-- two implementations of a sort function
-- this is an example only. Lua has now a built-in function "sort"
-- extracted from Programming Pearls, page 110
function qsort(x,l,u,f)
if l<u then
local m=math.random(u-(l-1))+l-1 -- choose a random pivot in range l..u
x[l],x[m]=x[m],x[l] -- swap pivot to first position
local t=x[l] -- pivot value
m=l
local i=l+1
while i<=u do
-- invariant: x[l+1..m] < t <= x[m+1..i-1]
if f(x[i],t) then
m=m+1
x[m],x[i]=x[i],x[m] -- swap x[i] and x[m]
end
i=i+1
end
x[l],x[m]=x[m],x[l] -- swap pivot to a valid place
-- x[l+1..m-1] < x[m] <= x[m+1..u]
qsort(x,l,m-1,f)
qsort(x,m+1,u,f)
end
end
function selectionsort(x,n,f)
local i=1
while i<=n do
local m,j=i,i+1
while j<=n do
if f(x[j],x[m]) then m=j end
j=j+1
end
x[i],x[m]=x[m],x[i] -- swap x[i] and x[m]
i=i+1
end
end
function show(m,x)
io.write(m,"\n\t")
local i=1
while x[i] do
io.write(x[i])
i=i+1
if x[i] then io.write(",") end
end
io.write("\n")
end
function testsorts(x)
local n=1
while x[n] do n=n+1 end; n=n-1 -- count elements
show("original",x)
qsort(x,1,n,function (x,y) return x<y end)
show("after quicksort",x)
selectionsort(x,n,function (x,y) return x>y end)
show("after reverse selection sort",x)
qsort(x,1,n,function (x,y) return x<y end)
show("after quicksort again",x)
end
-- array to be sorted
x={"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"}
testsorts(x)