Technique: Sentinels

A sentinel is an extra item added to one end of an array (or list) to ensure that a loop will terminate without having to include a separate check as in the SEQUENTIAL_SEARCH algorithm:

SEQUENTIAL_SEARCH: procedure expose A.
parse arg N, V
Np1 = N + 1; A.Np1 = V
do J = 1 until A.J = V; end
return J


It is elegant and a bit faster than the following solution. For N=15000, it averages about 20% faster, but it is the difference between 0.28 and 0.22 seconds.


procedure expose A.
parse arg N, V
do J = 1 to N
  if A.J = V then leave
return J


Now suppose that we have two sorted arrays (in ascending order) A.1,...,A.M and B.1,...,B.N of integers which we wish to merge into a third C. array of M+N elements. Using sentinels provides a way to write a very simple algorithm (compare with merging without sentinel: SPARSE_POLYADD).


MERGING: procedure expose A. B. C.
parse arg M, N
Mp1 = M + 1; Np1 = N + 1
if A.M > B.N
  then B.Np1 = A.M
  else A.Mp1 = B.N
K = 1; L = 1
do J = 1 to N + M
  if A.K < B.L
    then do
      C.J = A.K; K = K + 1
    else do
      C.J = B.L; L = L + 1



Kruse R. L. Data Structures and Program Design
Prentice Hall International Editions, ISBN 0-13-196049-0.
Sedgewick R., Algorithms
Addison-Wesley, Reading, Massachusetts, 1984

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last modified 8th August 2001
Copyright 2000-2001 Vladimir Zabrodsky
Czech Republic.