lua/test/sort.lua

   1 -- two implementations of a sort function
   2 -- this is an example only. Lua has now a built-in function "sort"
   3
   4 -- extracted from Programming Pearls, page 110
   5 function qsort(x,l,u,f)
   6  if l<u then
   7   local m=math.random(u-(l-1))+l-1      -- choose a random pivot in range l..u
   8   x[l],x[m]=x[m],x[l]                   -- swap pivot to first position
   9   local t=x[l]                          -- pivot value
  10   m=l
  11   local i=l+1
  12   while i<=u do
  13     -- invariant: x[l+1..m] < t <= x[m+1..i-1]
  14     if f(x[i],t) then
  15       m=m+1
  16       x[m],x[i]=x[i],x[m]               -- swap x[i] and x[m]
  17     end
  18     i=i+1
  19   end
  20   x[l],x[m]=x[m],x[l]                   -- swap pivot to a valid place
  21   -- x[l+1..m-1] < x[m] <= x[m+1..u]
  22   qsort(x,l,m-1,f)
  23   qsort(x,m+1,u,f)
  24  end
  25 end
  26
  27 function selectionsort(x,n,f)
  28  local i=1
  29  while i<=n do
  30   local m,j=i,i+1
  31   while j<=n do
  32    if f(x[j],x[m]) then m=j end
  33    j=j+1
  34   end
  35  x[i],x[m]=x[m],x[i]                    -- swap x[i] and x[m]
  36  i=i+1
  37  end
  38 end
  39
  40 function show(m,x)
  41  io.write(m,"\n\t")
  42  local i=1
  43  while x[i] do
  44   io.write(x[i])
  45   i=i+1
  46   if x[i] then io.write(",") end
  47  end
  48  io.write("\n")
  49 end
  50
  51 function testsorts(x)
  52  local n=1
  53  while x[n] do n=n+1 end; n=n-1         -- count elements
  54  show("original",x)
  55  qsort(x,1,n,function (x,y) return x<y end)
  56  show("after quicksort",x)
  57  selectionsort(x,n,function (x,y) return x>y end)
  58  show("after reverse selection sort",x)
  59  qsort(x,1,n,function (x,y) return x<y end)
  60  show("after quicksort again",x)
  61 end
  62
  63 -- array to be sorted
  64 x={"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"}
  65
  66 testsorts(x)