You can use try() or tryCatch(). The former is simpler, the latter is
more flexible.
For example:
result <-
try(read_html(?https://timeseries.sepa.org.uk/KiWIS/KiWIS?service=kisters&type=queryServices&datasource=0&request=getTimeseriesValues&ts_path=1/15016/Q/15m.Cmd&from=2015-01-01&to=2015-01-31&returnfields=Timestamp,Value,Quality%20Code?))
if (inherits(result, "try-error"))
message("year 2015 doesn't have data")
else {
... do the usual stuff ...
}
On 18/09/2023 12:54 p.m., Nick Wray wrote:> Hello I am downloading flow data from the UK national river flow archive
> (NRFA). I have code which works (thanks to previous help on r-help) but
> the problem is that before I make a call for the data for that particular
> year I don?t know whether the data exists for that particular year and
> catchment
>
> This is a typical url and it works if you paste it into the top line on a
> web page. 15007 is the code for the Pitnacree catchment in Scotland
>
>
https://timeseries.sepa.org.uk/KiWIS/KiWIS?service=kisters&type=queryServices&datasource=0&request=getTimeseriesValues&ts_path=1/15007/Q/15m.Cmd&from=2015-01-01&to=2015-01-31&returnfields=Timestamp,Value,Quality%20Code
>
> It works in R as well with
>
> page<-read_html("
>
https://timeseries.sepa.org.uk/KiWIS/KiWIS?service=kisters&type=queryServices&datasource=0&request=getTimeseriesValues&ts_path=1/15007/Q/15m.Cmd&from=2015-01-01&to=2015-01-31&returnfields=Timestamp,Value,Quality%20Code
> ")
>
> yiq<-page |>
>
> html_element("table") |>
>
> html_table(header = TRUE) |>
>
> (\(x) {
>
> hdr <- unlist(x[3, ])
>
> y <- x[-(1:3), ]
>
> names(y) <- hdr
>
> y
>
> })()
>
> print(nrow(yiq))
>
> yiq
>
>
>
> But if I try to move onto another catchment Kenmore 15006
>
> eg
>
>
>
>
https://timeseries.sepa.org.uk/KiWIS/KiWIS?service=kisters&type=queryServices&datasource=0&request=getTimeseriesValues&ts_path=1/15016/Q/15m.Cmd&from=2015-01-01&to=2015-01-31&returnfields=Timestamp,Value,Quality%20Code
>
> So in R
>
> read_html(?
>
https://timeseries.sepa.org.uk/KiWIS/KiWIS?service=kisters&type=queryServices&datasource=0&request=getTimeseriesValues&ts_path=1/15016/Q/15m.Cmd&from=2015-01-01&to=2015-01-31&returnfields=Timestamp,Value,Quality%20Code
> ?)
>
> This doesn?t work because there?s no data for this catchment for this year
>
> I am pasting in different years (from say 1961 to 2017) in a loop (and this
> works if the data sets exist) but the problem is that if the data is not
> there (and there doesn?t seem to be a way of determining this elsewhere)
> and so there?s nothing to read, an error message comes up and halts the
> program loop, so that manually I have to reset the url to try to find out
> whether there is data for the next year.
>
> What I?d like to know is whether there?s any way in R of seeing whether the
> data set exists, and if doesn?t, moving on the next possibility without
> halting?
>
> Any thoughts appreciated
>
> Thanks Nick Wray
>
> [[alternative HTML version deleted]]
>
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