Add optional argument to math parser function 'sort()' to specify sor…

…ting index.

CImg.h

@@ -22434,23 +22434,28 @@ namespace cimg_library {
               _cimg_mp_op("Function 'sort()'");
               s1 = ss5; while (s1<se1 && (*s1!=',' || level[s1 - expr._data]!=clevel1)) ++s1;
               arg1 = compile(ss5,s1,depth1,0,block_flags);
-              arg2 = arg4 = 1; arg3 = ~0U;
+              arg2 = arg4 = 1; arg3 = ~0U; arg5 = 0;
               if (s1<se1) {
                 s0 = ++s1; while (s0<se1 && (*s0!=',' || level[s0 - expr._data]!=clevel1)) ++s0;
                 arg2 = compile(s1,s0,depth1,0,block_flags);
                 if (s0<se1) {
                   s1 = ++s0; while (s1<se1 && (*s1!=',' || level[s1 - expr._data]!=clevel1)) ++s1;
                   arg3 = compile(s0,s1,depth1,0,block_flags);
-                  arg4 = s1<se1?compile(++s1,se1,depth1,0,block_flags):1;
+                  if (s1<se1) {
+                    s0 = ++s1; while (s0<se1 && (*s0!=',' || level[s0 - expr._data]!=clevel1)) ++s0;
+                    arg4 = compile(s1,s0,depth1,0,block_flags);
+                    arg5 = s0<se1?compile(++s0,se1,depth1,0,block_flags):0;
+                  }
                 }
               }
               _cimg_mp_check_type(arg1,1,2,0);
               _cimg_mp_check_type(arg2,2,1,0);
               if (arg3!=~0U) _cimg_mp_check_type(arg3,3,1,0);
               _cimg_mp_check_type(arg4,4,1,0);
+              _cimg_mp_check_type(arg5,5,1,0);
               p1 = size(arg1);
               pos = vector(p1);
-              CImg<ulongT>::vector((ulongT)mp_sort,pos,arg1,p1,arg2,arg3,arg4).move_to(code);
+              CImg<ulongT>::vector((ulongT)mp_sort,pos,arg1,p1,arg2,arg3,arg4,arg5).move_to(code);
               return_comp = true;
               _cimg_mp_return(pos);
             }
@@ -28556,15 +28561,16 @@ namespace cimg_library {
         const unsigned int
           siz = (unsigned int)mp.opcode[3],
           nb_elts = mp.opcode[5]==~0U?siz:(unsigned int)_mp_arg(5),
-          siz_elt = (unsigned int)_mp_arg(6);
+          siz_elt = (unsigned int)_mp_arg(6),
+          sort_index = std::min((unsigned int)_mp_arg(7),siz_elt - 1);
         const ulongT sn = siz_elt*nb_elts;
         if (sn>siz || siz_elt<1)
           throw CImgArgumentException("[" cimg_appname "_math_parser] CImg<%s>: Function 'sort()': "
                                       "Arguments 'nb_elts=%g' and 'siz_elt=%g' are invalid "
                                       "for sorting a vector of size %u.",
                                       mp.imgin.pixel_type(),_mp_arg(5),_mp_arg(6),siz);
         CImg<doubleT>(ptrd,siz_elt,nb_elts,1,1,true) = CImg<doubleT>(ptrs,siz_elt,nb_elts,1,1,true).
-          get_sort(is_increasing,siz_elt>1?'y':0);
+          shift(-(int)sort_index,0,0,0,2).get_sort(is_increasing,siz_elt>1?'y':0).shift((int)sort_index,0,0,0,2);
         if (sn<siz) CImg<doubleT>(ptrd + sn,siz - sn,1,1,1,true) = CImg<doubleT>(ptrs + sn,siz - sn,1,1,1,true);
         return cimg::type<double>::nan();
       }