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