whats wrong with this code ?!

*To*: mathgroup at smc.vnet.net*Subject*: [mg110456] whats wrong with this code ?!*From*: "becko" <becko565 at hotmail.com>*Date*: Sat, 19 Jun 2010 07:47:54 -0400 (EDT)

ok. I give up. I've been struggling with this the entire night. I have three functions: swap[..], split[..] and qksort[..]. The objective is to implement a recursive sort algorithm. I have tried to execute it on list={2,5,4,7,9,1};. But I keep getting the "Cannot take positions .. through .. in .." message. You may need to execute it a few times to see the error (because of it depends on the RandomInteger). Here are the three functions. Thanks in advance. swap[x_List,i_Integer,j_Integer]:=ReplacePart[x,{i->x[[j]],j->x[[i]]}] slowsort[x_List]:= Module[{z=x}, Do[ If[z[[j]]<z[[r]],z=swap[z,j,r]], {r,1,Length[z]-1},{j,r+1,Length[z]} ]; z ] split[x_List,left_Integer,right_Integer]:= Module[{L=RandomInteger[{left,right}],z,T,i=left}, T=x[[L]];z=swap[x,left,L]; Do[ If [ z[[j]]<T,z=swap[z,++i,j] ], {j,left+1,right} ]; z=swap[z,left,i]; {i,z} ] qksort[x_List,left_Integer,right_Integer]:= If[right-left>=1, Module[{i,z}, {i,z}=split[x,left,right]; {qksort[z,left,i-1][[left;;i-1]],z[[i]],qksort[z,i+1,right][[i+1;;right]]}//Flatten ], x ]