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POHow do I save warnings and errors as output from a function?
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copied!<p>I'm using <code>lapply</code> to run a complex function on a large number of items, and I'd like to save the output from each item (if any) together with any warnings/errors that were produced so that I can tell which item produced which warning/error. </p> <p>I found a way to catch warnings using <code>withCallingHandlers</code> (<a href="https://stackoverflow.com/questions/4947528">described here</a>). However, I need to catch errors as well. I can do it by wrapping it in a <code>tryCatch</code> (as in the code below), but is there a better way to do it?</p> <pre><code>catchToList <- function(expr) { val <- NULL myWarnings <- NULL wHandler <- function(w) { myWarnings <<- c(myWarnings, w$message) invokeRestart("muffleWarning") } myError <- NULL eHandler <- function(e) { myError <<- e$message NULL } val <- tryCatch(withCallingHandlers(expr, warning = wHandler), error = eHandler) list(value = val, warnings = myWarnings, error=myError) } </code></pre> <p>Sample output of this function is:</p> <pre><code>> catchToList({warning("warning 1");warning("warning 2");1}) $value [1] 1 $warnings [1] "warning 1" "warning 2" $error NULL > catchToList({warning("my warning");stop("my error")}) $value NULL $warnings [1] "my warning" $error [1] "my error" </code></pre> <p>There are several questions here on SO that discuss <code>tryCatch</code> and error handling, but none that I found that address this particular issue. See <a href="https://stackoverflow.com/questions/3903157">How can I check whether a function call results in a warning?</a>, <a href="https://stackoverflow.com/questions/4020239">warnings() does not work within a function? How can one work around this?</a>, and <a href="https://stackoverflow.com/questions/2589275">How to tell lapply to ignore an error and process the next thing in the list?</a> for the most relevant ones.</p>

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