Save warning messages within a loop to vector or data frame

loops, r, warnings

Solution

You can accomplish what you want with the `withCallingHandlers` function, like so

withCallingHandlers({
    for(i in 1:nrow(dat)){
        w <- length(warnings())
        dat$logk[i] <- log(dat$k[i])
    }
}, warning = function(w){
    dat$warnMsg[i] <<- w$message
    invokeRestart("muffleWarning")
})

Notice that you need the `<<-` operator in the warning handler to assign the message to the data frame in the global environment, since `<-` would create a local copy of `dat` within the handler and discard it upon exit.

Problem

How can warning messages be saved in a vector or data frame within a loop so that they can be associated with a specific iteration? Here's a toy example where I would like to store results and warning message (if any) from an operation (`log(k)`) in a data frame alongside the data (`k`): ``` dat <- data.frame(k=c(3,5,-2,7), logk=NA, warnMsg=NA) for(i in 1:nrow(dat)){ w <- length(warnings()) dat$logk[i] <- log(dat$k[i]) #if new warning, assign to data frame if(length(warnings()) > length (w)){ dat$warnMsg[i] <- warnings()[length(warnings())] } } ``` It seems this doesn't work because any warnings that are generated within the loop are not available to the `warnings()` call until the loop is over. Do I need to specify a different environment to access new warnings within the loop? I suspect there is also a better way to do this altogether using `try()` or `tryCatch()` but it isn't obvious to me yet.

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