72 lines
1.6 KiB
R
72 lines
1.6 KiB
R
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library(iterators)
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# This example was originally written and contributed
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# by Hadley Wickham, with minor modifications by
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# Revolution Analytics
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# Define a hasNext generic function
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hasNext <- function(obj, ...) {
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UseMethod('hasNext')
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}
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# Define a hasNext method for the "ihasNext" class
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hasNext.ihasNext <- function(obj, ...) {
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obj$hasNext()
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}
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# This function takes an iterator and returns an iterator that supports
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# the "hasNext" method. This simplifies manually calling the "nextElem"
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# method of the iterator, since you don't have to worry about catching
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# the "StopIteration" exception.
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ihasNext <- function(it) {
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it <- iter(it)
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# If "it" already has a hasNext function, return it unchanged
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if (!is.null(it$hasNext))
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return(it)
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cache <- NULL
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has_next <- NA
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nextEl <- function() {
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if (!hasNx())
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stop('StopIteration', call.=FALSE)
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# Reset the "has_next" flag and return the value
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has_next <<- NA
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cache
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}
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hasNx <- function() {
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# Check if you already know the answer
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if (!is.na(has_next))
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return(has_next)
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# Try to get the next element
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tryCatch({
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cache <<- nextElem(it)
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has_next <<- TRUE
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},
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error=function(e) {
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if (identical(conditionMessage(e), 'StopIteration')) {
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has_next <<- FALSE
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} else {
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stop(e)
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}
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})
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has_next
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}
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obj <- list(nextElem=nextEl, hasNext=hasNx)
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class(obj) <- c('ihasNext', 'abstractiter', 'iter')
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obj
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}
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# Create a "counting" iterator that has a hasNext method
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it <- ihasNext(icount(3))
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# Print the values of the iterator without the need for error handling
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while (hasNext(it))
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print(nextElem(it))
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