From 29a7027dbab2f98665355643738262df1aa68758 Mon Sep 17 00:00:00 2001
From: tqchen <tianqi.tchen@gmail.com>
Date: Mon, 1 Sep 2014 22:50:48 -0700
Subject: [PATCH] fix the zero length vector

---
 R-package/R/utils.R         | 8 +++++---
 R-package/src/xgboost_R.cpp | 7 ++++++-
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/R-package/R/utils.R b/R-package/R/utils.R
index 015026fe6..da602478a 100644
--- a/R-package/R/utils.R
+++ b/R-package/R/utils.R
@@ -121,8 +121,8 @@ xgb.iter.eval <- function(booster, watchlist, iter) {
       stop("xgb.eval: watch list can only contain xgb.DMatrix")
     }
   }
-  evnames <- list()
   if (length(watchlist) != 0) {
+    evnames <- list()
     for (i in 1:length(watchlist)) {
       w <- watchlist[i]
       if (length(names(w)) == 0) {
@@ -130,8 +130,10 @@ xgb.iter.eval <- function(booster, watchlist, iter) {
       }
       evnames <- append(evnames, names(w))
     }
+    msg <- .Call("XGBoosterEvalOneIter_R", booster, as.integer(iter), watchlist, 
+                 evnames, PACKAGE = "xgboost")
+  } else {
+    msg <- ""
   }
-  msg <- .Call("XGBoosterEvalOneIter_R", booster, as.integer(iter), watchlist, 
-               evnames, PACKAGE = "xgboost")
   return(msg)
 } 
diff --git a/R-package/src/xgboost_R.cpp b/R-package/src/xgboost_R.cpp
index ac5934d17..f255d233d 100644
--- a/R-package/src/xgboost_R.cpp
+++ b/R-package/src/xgboost_R.cpp
@@ -197,7 +197,12 @@ extern "C" {
     for (int i = 0; i < len; ++i){
       dvec.push_back(R_ExternalPtrAddr(VECTOR_ELT(dmats, i)));
     }
-    void *handle = XGBoosterCreate(&dvec[0], dvec.size());
+    void *handle;
+    if (dvec.size() == 0) {
+      handle = XGBoosterCreate(NULL, 0);
+    } else {
+      handle = XGBoosterCreate(&dvec[0], dvec.size());
+    }
     SEXP ret = PROTECT(R_MakeExternalPtr(handle, R_NilValue, R_NilValue));
     R_RegisterCFinalizerEx(ret, _BoosterFinalizer, TRUE);
     UNPROTECT(1);