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© 2000 Prentice Hall, Inc.
All rights reserved.
Chapter 5 - Functions
© 2000 Prentice Hall, Inc.
All rights reserved.
Introduction
• A function is a self contained block of code that
perform a...
© 2000 Prentice Hall, Inc.
All rights reserved.
Example
• void printfline(int l, char c);
int main()
{
printline(40, ‘-’);...
© 2000 Prentice Hall, Inc.
All rights reserved.
Program Modules in C
• Functions
– Modules in C
– Programs combine user-de...
© 2000 Prentice Hall, Inc.
All rights reserved.
Math Library Functions
• Math library functions
– perform common mathemati...
© 2000 Prentice Hall, Inc.
All rights reserved.
Functions
• Functions
– Modularize a program
– All variables declared insi...
© 2000 Prentice Hall, Inc.
All rights reserved.
Elements of user defined functions
(a) Function Definitions
• Function def...
© 2000 Prentice Hall, Inc.
All rights reserved.
Function Definitions
• Function definition format (continued)
return-value...
© 2000 Prentice Hall, Inc.
All rights reserved.
OutlineOutline
1. Function prototype
(3 parameters)
2. Input values
2.1 Ca...
© 2000 Prentice Hall, Inc.
All rights reserved.
Function Prototypes
• Function prototype
– Function name
– Parameters – wh...
© 2000 Prentice Hall, Inc.
All rights reserved.
• Function header :- Consist of return type. The function
name and the for...
© 2000 Prentice Hall, Inc.
All rights reserved.
Function call :- name of function followed by parameter list.
z = max(5,10...
© 2000 Prentice Hall, Inc.
All rights reserved.
• Prototyping
Function declaration is called function prototyping. Like
va...
© 2000 Prentice Hall, Inc.
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Category of functions
• Function with no argument no return type
void prin...
© 2000 Prentice Hall, Inc.
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Call by value vs call by reference
void swap(int a, int b);
void main()
{
...
© 2000 Prentice Hall, Inc.
All rights reserved.
5.7 Header Files
• Header files
– Contain function prototypes for library ...
© 2000 Prentice Hall, Inc.
All rights reserved.
5.8 Calling Functions: Call by Value and
Call by Reference
• Used when inv...
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All rights reserved.
5.9 Random Number Generation
• rand function
– Load <stdlib.h>
– Returns "...
© 2000 Prentice Hall, Inc.
All rights reserved.
5.9 Random Number Generation
• srand function
– <stdlib.h>
– Takes an inte...
© 2000 Prentice Hall, Inc.
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OutlineOutline
1. Initialize seed
2. Input value for seed
2.1 Use srand to...
© 2000 Prentice Hall, Inc.
All rights reserved.
OutlineOutline
Program Output
Enter seed: 867
2 4 6 1 6
1 1 3 6 2
Enter se...
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All rights reserved.
5.10 Example: A Game of Chance
• Craps simulator
• Rules
– Roll two dice
•...
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OutlineOutline
1. rollDice
prototype
1.1 Initialize variables
1.2 Seed sra...
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All rights reserved.
OutlineOutline
2.2 Print win/loss
Program Output
33 if ( sum == myPoint ) ...
© 2000 Prentice Hall, Inc.
All rights reserved.
OutlineOutline
Program Output
Player rolled 6 + 6 = 12
Player loses
Player...
© 2000 Prentice Hall, Inc.
All rights reserved.
5.11 Storage Classes
• Storage class specifiers
– Storage duration – how l...
© 2000 Prentice Hall, Inc.
All rights reserved.
5.11 Storage Classes
• Static storage
– Variables exist for entire program...
© 2000 Prentice Hall, Inc.
All rights reserved.
5.12 Scope Rules
• File scope
– Identifier defined outside function, known...
© 2000 Prentice Hall, Inc.
All rights reserved.
5.12 Scope Rules
• Block scope
– Identifier declared inside a block
• Bloc...
© 2000 Prentice Hall, Inc.
All rights reserved.
OutlineOutline
1. Function prototypes
1.1 Initialize global
variable
1.2 I...
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All rights reserved.
OutlineOutline
3.1 Function definitions
31
32 printf( "local x in main is ...
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OutlineOutline
Program Output
local x in outer scope of main is 5
local x ...
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All rights reserved.
5.13 Recursion
• Recursive functions
– Functions that call themselves
– Ca...
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All rights reserved.
5.13 Recursion
• Example: factorials
– 5! = 5 * 4 * 3 * 2 * 1
– Notice tha...
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All rights reserved.
5.14 Example Using Recursion: The
Fibonacci Series
• Fibonacci series: 0, ...
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5.14 Example Using Recursion: The
Fibonacci Series
• Set of recursive call...
© 2000 Prentice Hall, Inc.
All rights reserved.
OutlineOutline
1. Function prototype
1.1 Initialize variables
2. Input an ...
© 2000 Prentice Hall, Inc.
All rights reserved.
OutlineOutline
Program Output
Enter an integer: 2
Fibonacci(2) = 1
Enter a...
© 2000 Prentice Hall, Inc.
All rights reserved.
5.15 Recursion vs. Iteration
• Repetition
– Iteration: explicit loop
– Rec...
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Transcript of "Function"

  1. 1. © 2000 Prentice Hall, Inc. All rights reserved. Chapter 5 - Functions
  2. 2. © 2000 Prentice Hall, Inc. All rights reserved. Introduction • A function is a self contained block of code that perform a particular task. • Every program have a main function to indicate where the program has to begin its execution. • For a large and complex program the task of debugging testing and maintaining becomes difficult. • If a program is divided into functional parts (independently coded program called subprogram) debugging testing will be easy. All theses subprograms are integrated into a single unit. • Divide and conquer – Construct a program from smaller pieces or components • These smaller pieces are called modules – Each piece more manageable than the original program
  3. 3. © 2000 Prentice Hall, Inc. All rights reserved. Example • void printfline(int l, char c); int main() { printline(40, ‘-’); printf(“n hellon”); printline(40, ‘*’); printf(“n good n”); return 0; } void printline(int l, char c) { int k; for(k = 0; k < l; k++) printf(“%c”, c); printf(“n”); }
  4. 4. © 2000 Prentice Hall, Inc. All rights reserved. Program Modules in C • Functions – Modules in C – Programs combine user-defined functions with library functions • C standard library has a wide variety of functions • Function calls – Invoking functions • Provide function name and arguments (data) • Function performs operations or manipulations • Function returns results – Function call analogy: • Boss asks worker to complete task – Worker gets information, does task, returns result – Information hiding: boss does not know details
  5. 5. © 2000 Prentice Hall, Inc. All rights reserved. Math Library Functions • Math library functions – perform common mathematical calculations – #include <math.h> • Format for calling functions – FunctionName( argument ); • If multiple arguments, use comma-separated list – printf( "%.2f", sqrt( 900.0 ) ); • Calls function sqrt, which returns the square root of its argument • All math functions return data type double – Arguments may be constants, variables, or expressions
  6. 6. © 2000 Prentice Hall, Inc. All rights reserved. Functions • Functions – Modularize a program – All variables declared inside functions are local variables • Known only in function defined – Parameters • Communicate information between functions • Local variables • Benefits of functions – Divide and conquer • Manageable program development – Software reusability • Use existing functions as building blocks for new programs • Abstraction - hide internal details (library functions) – Avoid code repetition
  7. 7. © 2000 Prentice Hall, Inc. All rights reserved. Elements of user defined functions (a) Function Definitions • Function definition format return-value-type function-name( parameter-list ) { declarations and statements } – Function-name: any valid identifier – Return-value-type: data type of the result (default int) • void – indicates that the function returns nothing – Parameter-list: comma separated list, declares parameters • A type must be listed explicitly for each parameter unless, the parameter is of type int
  8. 8. © 2000 Prentice Hall, Inc. All rights reserved. Function Definitions • Function definition format (continued) return-value-type function-name( parameter-list ) { declarations and statements } – Declarations and statements: function body (block) • Variables can be declared inside blocks (can be nested) • Functions can not be defined inside other functions – Returning control • If nothing returned – return; – or, until reaches right brace • If something returned – return expression;
  9. 9. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline 1. Function prototype (3 parameters) 2. Input values 2.1 Call function 3. Function definition Program Output 1 /* Fig. 5.4: fig05_04.c 2 Finding the maximum of three integers */ 3 #include <stdio.h> 4 5 int maximum( int, int, int ); /* function prototype */ 6 7 int main() 8 { 9 int a, b, c; 10 11 printf( "Enter three integers: " ); 12 scanf( "%d%d%d", &a, &b, &c ); 13 printf( "Maximum is: %dn", maximum( a, b, c ) ); 14 15 return 0; 16 } 17 18 /* Function maximum definition */ 19 int maximum( int x, int y, int z ) 20 { 21 int max = x; 22 23 if ( y > max ) 24 max = y; 25 26 if ( z > max ) 27 max = z; 28 29 return max; 30 } Enter three integers: 22 85 17 Maximum is: 85
  10. 10. © 2000 Prentice Hall, Inc. All rights reserved. Function Prototypes • Function prototype – Function name – Parameters – what the function takes in – Return type – data type function returns (default int) – Used to validate functions – Prototype only needed if function definition comes after use in program – The function with the prototype int maximum( int, int, int ); • Takes in 3 ints • Returns an int • Promotion rules and conversions – Converting to lower types can lead to errors
  11. 11. © 2000 Prentice Hall, Inc. All rights reserved. • Function header :- Consist of return type. The function name and the formal parameter list. A ; is not used at the end of function header. • Function body :- contains the declrations and statements necessary for performing the required task. long int fact(int n) { long int f =1; int k; for(k = 1; k <= n; k++) f = f * k; return(f); }
  12. 12. © 2000 Prentice Hall, Inc. All rights reserved. Function call :- name of function followed by parameter list. z = max(5,10); return statement :- when a return is encountered, the control is immediately passed back to the calling function. return; return(expression); int max(int x, int y) { if(x > y) return(x); else return(y); }
  13. 13. © 2000 Prentice Hall, Inc. All rights reserved. • Prototyping Function declaration is called function prototyping. Like variable function must be declared before they are invoked. void printline(int l, char c); void printline(int, char) Parameter used in prototyping and function definition are called formal parameters and those used in function calls are called actual parameter. Actual parameters used in a calling statement may be constant, variable or expression. The formal and actual parameters must match exactly in type order and number. Their names may be different.
  14. 14. © 2000 Prentice Hall, Inc. All rights reserved. Category of functions • Function with no argument no return type void printline(void); • Argument but no return value void printline(int l, char c); • Argument with return value long int fact(int n); • No argument but return a value int getnumber(void); • Function that return multiple value. void interchange(int *x, int *y);
  15. 15. © 2000 Prentice Hall, Inc. All rights reserved. Call by value vs call by reference void swap(int a, int b); void main() { int x = 30, y = 40; printf(“%d %d”, x, y); swap(x, y); printf(“%d %d”, x, y); } void swap(int a, int b) { int t; t = a a = b; b = t; printf(%d %d”, a, b); } void swap(int *a, int *b); void main() { int x = 30, y = 40; printf(“%d %d”, x, y); swap(&x, &y); printf(“%d %d”, x, y); } void swap(int *a, int *b) { int t; t = *a *a = *b; *b = t; printf(%d %d”, *a, *b); }
  16. 16. © 2000 Prentice Hall, Inc. All rights reserved. 5.7 Header Files • Header files – Contain function prototypes for library functions – <stdlib.h> , <math.h> , etc – Load with #include <filename> #include <math.h> • Custom header files – Create file with functions – Save as filename.h – Load in other files with #include "filename.h" – Reuse functions
  17. 17. © 2000 Prentice Hall, Inc. All rights reserved. 5.8 Calling Functions: Call by Value and Call by Reference • Used when invoking functions • Call by value – Copy of argument passed to function – Changes in function do not effect original – Use when function does not need to modify argument • Avoids accidental changes • Call by reference – Passes original argument – Changes in function effect original – Only used with trusted functions • For now, we focus on call by value
  18. 18. © 2000 Prentice Hall, Inc. All rights reserved. 5.9 Random Number Generation • rand function – Load <stdlib.h> – Returns "random" number between 0 and RAND_MAX (at least 32767) i = rand(); – Pseudorandom • Preset sequence of "random" numbers • Same sequence for every function call • Scaling – To get a random number between 1 and n 1 + ( rand() % n ) • rand() % n returns a number between 0 and n - 1 • Add 1 to make random number between 1 and n 1 + ( rand() % 6) – number between 1 and 6
  19. 19. © 2000 Prentice Hall, Inc. All rights reserved. 5.9 Random Number Generation • srand function – <stdlib.h> – Takes an integer seed and jumps to that location in its "random" sequence srand( seed ); – srand( time( NULL ) ); //load <time.h> • time( NULL ) – Returns the time at which the program was compiled in seconds – “Randomizes" the seed
  20. 20. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline 1. Initialize seed 2. Input value for seed 2.1 Use srand to change random sequence 2.2 Define Loop 3. Generate and output random numbers 1 /* Fig. 5.9: fig05_09.c 2 Randomizing die-rolling program */ 3 #include <stdlib.h> 4 #include <stdio.h> 5 6 int main() 7 { 8 int i; 9 unsigned seed; 10 11 printf( "Enter seed: " ); 12 scanf( "%u", &seed ); 13 srand( seed ); 14 15 for ( i = 1; i <= 10; i++ ) { 16 printf( "%10d", 1 + ( rand() % 6 ) ); 17 18 if ( i % 5 == 0 ) 19 printf( "n" ); 20 } 21 22 return 0; 23 }
  21. 21. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline Program Output Enter seed: 867 2 4 6 1 6 1 1 3 6 2 Enter seed: 67 6 1 4 6 2 1 6 1 6 4 Enter seed: 67 6 1 4 6 2 1 6 1 6 4
  22. 22. © 2000 Prentice Hall, Inc. All rights reserved. 5.10 Example: A Game of Chance • Craps simulator • Rules – Roll two dice • 7 or 11 on first throw, player wins • 2, 3, or 12 on first throw, player loses • 4, 5, 6, 8, 9, 10 - value becomes player's "point" – Player must roll his point before rolling 7 to win
  23. 23. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline 1. rollDice prototype 1.1 Initialize variables 1.2 Seed srand 2. Define switch statement for win/loss/continue 2.1 Loop 1 /* Fig. 5.10: fig05_10.c 2 Craps */ 3 #include <stdio.h> 4 #include <stdlib.h> 5 #include <time.h> 6 7 int rollDice( void ); 8 9 int main() 10 { 11 int gameStatus, sum, myPoint; 12 13 srand( time( NULL ) ); 14 sum = rollDice(); /* first roll of the dice */ 15 16 switch ( sum ) { 17 case 7: case 11: /* win on first roll */ 18 gameStatus = 1; 19 break; 20 case 2: case 3: case 12: /* lose on first roll */ 21 gameStatus = 2; 22 break; 23 default: /* remember point */ 24 gameStatus = 0; 25 myPoint = sum; 26 printf( "Point is %dn", myPoint ); 27 break; 28 } 29 30 while ( gameStatus == 0 ) { /* keep rolling */ 31 sum = rollDice(); 32
  24. 24. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline 2.2 Print win/loss Program Output 33 if ( sum == myPoint ) /* win by making point */ 34 gameStatus = 1; 35 else 36 if ( sum == 7 ) /* lose by rolling 7 */ 37 gameStatus = 2; 38 } 39 40 if ( gameStatus == 1 ) 41 printf( "Player winsn" ); 42 else 43 printf( "Player losesn" ); 44 45 return 0; 46 } 47 48 int rollDice( void ) 49 { 50 int die1, die2, workSum; 51 52 die1 = 1 + ( rand() % 6 ); 53 die2 = 1 + ( rand() % 6 ); 54 workSum = die1 + die2; 55 printf( "Player rolled %d + %d = %dn", die1, die2, workSum ); 56 return workSum; 57 } Player rolled 6 + 5 = 11 Player wins
  25. 25. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline Program Output Player rolled 6 + 6 = 12 Player loses Player rolled 4 + 6 = 10 Point is 10 Player rolled 2 + 4 = 6 Player rolled 6 + 5 = 11 Player rolled 3 + 3 = 6 Player rolled 6 + 4 = 10 Player wins Player rolled 1 + 3 = 4 Point is 4 Player rolled 1 + 4 = 5 Player rolled 5 + 4 = 9 Player rolled 4 + 6 = 10 Player rolled 6 + 3 = 9 Player rolled 1 + 2 = 3 Player rolled 5 + 2 = 7 Player loses
  26. 26. © 2000 Prentice Hall, Inc. All rights reserved. 5.11 Storage Classes • Storage class specifiers – Storage duration – how long an variable exists in memory – Scope – where variable can be referenced in program – Linkage – specifies the files in which an identifier is known • Automatic storage – variable created and destroyed within its block – auto: default for local variables auto double x, y; – register: tries to put variable into high-speed registers • Can only be used for automatic variables register int counter = 1;
  27. 27. © 2000 Prentice Hall, Inc. All rights reserved. 5.11 Storage Classes • Static storage – Variables exist for entire program execution – Default value of zero – static: local variables defined in functions. • Keep value after function ends • Only known in their own function – extern: default for global variables and functions • Known in any function
  28. 28. © 2000 Prentice Hall, Inc. All rights reserved. 5.12 Scope Rules • File scope – Identifier defined outside function, known in all functions – Used for global variables, function definitions, function prototypes • Function scope – Can only be referenced inside a function body – Used only for labels (start:, case: , etc.)
  29. 29. © 2000 Prentice Hall, Inc. All rights reserved. 5.12 Scope Rules • Block scope – Identifier declared inside a block • Block scope begins at declaration, ends at right brace – Used for variables, function parameters (local variables of function) – Outer blocks "hidden" from inner blocks if there is a variable with the same name in the inner block • Function prototype scope – Used for identifiers in parameter list
  30. 30. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline 1. Function prototypes 1.1 Initialize global variable 1.2 Initialize local variable 1.3 Initialize local variable in block 2. Call functions 3. Output results 1 /* Fig. 5.12: fig05_12.c 2 A scoping example */ 3 #include <stdio.h> 4 5 void a( void ); /* function prototype */ 6 void b( void ); /* function prototype */ 7 void c( void ); /* function prototype */ 8 9 int x = 1; /* global variable */ 10 11 int main() 12 { 13 int x = 5; /* local variable to main */ 14 15 printf("local x in outer scope of main is %dn", x ); 16 17 { /* start new scope */ 18 int x = 7; 19 20 printf( "local x in inner scope of main is %dn", x ); 21 } /* end new scope */ 22 23 printf( "local x in outer scope of main is %dn", x ); 24 25 a(); /* a has automatic local x */ 26 b(); /* b has static local x */ 27 c(); /* c uses global x */ 28 a(); /* a reinitializes automatic local x */ 29 b(); /* static local x retains its previous value */ 30 c(); /* global x also retains its value */
  31. 31. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline 3.1 Function definitions 31 32 printf( "local x in main is %dn", x ); 33 return 0; 34 } 35 36 void a( void ) 37 { 38 int x = 25; /* initialized each time a is called */ 39 40 printf( "nlocal x in a is %d after entering an", x ); 41 ++x; 42 printf( "local x in a is %d before exiting an", x ); 43 } 44 45 void b( void ) 46 { 47 static int x = 50; /* static initialization only */ 48 /* first time b is called */ 49 printf( "nlocal static x is %d on entering bn", x ); 50 ++x; 51 printf( "local static x is %d on exiting bn", x ); 52 } 53 54 void c( void ) 55 { 56 printf( "nglobal x is %d on entering cn", x ); 57 x *= 10; 58 printf( "global x is %d on exiting cn", x ); 59 }
  32. 32. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline Program Output local x in outer scope of main is 5 local x in inner scope of main is 7 local x in outer scope of main is 5 local x in a is 25 after entering a local x in a is 26 before exiting a local static x is 50 on entering b local static x is 51 on exiting b global x is 1 on entering c global x is 10 on exiting c local x in a is 25 after entering a local x in a is 26 before exiting a local static x is 51 on entering b local static x is 52 on exiting b global x is 10 on entering c global x is 100 on exiting c local x in main is 5
  33. 33. © 2000 Prentice Hall, Inc. All rights reserved. 5.13 Recursion • Recursive functions – Functions that call themselves – Can only solve a base case – Divide a problem up into • What it can do • What it cannot do – What it cannot do resembles original problem – The function launches a new copy of itself (recursion step) to solve what it cannot do – Eventually base case gets solved • Gets plugged in, works its way up and solves whole problem
  34. 34. © 2000 Prentice Hall, Inc. All rights reserved. 5.13 Recursion • Example: factorials – 5! = 5 * 4 * 3 * 2 * 1 – Notice that • 5! = 5 * 4! • 4! = 4 * 3! ... – Can compute factorials recursively – Solve base case (1! = 0! = 1) then plug in • 2! = 2 * 1! = 2 * 1 = 2; • 3! = 3 * 2! = 3 * 2 = 6;
  35. 35. © 2000 Prentice Hall, Inc. All rights reserved. 5.14 Example Using Recursion: The Fibonacci Series • Fibonacci series: 0, 1, 1, 2, 3, 5, 8... – Each number is the sum of the previous two – Can be solved recursively: • fib( n ) = fib( n - 1 ) + fib( n – 2 ) – Code for the fibaonacci function long fibonacci( long n ) { if (n == 0 || n == 1) // base case return n; else return fibonacci( n - 1) + fibonacci( n – 2 ); }
  36. 36. © 2000 Prentice Hall, Inc. All rights reserved. 5.14 Example Using Recursion: The Fibonacci Series • Set of recursive calls to function fibonacci f( 3 ) f( 1 )f( 2 ) f( 1 ) f( 0 ) return 1 return 1 return 0 return + +return
  37. 37. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline 1. Function prototype 1.1 Initialize variables 2. Input an integer 2.1 Call function fibonacci 2.2 Output results. 3. Define fibonacci recursively Program Output 1 /* Fig. 5.15: fig05_15.c 2 Recursive fibonacci function */ 3 #include <stdio.h> 4 5 long fibonacci( long ); 6 7 int main() 8 { 9 long result, number; 10 11 printf( "Enter an integer: " ); 12 scanf( "%ld", &number ); 13 result = fibonacci( number ); 14 printf( "Fibonacci( %ld ) = %ldn", number, result ); 15 return 0; 16 } 17 18 /* Recursive definition of function fibonacci */ 19 long fibonacci( long n ) 20 { 21 if ( n == 0 || n == 1 ) 22 return n; 23 else 24 return fibonacci( n - 1 ) + fibonacci( n - 2 ); 25 } Enter an integer: 0 Fibonacci(0) = 0 Enter an integer: 1 Fibonacci(1) = 1
  38. 38. © 2000 Prentice Hall, Inc. All rights reserved. OutlineOutline Program Output Enter an integer: 2 Fibonacci(2) = 1 Enter an integer: 3 Fibonacci(3) = 2 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) = 832040 Enter an integer: 35 Fibonacci(35) = 9227465
  39. 39. © 2000 Prentice Hall, Inc. All rights reserved. 5.15 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 – Choice between performance (iteration) and good software engineering (recursion)
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