Backtrack search-algorithm

BACKTRACKING
 Suppose you have to make a series of decisions,
among various choices, where
 You don’t have enough information to know what to
choose
 Each decision leads to a new set of choices
 Some sequence of choices (possibly more than one)
may be a solution to your problem
 Backtracking is a methodical way of trying out
various sequences of decisions, until you find the
correct one that “works”.

BACKTRACKING
 Backtracking is used to solve problems in which a
sequence of objects is chosen from a specified set
so that the sequence satisfies some criterion.
 Backtracking is a modified depth-first search of a
tree.
 It is the procedure whereby, after determining that a
node can lead to nothing but dead nodes, we go
back (“backtrack”) to the node’s parent and proceed
with the search on the next child.

BACKTRACK ALGORITHM
 Based on depth-first recursive search
 Approach
1. Tests whether solution has been found
2. If found solution, return it
3. Else for each choice that can be made
a) Make that choice
b) Recursive
c) If recursion returns a solution, return it
4. If no choices remain, return failure
 Some times called “search tree”

IMPROVING BACKTRACKING
 Search pruning will help us to reduce the search
space and hence get a solution faster.
 The idea is to avoid those paths that may not lead
to a solutions as early as possible by finding
contradictions so that we can backtrack
immediately without the need to build a hopeless
solution vector.

BACKTRACKING EXAMPLES
 The backtracking can be used in this cases:
Solving a maze
Coloring a map
Solving a puzzle
N queens problem etc.,

BACKTRACKING EXAMPLE—8 QUEENS
PROBLEM
 The 8-queens problem is a classical combinatorial
problem in which it is required to place eight
queens on an 8 x 8 chessboard so no two can
attack each other.
 A queen can attack another queen if it exists in the
same row, column or diagonal as the queen.
7

PROBLEM(CONT…)
 This problem can be solved by trying to place the
first queen, then the second queen so that it cannot
attack the first, and then the third so that it is not
conflicting with previously placed queens.

PROBLEM(CONT…)
 It is an empty 8 x 8
chess board. We have
to place the queens in
this board.

PROBLEM(CONT…)
 We have placed the
first queen on the
chess board

PROBLEM(CONT…)
 Then we have placed
the second queen on
the board.
 The darken place
should not have the
queens because they
are horizontal, vertical,
diagonal to the placed
queens.

PROBLEM(CONT…)
 We have placed the
third queen on board.

PROBLEM(CONT…)
 We have placed the 4th
queen on the board.
 We have placed that in
the wrong spot, so we
backtrack and change
the place of that one.

PROBLEM(CONT…)
 In this way, we have to
continue the process
untill our is reached ie.,
we must place 8
queens on the board.

PROBLEM(CONT…)
 Backtracking provides the hope to solve some
problem instances of nontrivial sizes by pruning
non-promising branches of the state-space tree.
 The success of backtracking varies from problem
to problem and from instance to instance.
 Backtracking possibly generates all possible
candidates in an exponentially growing state-
space tree.

PARALLELIZING BACKTRACK ALGORITHM
 First, we have to parallelize the root node of the
algorithm.
 Then the sub nodes and the child nodes should be
parallelized independently using the other
processors.
 For example, if we take the 8 queens problem then
it can be easily implemented in parallel.
 The N-queens problem can be parallelized in this
way.

 The solutions to the n-queens problem can be
generated in parallel by using the master-worker
technique.
 The manager generates the upper portion of the
search tree by generating those nodes of fixed
depth d, for some d.
 The manager dynamically passes each of these
sequences to an idle worker, who in turn continues
to search for sequences with n-queens property
that contain the fixed subsequence of length d.
 The master-worker technique is particularly well-
suited for implementation with MPI

 Parallelizing the backtrack algorithm will gives us a
good speedup and efficiency when compared to the
normal algorithm.
 The speedup and the efficiency will gets drastically
increased when it is done in the parallel.

Backtrack search-algorithm

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