From b056321f3a322b14ea007bfc47e49715736bb04b Mon Sep 17 00:00:00 2001
From: Spark <43844351+keepmine@users.noreply.github.com>
Date: Fri, 8 Mar 2019 22:18:09 +0800
Subject: [PATCH] Update stack.c

I think you should test the top before swap the two top elements
---
 Chapter4/Exercise 4-04/stack.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/Chapter4/Exercise 4-04/stack.c b/Chapter4/Exercise 4-04/stack.c
index 6ea7ad3..6c03606 100644
--- a/Chapter4/Exercise 4-04/stack.c	
+++ b/Chapter4/Exercise 4-04/stack.c	
@@ -76,15 +76,21 @@ void duplicate(void)
 
 /* swap:  swap the top two elements of the stack */
 void swap(void)
-{
-    double top1 = pop();
-    double top2 = pop();
-    push(top1);
-    push(top2);
+ {
+    double top1;
+    double top2;
+    if (top > 1) {
+        top1 = pop();
+        top2 = pop();
+        push(top1);
+        push(top2);
+    }
+    else
+        printf("erorr: there is only one or no element in stack\n");
 }
 
 /* clear: clear stack */
 void clear(void)
 {
     sp = 0;
-}
\ No newline at end of file
+}