[Rd] warnings() generates error if there never were any (PR#4389)

[gregory_r_warnes at groton.pfizer.com]

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In debugging a long-running function where I occasionally call warnings() to
write out any warnings that have occurred, I discovered that calling
warnings() before any warnings have ever been generated causes an error:

> warnings()
Error in warnings() : Object "last.warning" not found


The problem is that warnings() attemts to check the length of the variable
'last.warning' without first checking for its existance.  

This problem exists in R 1.7.1 as well as in the new 1.8.0 beta code.

A simple patch fixes the problem:

--- warnings.R.orig     2003-10-02 02:35:04.359451000 -0400
+++ warnings.R  2003-10-02 02:42:08.900717000 -0400
@@ -1,6 +1,6 @@
 warnings <- function(...)
 {
-    if(!(n <- length(last.warning)))
+    if(!exists("last.warning") || !(n <- length(last.warning)))
        return()
     names <- names(last.warning)
     cat("Warning message", if(n > 1)"s", ":\n", sep="")


-Greg



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