diff --git a/nasal_parse.h b/nasal_parse.h
index 8600da9..f5f4128 100644
--- a/nasal_parse.h
+++ b/nasal_parse.h
@@ -802,26 +802,55 @@ nasal_ast nasal_parse::additive_expr()
         else
             die(error_line,"expected calculation");
         // pre-calculation
-        if(tmp.get_type()!=ast_link && tmp.get_children()[0].get_type()==ast_number && tmp.get_children()[1].get_type()==ast_number)
+        if(tmp.get_type()!=ast_link)
         {
-            double num1=tmp.get_children()[0].get_num();
-            double num2=tmp.get_children()[1].get_num();
-            double num;
-            if(tmp.get_type()==ast_add)
-                num=num1+num2;
-            else if(tmp.get_type()==ast_sub)
-                num=num1-num2;
+            int type1=tmp.get_children()[0].get_type();
+            int type2=tmp.get_children()[1].get_type();
+            double num1,num2,num;
+            if(type1==ast_number && type2==ast_number)
+            {
+                num1=tmp.get_children()[0].get_num();
+                num2=tmp.get_children()[1].get_num();
+            }
+            else if(type1==ast_string && type2==ast_number)
+            {
+                std::string str=tmp.get_children()[0].get_str();
+                num1=trans_string_to_number(str);
+                num2=tmp.get_children()[1].get_num();
+                if(std::isnan(num1))
+                    die(tmp.get_children()[0].get_line(),"plus/minus a string: "+str);
+            }
+            else if(type1==ast_number && type2==ast_string)
+            {
+                std::string str=tmp.get_children()[1].get_str();
+                num1=tmp.get_children()[0].get_num();
+                num2=trans_string_to_number(str);
+                if(std::isnan(num2))
+                    die(tmp.get_children()[1].get_line(),"plus/minus a string: "+str);
+            }
+            else if(type1==ast_string && type2==ast_string)
+            {
+                std::string str1=tmp.get_children()[0].get_str();
+                std::string str2=tmp.get_children()[1].get_str();
+                num1=trans_string_to_number(str1);
+                num2=trans_string_to_number(str2);
+                if(std::isnan(num1))
+                    die(tmp.get_children()[0].get_line(),"plus/minus a string: "+str1);
+                if(std::isnan(num2))
+                    die(tmp.get_children()[1].get_line(),"plus/minus a string: "+str2);
+            }
+            num=(tmp.get_type()==ast_add? num1+num2:num1-num2);
             tmp.set_type(ast_number);
             tmp.set_num(num);
             tmp.get_children().clear();
         }
-        else if(tmp.get_type()==ast_link)
+        else
         {
             int type1=tmp.get_children()[0].get_type();
             int type2=tmp.get_children()[1].get_type();
+            std::string s1,s2;
             if((type1==ast_number || type1==ast_string) && (type2==ast_number || type2==ast_string))
             {
-                std::string s1,s2;
                 if(type1==ast_number)
                     s1=trans_number_to_string(tmp.get_children()[0].get_num());
                 else
@@ -830,11 +859,17 @@ nasal_ast nasal_parse::additive_expr()
                     s2=trans_number_to_string(tmp.get_children()[1].get_num());
                 else
                     s2=tmp.get_children()[1].get_str();
-                s1+=s2;
-                tmp.set_type(ast_string);
-                tmp.set_str(s1);
-                tmp.get_children().clear();
             }
+            else if(type1==ast_string && type2==ast_string)
+            {
+                s1=tmp.get_children()[0].get_str();
+                s2=tmp.get_children()[1].get_str();
+                s1+=s2;
+            }
+            s1+=s2;
+            tmp.set_type(ast_string);
+            tmp.set_str(s1);
+            tmp.get_children().clear();
         }
         node=tmp;
         ++ptr;
@@ -866,15 +901,45 @@ nasal_ast nasal_parse::multive_expr()
             break;
         }
         // pre-calculation
-        if(tmp.get_children()[0].get_type()==ast_number && tmp.get_children()[1].get_type()==ast_number)
+        int type1=tmp.get_children()[0].get_type();
+        int type2=tmp.get_children()[1].get_type();
+        double num1,num2,num;
+        if(type1==ast_number && type2==ast_number)
         {
-            double num1=tmp.get_children()[0].get_num();
-            double num2=tmp.get_children()[1].get_num();
-            double num=(tmp.get_type()==ast_mult? num1*num2:num1/num2);
-            tmp.set_type(ast_number);
-            tmp.set_num(num);
-            tmp.get_children().clear();
+            num1=tmp.get_children()[0].get_num();
+            num2=tmp.get_children()[1].get_num();
         }
+        else if(type1==ast_string && type2==ast_number)
+        {
+            std::string str=tmp.get_children()[0].get_str();
+            num1=trans_string_to_number(str);
+            num2=tmp.get_children()[1].get_num();
+            if(std::isnan(num1))
+                die(tmp.get_children()[0].get_line(),"multiply a string: "+str);
+        }
+        else if(type1==ast_number && type2==ast_string)
+        {
+            std::string str=tmp.get_children()[1].get_str();
+            num1=tmp.get_children()[0].get_num();
+            num2=trans_string_to_number(str);
+            if(std::isnan(num2))
+                die(tmp.get_children()[1].get_line(),"multiply a string: "+str);
+        }
+        else if(type1==ast_string && type2==ast_string)
+        {
+            std::string str1=tmp.get_children()[0].get_str();
+            std::string str2=tmp.get_children()[1].get_str();
+            num1=trans_string_to_number(str1);
+            num2=trans_string_to_number(str2);
+            if(std::isnan(num1))
+                die(tmp.get_children()[0].get_line(),"multiply a string: "+str1);
+            if(std::isnan(num2))
+                die(tmp.get_children()[1].get_line(),"multiply a string: "+str2);
+        }
+        num=(tmp.get_type()==ast_mult? num1*num2:num1/num2);
+        tmp.set_type(ast_number);
+        tmp.set_num(num);
+        tmp.get_children().clear();
         node=tmp;
         ++ptr;
     }
@@ -912,6 +977,37 @@ nasal_ast nasal_parse::unary()
         node.set_num(num);
         node.get_children().clear();
     }
+    else if(node.get_children()[0].get_type()==ast_string)
+    {
+        std::string str=node.get_children()[0].get_str();
+        double num=trans_string_to_number(str);
+        if(node.get_type()==ast_unary_sub)
+        {
+            if(std::isnan(num))
+                die(node.get_line(),"cannot get negative value of this string: "+str);
+            else
+            {
+                node.set_type(ast_number);
+                node.set_num(-num);
+                node.get_children().clear();
+            }
+        }
+        else if(node.get_type()==ast_unary_not)
+        {
+            if(std::isnan(num))
+            {
+                node.set_type(ast_number);
+                node.set_num(!str.length());
+                node.get_children().clear();
+            }
+            else
+            {
+                node.set_type(ast_number);
+                node.set_num(num==0);
+                node.get_children().clear();
+            }
+        }
+    }
     return node;
 }
 nasal_ast nasal_parse::scalar()
diff --git a/nasal_runtime.h b/nasal_runtime.h
index 4093828..56a46e6 100644
--- a/nasal_runtime.h
+++ b/nasal_runtime.h
@@ -725,7 +725,7 @@ int nasal_runtime::call_vector(nasal_ast& node,int base_value_addr,int local_sco
         nasal_ast& tmp=node.get_children()[0];
         if(tmp.get_type()==ast_subvec)
         {
-            die(tmp.get_line(),"cannot slice string (use subvec)");
+            die(tmp.get_line(),"cannot slice string (use substr)");
             return -1;
         }
         int index_value_addr=calculation(tmp,local_scope_addr);
@@ -1486,6 +1486,8 @@ void nasal_runtime::multi_assignment(nasal_ast& node,int local_scope_addr)
             die(value_node.get_line(),"size of calls and size of values do not match");
             return;
         }
+        // use value_table to store values temprorarily
+        // so that (a,b)=(b,a) can run correctly
         std::vector<int> value_table;
         for(int i=0;i<id_size;++i)
         {