Question

Two important functions that convert pairlists to pairs of lists and vice versa are Zip and UnZip.

a)      Implement a function Zip that takes a pair Xs#Ys of two lists Xs and Ys (of the same length) and returns a pairlist, where the first field of each pair is taken from Xs and the second from Ys. For example, {Zip [a b c]#[1 2 3]} returns the pairlist [a#1 b#2 c#3].

The function UnZip does the inverse, for example {UnZip [a#1 b#2 c#3]} returns [a b c]#[1 2 3]. Give a specification and implementation of UnZip.

OZ programming language

Answer 1

a) Implement a function Zip that takes a pair Xs#Ys of two lists Xs and Ys (of the same length) and returns a pairlist, where the first field of each pair is taken from Xs and the second from Ys. For example, {Zip [a b c]#[1 2 3]} returns the pairlist [a#1 b#2 c#3]

Answer:------------
a). The first solution refers to the case when the lists have the same length:
declare
fun {Zip Xs#Ys}
case Xs#Ys
of nil#nil then nil
[ ] (X|Xr)#(Y|Yr) then X#Y|{Zip Xr#Yr}
end
end
{Browse {Zip [a b c]#[1 2 3]}}
a).The second solution covers the cases when the lists may have also different
lengths:
declare
fun {Zip X}
case X of nil#nil then nil
[ ] X#nil then {Browse ’First list is too long’} X
[ ] nil#X then {Browse ’Second list is too long’} X
[ ] (H1|T1)#(H2|T2) then
H1#H2|{Zip T1#T2}
end
end
{Browse {Zip [a b c]#[1 2 3]}}
{Browse {Zip [a b c d]#[1 2 3]}}
{Browse {Zip [a b c]#[1 2 3 4]}}

b).The function UnZip does the inverse, for example {UnZip [a#1 b#2 c#3]} returns [a b c]#[1 2 3]. Give a specification and implementation of UnZip.

Answer:----
b). A condensed solution:
declare
fun {UnZip XYs}
case XYs
of nil then nil#nil
[ ] X#Y|XYr then
Xr#Yr={UnZip XYr}
in
(X|Xr)#(Y|Yr)
end
end
{Browse {UnZip [a#1 b#2 c#3]}}
b). An equivalent solution, where the code is explained in some details:
declare
fun {Unzip X}
case X
of nil then nil#nil
[] (H1#H2|T) then
Local Xr Yr %Xr and Yr are local variables
in
Xr#Yr={Unzip T} %when coming back from the recursion,
%Xr and Yr will be bound
(H1|Xr)#(H2|Yr) %construct the returned value
end
end
end
{Browse {Unzip [a#1 b#2 c#3]}}