support trailing brackets for sort() and rsort()

src/variable.cpp

@@ -2288,7 +2288,7 @@ double Variable::evaluate(char *str, Tree **tree, int ivar)
 
           if (math_function(word,contents,tree,treestack,ntreestack,argstack,nargstack,ivar));
           else if (group_function(word,contents,tree,treestack,ntreestack,argstack,nargstack,ivar));
-          else if (special_function(std::string(word),contents,tree,treestack,ntreestack,argstack,nargstack,ivar));
+          else if (special_function(std::string(word),contents,tree,treestack,ntreestack,argstack,nargstack,ivar,str,i,ptr));
           else if (feature_function(word,contents,tree,treestack,ntreestack,argstack,nargstack,ivar));
           else print_var_error(FLERR,fmt::format("Invalid math/group/special/feature function '{}()' "
                                                  "in variable formula", word),ivar);
@@ -4271,7 +4271,7 @@ Region *Variable::region_function(char *id, int ivar)
 ------------------------------------------------------------------------- */
 
 int Variable::special_function(const std::string &word, char *contents, Tree **tree, Tree **treestack,
-                               int &ntreestack, double *argstack, int &nargstack, int ivar)
+                               int &ntreestack, double *argstack, int &nargstack, int ivar, char *str, int &istr, char *&ptr)
 {
   double sx,sxx;
   double value,sy,sxy;
@@ -4569,12 +4569,12 @@ int Variable::special_function(const std::string &word, char *contents, Tree **t
       if (method == SORT) std::sort(unsorted.begin(), unsorted.end(), std::less<double>());
       if (method == RSORT) std::sort(unsorted.begin(), unsorted.end(), std::greater<double>());
 
-      double *newvec;
-      memory->create(newvec,nvec,"variable:values");
-      for (int m = 0; m < nvec; m++)
-        newvec[m] = unsorted[m];
-
       if (tree) {
+        double *newvec;
+        memory->create(newvec,nvec,"variable:values");
+        for (int m = 0; m < nvec; m++)
+          newvec[m] = unsorted[m];
+
         auto newtree = new Tree();
         newtree->type = VECTORARRAY;
         newtree->array = newvec;
@@ -4582,8 +4582,22 @@ int Variable::special_function(const std::string &word, char *contents, Tree **t
         newtree->nstride = 1;
         newtree->selfalloc = 1;
         treestack[ntreestack++] = newtree;
+
       } else {
-        error->all(FLERR, "Cannot use argstack for sorted vector");
+
+        // process one pair of trailing brackets
+        // point istr beyond last bracket
+        // nbracket = # of bracket pairs
+        // index    = int inside each bracket pair, vector index
+
+        if (str[istr] == '[') {
+          ptr = &str[istr];
+          int index = int_between_brackets(ptr,1);
+          if (index > nvec)
+            print_var_error(FLERR, fmt::format("index {} exceeds vector size of {}\n", index, nvec),ivar);
+          istr = ptr-str+1;
+          argstack[nargstack++] = unsorted[index-1];
+        }
       }
     } else {
 

src/variable.h

@@ -143,7 +143,8 @@ class Variable : protected Pointers {
   int math_function(char *, char *, Tree **, Tree **, int &, double *, int &, int);
   int group_function(char *, char *, Tree **, Tree **, int &, double *, int &, int);
   Region *region_function(char *, int);
-  int special_function(const std::string &, char *, Tree **, Tree **, int &, double *, int &, int);
+  int special_function(const std::string &, char *, Tree **, Tree **, int &, double *, int &,
+                       int, char *, int &, char *&);
   int feature_function(char *, char *, Tree **, Tree **, int &, double *, int &, int);
   void peratom2global(int, char *, double *, int, tagint, Tree **, Tree **, int &, double *, int &);
   void custom2global(int *, double *, int, tagint, Tree **, Tree **, int &, double *, int &);

unittest/commands/test_variables.cpp

@@ -340,6 +340,9 @@ TEST_F(VariableTest, Expressions)
     command("variable vec4   vector    '[1, 5, 2.5, -10, -5, 20, 120, 4, 3, 3]'");
     command("variable sort   vector    sort(v_vec4)");
     command("variable rsrt   vector    rsort(v_vec4)");
+    command("variable max2   equal     sort(v_vec4)[2]");
+    command("variable rmax   equal     rsort(v_vec4)[1]");
+    command("variable xxxl   equal     rsort(v_vec4)[11]");
     command("variable isrt   vector    sort(v_one)");
     variable->set("dummy  index     1 2");
     END_HIDE_OUTPUT();
@@ -372,6 +375,8 @@ TEST_F(VariableTest, Expressions)
     ASSERT_DOUBLE_EQ(variable->compute_equal("v_vec3"), 0.5);
     EXPECT_THAT(variable->retrieve("sort"), StrEq("[-10,-5,1,2.5,3,3,4,5,20,120]"));
     EXPECT_THAT(variable->retrieve("rsrt"), StrEq("[120,20,5,4,3,3,2.5,1,-5,-10]"));
+    ASSERT_DOUBLE_EQ(variable->compute_equal("v_max2"), -5);
+    ASSERT_DOUBLE_EQ(variable->compute_equal("v_rmax"), 120);
 
     TEST_FAILURE(".*ERROR: Variable six: Invalid thermo keyword 'XXX' in variable formula.*",
                  command("print \"${six}\""););
@@ -385,6 +390,8 @@ TEST_F(VariableTest, Expressions)
                  command("print \"${err3}\""););
     TEST_FAILURE(".*ERROR: Variable one: Mis-matched special function variable in variable formula.*",
                  command("print \"${isrt}\""););
+    TEST_FAILURE(".*ERROR: Variable vec4: index 11 exceeds vector size of 10.*",
+                 command("print \"${xxxl}\""););
 }
 
 TEST_F(VariableTest, Functions)