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
5function 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
25end
26
27function 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
38end
39
40function 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")
49end
50
51function 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)
61end
62
63-- array to be sorted
64x={"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"}
65
66testsorts(x)
67