Source code in the xpl0 programming language

include xpllib; \for Print

int Items, Size, I, J, Gap, JG, T, C, Count;
[Items:= ["violet", "red", "green", "indigo", "blue", "yellow", "orange"];
Size:= 7;
Count:= 0;
Gap:= Size>>1;
while Gap > 0 do
    [for I:= Gap to Size-1 do
        [J:= I - Gap;
        loop    [JG:= J + Gap;
                Count:= Count+1;
                Print("%2d: Is %6s less than %6s (y/n)? ",
                        Count, Items(J), Items(JG));
                repeat  OpenI(1);
                        C:= ChIn(1);
                until   C=^y or C=^n;
                ChOut(0, C);  CrLf(0);
                if C = ^y then quit;
                T:= Items(J);  Items(J):= Items(JG);  Items(JG):= T;
                J:= J - Gap;
                if J < 0 then quit;
                ];
        ];
    Gap:= Gap>>1;
    ];
Print("The colors of the rainbow, in sorted order, are:\n");
for I:= 0 to Size-2 do Print("%s, ", Items(I));
Print("%s\n", Items(I));
]