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Bilgisayar Mühendisliği Bölümü

DATA STRUCTURES AND ALGORITHMS Lecture Notes 9 Recursion Spring 2008

GIT – Computer Engineering Department

Chapter Outline  Thinking recursively  Tracing execution of a recursive method  Writing recursive algorithms – Methods for searching arrays

   

Solving the Towers of Hanoi problem with recursion Processing 2-D images with recursion Backtracking to solve search problems, as in mazes Recursion vs. Iteration

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Recursive Thinking  Recursion is: – a problem-solving approach, that can ... – generate simple solutions to ... – certain kinds of problems that ... – would be difficult to solve in other ways

 Recursion splits a problem: – Into one or more simpler versions of itself

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Recursive Thinking: An Example Strategy for processing nested dolls: 1. if there is only one doll 2. do what it needed for it else 3. do what is needed for the outer doll 4. Process the inner nest in the same way

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Recursive Thinking: Another Example Strategy for searching a sorted array: 1. if the array is empty 2. return -1 as the search result (not present) 3. else if the middle element == target 4. return subscript of the middle element 5. else if target < middle element 6. recursively search elements before middle 7. else 8. recursively search elements after the middle

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Recursive Thinking: The General Approach 1. if problem is “small enough” 2. solve it directly 3. else 4. break into one or more smaller subproblems 5. solve each subproblem recursively 6. combine results into solution to whole problem

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Requirements for Recursive Solution  At least one “small” case that you can solve directly  A way of breaking a larger problem down into: – One or more smaller subproblems – Each of the same kind as the original

 A way of combining subproblem results into an overall solution to the larger problem

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Tracing a Recursive Method Overall result

length(“ace”) 3

2

1

return 1 + length(“ce”)

return 1 + length(“e”)

return 1 + length(“”) 0

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Tracing a Recursive Method (2)

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Recursive Definitions of Mathematical Formulas  Mathematicians often use recursive definitions  These lead very naturally to recursive algorithms  Examples include: – Factorial – Powers – Greatest common divisor

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Recursive Definitions: Factorial  0! = 1  n! = n x (n-1)!

 If a recursive function never reaches its base case, a stack overflow error occurs GIT – Computer Engineering Department

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Recursive Definitions: Factorial Code int factorial (int n) { if (n == 0) // or: throw exc. if < 0 return 1; else return n * factorial(n-1); }

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Recursive Definitions: Power  x0 = 1  xn = x × xn-1 double power(double x, int n) { if (n <= 0) // or: throw exc. if < 0 return 1; else return x * power(x, n-1); }

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Recursive Definitions: Greatest Common Divisor

Definition of gcd(m, n), for integers m > n > 0:  gcd(m, n) = n, if n divides m evenly  gcd(m, n) = gcd(n, m % n), otherwise int gcd (int m, int n) { if (m < n) return gcd(n, m); else if (m % n == 0) // could check n>0 return n; else return gcd(n, m % n); }

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Recursive Definitions: Fibonacci Series Definition of fibi, for integer i > 0:  fib1 = 1  fib2 = 1  fibn = fibn-1 + fibn-2, for n > 2

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Fibonacci Series Code int fib (int n) { if (n <= 2) return 1; else return fib(n-1) + fib(n-2); }

This is straightforward, but an inefficient recursion ...

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Efficiency of Recursion: Inefficient Fibonacci

# calls apparently O(2n) – big!

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Efficient Fibonacci  Strategy: keep track of: – Current Fibonacci number – Previous Fibonacci number – # left to compute

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Efficient Fibonacci: Code int fibonacci_start (int n) { return fibo(1, 0, n); } int fibo (int curr, int prev, int n) { if (n <= 1) return curr; else return fibo(curr+prev, curr, n-1); }

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Efficient Fibonacci: A Trace

Performance is O(n)

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Recursive Array Search  Can search an array using recursion  Simplest way is linear search – Examine one element at a time – Start with the first element, end with the last

 One base case for recursive search: empty array – Result is -1 (negative, not an index  not found)

 Another base case: current element matches target – Result is index of current element

 Recursive case: search rest, without current element

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Recursive Array Search: Linear Algorithm 1. if the array is empty 2. return -1 3. else if first element matches target 4. return index of first element 5. else 6. return result of searching rest of the array, excluding the first element

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Linear Array Search Code template int linear_search ( const std::vector& items, const Item_Type& target) { return linear_search(items, target, 0); } template int linear_search ( const std::vector& items, const Item_Type& target, size_t pos_first) { if (pos_first == items.size()) return -1; else if (target == items[pos_first]) return pos_first; else return linear_search(items, target, pos_first+1); }

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Array Search: Getting Better Performance  Item not found: O(n)  Item found: n/2 on average, still O(n)  How can we perhaps do better than this? – What if the array is sorted? – Can compare with middle item – Get two subproblems of size ≤ n/2

 What performance would this have? – Divide by 2 at each recursion  O(log n) – Much better! GIT – Computer Engineering Department

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Binary Search Algorithm  Works only on sorted arrays!  Base cases: – Empty (sub)array – Current element matches the target

 Check middle element with the target  Consider only the array half where target can still lie

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Binary Search Algorithm Steps 1. 2. 3. 4. 5. 6. 7. 8.

if array is empty return -1 as result else if middle element matches return index of middle element as result else if target < middle element return result of searching lower portion of array else return result of searching upper portion of array

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Binary Search Example

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Binary Search Code template int binary_search(const std::vector& items, int first, int last, const Item_Type& target) { if (first > last) return -1; // Base case for unsuccessful search. else { int middle = (first + last) / 2; // Next probe index. if (target < items[middle]) return binary_search(items, first, middle - 1, target); else if (items[middle] < target) return binary_search(items, middle + 1, last, target); else return middle; // Base case for successful search. } }

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Binary Search Code (2) template int binary_search(const std::vectoritems, const Item_Type& target) { return binary_search(items, 0, items.size() - 1, target); }



C++ Standard library function binary_search defined in does this.

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Problem Solving with Recursion  Towers of Hanoi  Counting grid squares in a blob  Backtracking, as in maze search

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Towers of Hanoi: Description Goal: Move entire tower to another peg Rules: 1. You can move only the top disk from a peg. 2. You can only put a smaller on a larger disk (or on an empty peg)

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Towers of Hanoi: Solution Strategy

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Towers of Hanoi: Solution Strategy (2)

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Towers of Hanoi: Program Specification Problem Inputs Number of disks (an integer) Letter of starting peg: L (left), M (middle), or R (right) Letter of destination peg (L, M, or R), but different from starting peg. Letter of temporary peg (L, M, or R), but different from starting peg and destination peg.

Problem Outputs A list of moves

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Towers of Hanoi: Program Specification (2)

Function

Behavior

void show_moves(int n, char start_peg, char dest_peg, chat temp_peg)

Builds a string containing all moves for a game with n disks on start_peg that will be moved to dest_peg, using temp_peg for temporary storage of disks being moved.

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Towers of Hanoi: Recursion Structure move(n, src, dst, tmp) = if n == 1: move disk 1 from src to dst otherwise: move(n-1, src, tmp, dst) move disk n from src to dst move(n-1, tmp, dst, src)

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Towers of Hanoi: Code void show_moves(int n, char start_peg, char dest_peg, char temp_peg) { if (n == 1) { cout << "Move disk 1 from peg " << start_peg << " to peg " << dest_peg << "\n"; } else { // Recursive step show_moves(n - 1, start_peg, temp_peg, dest_peg); cout << "Move disk " << n << " from peg " << start_peg << " to peg " << dest_peg << "\n"; show_moves(n - 1, temp_peg, dest_peg, start_peg); } }

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Towers of Hanoi: Performance Analysis How big will the output be for a tower of size n? We’ll just count lines; call this L(n).  For n = 1, one line: L(1) = 1  For n > 1, one line plus twice L for next smaller size: L(n+1) = 2 x L(n) + 1

Solving this gives L(n) = 2n – 1 = O(2n) So, don’t try this for very large n.

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Counting Cells in a Blob  Desire: Process an image presented as a two-dimensional array of color values  Information in the image may come from – X-Ray – MRI – Satellite imagery – Etc.

 Goal: Determine size of any area considered abnormal because of its color values GIT – Computer Engineering Department

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Counting Cells in a Blob (2)  A blob is a collection of contiguous cells that are abnormal  By contiguous we mean cells that are adjacent, horizontally, vertically, or diagonally

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Counting Cells in a Blob: Example

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Counting Cells in a Blob: Recursive Algorithm Algorithm count_cells(x, y):  if (x, y) outside grid  return 0  else if color at (x, y) normal  return 0  else  Set color at (x, y) to “Temporary” (normal)  return 1 + sum of count_cells on neighbors

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Count Cells Code int count_cells(color grid[ROW_SIZE][COL_SIZE], int r, int c) { if (r < 0 || r >= ROW_SIZE || c < { return 0; } else if (grid[r][c] != ABNORMAL) { return 0; } else { grid[r][c] = TEMPORARY; return 1 + count_cells(grid, r - 1, c + count_cells(grid, r - 1, c + count_cells(grid, r + 1, c + count_cells(grid, r + 1, c }

0 || c >= COL_SIZE)

+ + -

1) 1) 1) 1)

+ + + +

count_cells(grid, count_cells(grid, count_cells(grid, count_cells(grid,

r - 1, r, c + r + 1, r, c -

c) 1) c) 1);

}

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Backtracking  Backtracking: systematic trial and error search for solution to a problem – Example: Finding a path through a maze

 In walking through a maze, probably walk a path as far as you can go – Eventually, reach destination or dead end – If dead end, must retrace your steps – Loops: stop when reach place you’ve been before

 Backtracking systematically tries alternative paths and eliminates them if they don’t work

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Backtracking (2)  If you never try exact same path more than once, and  You try all possibilities,  You will eventually find a solution path if one exists  Problems solved by backtracking: a set of choices  Recursion implements backtracking straightforwardly – Activation frame remembers choice made at that decision point

 A chess playing program likely involves backtracking

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Maze Solving Algorithm: findPath(x, y) 1. 2. 3. 4. 5. 6. 7. 8. 9.

if (x,y) outside grid, return false if (x,y) barrier or visited, return false if (x,y) is maze exit, color PATH and return true else: set (x,y) color to PATH (“optimistically”) for each neighbor of (x,y) if findPath(neighbor), return true set (x,y) color to TEMPORARY (“visited”) return false

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Maze Solving Code bool findMazePath(color grid[ROW_SIZE][COL_SIZE], int r, int c) { if (r < 0 || c < 0 || r >= ROW_SIZE || c >= COL_SIZE) return false; // Cell is out of bounds. else if (grid[r][c] != BACKGROUND) return false; // Cell is on barrier or dead end. else if (r == ROW_SIZE - 1 && c == COL_SIZE - 1) { grid[r][c] = PATH; // Cell is on path return true; // and is maze exit. } else . . . }

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Maze Solving Code (2) {

// Recursive case. // Attempt to find a path from each neighbor. // Tentatively mark cell as on path. grid[r][c] = PATH; if (findMazePath(grid, r - 1, c) || findMazePath(grid, r + 1, c) || findMazePath(grid, r, c - 1) || findMazePath(grid, r, c + 1 ) ) { return true; } else { grid[r][c] = TEMPORARY; // Dead end. return false; }

}

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Recursion Versus Iteration  Recursion and iteration are similar  Iteration: – Loop repetition test determines whether to exit

 Recursion: – Condition tests for a base case

 Recursive method often slower than iterative; why? – Overhead for loop repetition smaller than overhead for call and return

 If easier to develop algorithm using recursion, – Then code it as a recursive method (simpler than iterative) • easier to write, read, and debug

– Software engineering benefit probably outweighs ... • reduction in efficiency

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Recursion Versus Iteration  Recursive subroutines are implemented using stacks – Programmer who use recursion does not have to think about stacks – This is done automatically

 Remember the string size example:

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Recursion  Iteration  It is possible for a programmer to implement a recursive algorithm iteratively “by hand” – using a stack structure – a non-recursive routine might be faster than a recursive routine

 Advantages – – – – –

have advantages of iterative solutions no additional function calls protect from stack overflow run faster use less memory

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Recursion  Iteration  When recursion involves single call that is at the end ...  It is called tail recursion and it easy to make iterative:

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Example: Print an array of integers void Print(int X[],int N) { if (N == 0) cout << X[0] << "\n"; //stopping case else { cout << X[N] << "\n"; PrintBack(X, N - 1); //recursive step } }

void Print(int X[],int N) { while(N > 0){ cout << X[N] << "\n"; N = N - 1; } cout << X[0] << "\n"; }

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Example: String length int size(string str) { if (str == "") return 0; else return 1 + size(str.substr(1)); } int iterSize(string str) { int len = 0; while (str == ""){ len=len+1 str = str.substr(1); } } GIT – Computer Engineering Department

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Example: Factorial int factorial (int n) { if (n == 0) // or: throw exc. if < 0 return 1; else return n * factorial(n-1); } int iterFact (int n) { int result = 1; for (int k = 1; k <= n; k++) { result = result * k; } return result; }

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Example: Binary Search template int binary_search(const std::vector& items, int first, int last, const Item_Type& target) { int middle; while (first <= last){ middle = (first + last) / 2; if (target < items[middle]) middle = middle - 1; else if (items[middle] < target) first = middle + 1; else return middle; } return -1; }

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Example: Binary Search template int binary_search(const std::vector& items, int first, int last, const Item_Type& target) { if (first > last) return -1; // Base case for unsuccessful search. else { int middle = (first + last) / 2; // Next probe index. if (target < items[middle]) return binary_search(items, first, middle - 1, target); else if (items[middle] < target) return binary_search(items, middle + 1, last, target); else return middle; // Base case for successful search. } }

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Recursion  Iteration: using a stack  If not tail recursion but recursion is at the end:

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Example: String length int size(string str) { { ParamStack.Push(str); while (!ParamStack.IsEmpty()){ ParamStack.Pop(str); if (str == "") len =0; else { len=len+1; ParamStack.Push(str.substr(1)); } } int size(string str) { } if (str == "") return 0; else return 1 + size(str.substr(1)); } GIT – Computer Engineering Department

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Example: Print an array of integers void Print(int X[],int N) { ParamStack.Push(N); while (!ParamStack.IsEmpty()){ ParamStack.Pop(N); if(N == 0) cout << X[0] << "\n"; else { cout << X[N] << "\n"; ParamStack.Push(N - 1); } void Print(int X[],int N) { } if (N == 0) } cout << X[0] << "\n"; else { cout << X[N] << "\n"; PrintBack(X, N - 1); }

} GIT – Computer Engineering Department

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Towers of Hanoi (Recursive)

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Towers of Hanoi (Iterative)

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Other Examples  Counting Cells in a Blob – Can we use the same method?

 What about maze solving? – Recursive calls are not at the end?

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Example: Print an array elements in reverse printback(a,n) if(n==0) return else printback(a+1,n-1) print(*a) printback(a,n) Stack s; s.push(a,n,0) while s.notempty a,n,i=s.pop() if(n==0) continue if(i==0) s.push(a,n,i+1) s.push(a+1,n-1,0) else if(i==1) print(*a) else ERROR GIT – Computer Engineering Department

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Example: Counting nodes in a tree // Counts the number of nodes in a tree Recursive count: int count(TreeNode *t) { int a=0; if t==NULL return 0; else{ a+=count(t->left); a+=count(t->right); a=a+1; return a; } }

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Example: Counting nodes in a tree Iterative count: s.push(t,0,0,NULL) while stack not empty { t,a,i,r=s.pop() if(t==NULL){ t’,a’,i’,r’=s.pop() r’=0 s.push(t’,a’,i’,r’) continue }

else{ if(i==0){ s.push(t,a,i+1,r) s.push(t->left,0,0,NULL) } else if(i==1){ a=a+r s.push(t,a,i+1,NULL) s.push(t->right,0,0,NULL) } else if(i==2){ a=a+r a=a+1 t’,a’,i’,r’=s.pop() r’=a s.push(t’,a’,i’,r’) } } }

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