }

  reset = function() {
    state <<- state.init
    counter <<- 0L
    invisible(TRUE)
  }


  state.last = sort(vapply(li, length, 1L), decreasing=TRUE)
  state.init = setNames(c(0L, rep.int(1L, length(state.last) - 1L)), names(state.last))
  counter.max = prod(state.last)
  if (counter.max > .Machine$integer.max)
    stop("The generated design is too big. Designs with up to ",
      .Machine$integer.max, " rows are supported!")
  counter.max = as.integer(counter.max)

  state = state.init
  counter = 0L

  list(nextElem = nextElem,
       hasNext = hasNext,
       reset = reset,
       n.states = counter.max)
}

it = cart_iter(list(x = 1:10, b = 1:2))
while(it$hasNext()) {
  print(it$nextElem())
}