Courses/CS 460/Fall 2005/Homework/Sean Tseng/Oct 8

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Contents 1 Homework 2 1.1 Take 1.2 Drop 1.3 Sub 1.4 All 1.5 Some

[edit] Homework 2

[edit] Take

local A B
   proc {Take Xs I Ys }
      if I == 0 then Ys = nil
      elseif I > {List.length Xs} then Ys = Xs
      else Zs Zr in
	 Xs = Zs|Zr
	 if I == 1 then Ys = [Zs]
	 else Ys = Zs | {Take Zr (I-1)}
	 end
      end
   end in
   A = [6 2 3 7]
   {Take A 1 B}
   {Browse B}
end

[edit] Drop

local A B
   proc {Drop Xs I Ys}
      if I == 0 then Ys = Xs
      elseif I >= {List.length Xs} then Ys = nil
      else Zr in
	 Xs = _|Zr
	 Ys = {Drop Zr (I-1)}
      end
   end in
   A = [6 2 3 7]
   {Drop A 2 B}
   {Browse B}
end   

[edit] Sub

local A B
   proc {Sub Xs Ys B}
      case Xs#Ys of
	   _#nil then B = true
	 []nil#_ then B = false
         [](X1|Xr) # (Y1|Yr) then
	 if X1 == Y1 then B = {Sub Xr Yr}
	 else B = {Sub Xr Ys}
	 end
      end
   end in
   A = [a 3 4 g]
   {Sub A [3 g] B}
   {Browse B}
end

[edit] All

local X Y
   proc {All Xs P B}
      case Xs of
	 nil then B = true
         []X1|Xr then if {P X1} == false then B = false
	      else B = {All Xr P}
	      end
      end
   end in
   X = [4 ~2 2 1]
   {All X fun {$ A} if A < 5 then true else false end end Y}
   {Browse Y}
end

[edit] Some

local X Y
   proc {Some Xs P B}
      case Xs of
	 nil then B = false
         []X1|Xr then if {P X1} == true then B = true
	      else B = {Some Xr P}
	      end
      end
   end in
   X = [42 6 21]
   {Some X fun {$ A} if A < 5 then true else false end end Y}
   {Browse Y}
end