 2003 Prentice Hall, Inc. All rights reserved. 1 Recursion Recursive functions –Functions that call themselves –Can only solve a base case If not base case –Break problem into smaller problem(s) –Launch new copy of function to work on the smaller problem (recursive call/recursive step) Slowly converges towards base case Function makes call to itself inside the return statement –Eventually base case gets solved Answer works its way back up to solve the entire problem

 2003 Prentice Hall, Inc. All rights reserved. 2 Recursion Example: factorial n! = n * ( n – 1 ) * ( n – 2 ) * … * 1 –Recursive relationship ( n! = n * ( n – 1 )! ) 5! = 5 * 4! 4! = 4 * 3!… –Base case (1! = 0! = 1)

 2003 Prentice Hall, Inc. All rights reserved. Outline // Recursive factorial function. 3 #include 4 5 using std::cout; 6 using std::endl; 7 8 #include 9 10 using std::setw; unsigned long factorial( unsigned long ); // function prototype int main() 15 { 16 // Loop 10 times. During each iteration, calculate 17 // factorial( i ) and display result. 18 for ( int i = 0; i <= 10; i++ ) 19 cout << setw( 2 ) << i << "! = " 20 << factorial( i ) << endl; return 0; // indicates successful termination } // end main Data type unsigned long can hold an integer from 0 to 4 billion.

 2003 Prentice Hall, Inc. All rights reserved. Outline // recursive definition of function factorial 27 unsigned long factorial( unsigned long number ) 28 { 29 // base case 30 if ( number <= 1 ) 31 return 1; // recursive step 34 else 35 return number * factorial( number - 1 ); } // end function factorial 0! = 1 1! = 1 2! = 2 3! = 6 4! = 24 5! = 120 6! = 720 7! = ! = ! = ! = The base case occurs when we have 0! or 1!. All other cases must be split up (recursive step).

 2003 Prentice Hall, Inc. All rights reserved. 5 Example Using Recursion: Fibonacci Series Fibonacci series: 0, 1, 1, 2, 3, 5, 8... –Each number sum of two previous ones –Example of a recursive formula: fib(n) = fib(n-1) + fib(n-2) C++ code for Fibonacci function long fibonacci( long n ) { if ( n == 0 || n == 1 ) // base case return n; else return fibonacci( n - 1 ) + fibonacci( n – 2 ); }

 2003 Prentice Hall, Inc. All rights reserved. 6 Example Using Recursion: Fibonacci Series f( 3 ) f( 1 ) f( 2 ) f( 1 )f( 0 )return 1 return 0 return + +

 2003 Prentice Hall, Inc. All rights reserved. Outline // Recursive fibonacci function. 3 #include 4 5 using std::cout; 6 using std::cin; 7 using std::endl; 8 9 unsigned long fibonacci( unsigned long ); // function prototype int main() 12 { 13 unsigned long result, number; // obtain integer from user 16 cout << "Enter an integer: "; 17 cin >> number; // calculate fibonacci value for number input by user 20 result = fibonacci( number ); // display result 23 cout << "Fibonacci(" << number << ") = " << result << endl; return 0; // indicates successful termination The Fibonacci numbers get large very quickly, and are all non-negative integers. Thus, we use the unsigned long data type.

 2003 Prentice Hall, Inc. All rights reserved. Outline } // end main // recursive definition of function fibonacci 30 unsigned long fibonacci( unsigned long n ) 31 { 32 // base case 33 if ( n == 0 || n == 1 ) 34 return n; // recursive step 37 else 38 return fibonacci( n - 1 ) + fibonacci( n - 2 ); } // end function fibonacci Enter an integer: 0 Fibonacci(0) = 0 Enter an integer: 1 Fibonacci(1) = 1 Enter an integer: 2 Fibonacci(2) = 1 Enter an integer: 3 Fibonacci(3) = 2

 2003 Prentice Hall, Inc. All rights reserved. Outline 9 Enter an integer: 4 Fibonacci(4) = 3 Enter an integer: 5 Fibonacci(5) = 5 Enter an integer: 6 Fibonacci(6) = 8 Enter an integer: 10 Fibonacci(10) = 55 Enter an integer: 20 Fibonacci(20) = 6765 Enter an integer: 30 Fibonacci(30) = Enter an integer: 35 Fibonacci(35) =

 2003 Prentice Hall, Inc. All rights reserved. 10 Recursion vs. Iteration Repetition –Iteration: explicit loop –Recursion: repeated function calls Termination –Iteration: loop condition fails –Recursion: base case recognized Both can have infinite loops Balance between performance (iteration) and good software engineering (recursion)

 2003 Prentice Hall, Inc. All rights reserved. Quick Sort Divide and conquer, recursive sorting algorithm O(n log n) comparisons to sort n items –O(n 2 ) for the worst case Fast sorting for large array sizes Strategy: 1. Pick a value from the list, called pivot (i.e. median or first) 2. Partition the list according to the following: (values less than pivot) pivot (values larger than pivot) 3. Recursively sort the two sub-lists. Base cases are lists of size zero or one. 11

 2003 Prentice Hall, Inc. All rights reserved. Quick Sort Sample Function void quicksort(int data[],int first, int last) { int pivot; if(last > first) { // list has two or more elements pivot=partition(data, first, last); // find pivot and split list into 2 sub lists quicksort(data, first, pivot - 1); // apply quick sort to the left sub list quicksort(data, pivot + 1, last); // apply quick sort to the right sub list }//IF }//QUICKSORT 12

 2003 Prentice Hall, Inc. All rights reserved. Quick Sort Recursion Tree 13

 2003 Prentice Hall, Inc. All rights reserved. File Handling Accessing a data file for input: 1) Include the file stream header file: #include using namespace std; 2) Associate an input file name with an input file stream for accessing the data file. ifstream fin(“datafile.in”); You may explicitly indicate the mode of file access as input: ifstream fin(“datafile.in”,ios::in); 14

 2003 Prentice Hall, Inc. All rights reserved. File Handling 3) Use the associated file name to access the data: fin>>accountNum>>balance; or while(fin>>accountNum[i]>>balance[i]){ … } to retrieve data until end of file (EOF) is reached 4) Close the file indicating that the data access is terminated. fin.close( ); 15

 2003 Prentice Hall, Inc. All rights reserved. File Handling Accessing a data file for output: 1) Include the file stream header file: #include using namespace std; 2) Associate an output file name with an output file stream: ofstream fout(“datafile.out”,ios::out); 16

 2003 Prentice Hall, Inc. All rights reserved. File Handling 3) Use the associated file name to output the data: fout<<accountNum<< “ ”<<balance<<endl; 4) Close the file indicating that the data access is terminated. fout.close( ); 17

 2003 Prentice Hall, Inc. All rights reserved. Mode of Opening a File Input –ifstream fin(“datafile.in”, ios::in); Output –ofstream fout(“datafile.out”, ios::out); Both input and output –fstream acctFile(“accountfile.txt”, ios::in|ios::out); Append –ofstream fout(“accountfile.txt”, ios::app); 18