Data commit

Task/Van-Eck-sequence/ALGOL-W/van-eck-sequence.alg

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+begin % find elements of the Van Eck Sequence - first term is 0, following  %
+      % terms are 0 if the previous was the first appearance of the element %
+      %        or how far back in the sequence the last element appeared    %
+    % sets s to the first n elements of the Van Eck sequence                %
+    procedure VanEck ( integer array s ( * ) ; integer value n ) ;
+    begin
+        integer array pos ( 0 :: n );
+        for i := 1 until n do s(   i ) := 0;
+        for i := 0 until n do pos( i ) := 0;
+        for i := 2 until n do begin
+            integer j, prev;
+            j    := i - 1;
+            prev := s( j );
+            if pos( prev ) not = 0 then begin
+                    % not a new element                                     %
+                    s( i ) := j - pos( prev )
+            end if_pos_prev_ne_0 ;
+            pos( prev ) := j
+        end for_j;
+    end VanEck ;
+    % construct the first 1000 terms of the sequence                        %
+    integer MAX_VAN_ECK;
+    MAX_VAN_ECK := 1000;
+    begin
+        integer array seq ( 1 :: MAX_VAN_ECK );
+        VanEck( seq, MAX_VAN_ECK );
+        % show the first and last 10 elements                               %
+        for i := 1 until 10 do writeon( i_w := 1, s_w := 0, " ", seq( i ) );
+        write();
+        for i := MAX_VAN_ECK - 9 until MAX_VAN_ECK do writeon( i_w := 1, s_w := 0, " ", seq( i ) );
+        write()
+    end
+end.