43 lines
1.0 KiB
R
43 lines
1.0 KiB
R
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library(foreach)
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library(RSQLite)
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# Define a simple iterator for a query result, which is
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# just a wrapper around the fetch function
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iquery <- function(con, statement, ..., n=1) {
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rs <- dbSendQuery(con, statement, ...)
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nextEl <- function() {
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r <- fetch(rs, n)
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if (nrow(r) == 0) {
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dbClearResult(rs)
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stop('StopIteration')
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}
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r
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}
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obj <- list(nextElem=nextEl)
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class(obj) <- c('abstractiter', 'iter')
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obj
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}
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# create a SQLite instance and create one connection.
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m <- dbDriver('SQLite')
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# initialize a new database to a tempfile and copy some data.frame
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# from the base package into it
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tfile <- tempfile()
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con <- dbConnect(m, dbname=tfile)
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data(USArrests)
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dbWriteTable(con, 'USArrests', USArrests)
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# issue the query, and then iterate over the results
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it <- iquery(con, 'select * from USArrests', n=10)
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r <- foreach(r=it, .combine='rbind') %do% {
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state <- r$row_names
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crime <- r$Murder + r$Assault + r$Rape
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data.frame(state=state, crime=crime)
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}
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print(r)
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# clean up
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dbDisconnect(con)
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file.remove(tfile)
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