Week 1-2 - Functions: 2003 Prentice Hall, Inc. All Rights Reserved

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Week 1-2 - Functions Outline • Introduction • Program Components in C++ • Functions • Function Definitions • Function Prototypes • Math Library Functions • Storage Classes • Scope Rules • Recursion • Example Using Recursion: The Factorial Number • Functions with Empty Parameter Lists • References and Reference Parameters • Call by value • Call by reference • Default Arguments • Function Overloading

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Introduction • Reason of using functions – Divide and conquer • Construct a program from smaller pieces or components • Each piece more manageable than the original program

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Program Components in C++ • Modules: functions and classes • Programs use new and “prepackaged” modules – New: programmer-defined functions, classes (user-defined functions) – Prepackaged: from the standard library (Reserved functions)

• Functions invoked by function call – Function name and information (arguments) it needs

• Function definitions – Only written once – Hidden from other functions

• Boss to worker correspondence – A boss (the calling function or caller) asks a worker (the called function) to perform a task and return (i.e., report back) the results when the task is done.  2003 Prentice Hall, Inc. All rights reserved.

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Functions • Functions – Modularize a program – Software reusability • Call function multiple times

• Local variables – Known only in the function in which they are defined – All variables declared in function definitions are local variables

• Parameters – Local variables passed to function when called – Provide outside information

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Function Definitions • Function prototype – Tells compiler argument type and return type of function – int square( int ); • Function takes an int and returns an int

– Explained in more detail later

• Calling/invoking a function – square(x); – Parentheses an operator used to call function • Pass argument x • Function gets its own copy of arguments

– After finished, passes back result

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Function Definitions • Format for function definition return-value-type function-name( parameter-list ) { declarations and statements } – Parameter list • Comma separated list of arguments – Data type needed for each argument • If no arguments, use void or leave blank

– Return-value-type • Data type of result returned (use void if nothing returned)

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Function Definitions • Example function int square( int y ) { return y * y; }

• return keyword – Returns data, and control goes to function’s caller • If no data to return, use return;

– Function ends when reaches right brace • Control goes to caller

• Functions cannot be defined inside other functions • Next: program examples  2003 Prentice Hall, Inc. All rights reserved.

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// Fig. 3.3: fig03_03.cpp // Creating and using a programmer-defined function. #include using std::cout; using std::endl; int square( int );

//

Function prototype: specifies data types of arguments and return values. square expects and int, and returns function prototype an int.

Outline fig03_03.cpp (1 of 2)

int main() { Parentheses () cause function // loop 10 times and calculate and output to be called. When done, it // square of x each time returns the result. for ( int x = 1; x <= 10; x++ ) cout << square( x ) << " "; // function call cout << endl; return 0;

// indicates successful termination

} // end main

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// square function definition returns square of an integer int square( int y ) // y is a copy of argument to function { return y * y; // returns square of y as an int } // end function square

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Definition of square. y is a copy of the argument passed. Returns y * y, or y squared.

Outline fig03_03.cpp (2 of 2) fig03_03.cpp output (1 of 1)

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// Fig. 3.4: fig03_04.cpp // Finding the maximum of three floating-point numbers. #include

Outline fig03_04.cpp (1 of 2)

using std::cout; using std::cin; using std::endl; double maximum( double, double, double ); // function prototype int main() { double number1; double number2; double number3;

Function maximum takes 3 arguments (all double) and returns a double.

cout << "Enter three floating-point numbers: "; cin >> number1 >> number2 >> number3; // number1, number2 and number3 are arguments to // the maximum function call cout << "Maximum is: " << maximum( number1, number2, number3 ) << endl; return 0;

// indicates successful termination

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Outline

} // end main

Comma separated list for multiple parameters.

// function maximum definition; // x, y and z are parameters double maximum( double x, double y, double z ) { double max = x; // assume x is largest if ( y > max ) max = y;

// if y is larger, // assign y to max

if ( z > max ) max = z;

// if z is larger, // assign z to max

return max;

// max is largest value

fig03_04.cpp (2 of 2) fig03_04.cpp output (1 of 1)

} // end function maximum

Enter three floating-point numbers: 99.32 37.3 27.1928 Maximum is: 99.32 Enter three floating-point numbers: 1.1 3.333 2.22 Maximum is: 3.333 Enter three floating-point numbers: 27.9 14.31 88.99 Maximum is: 88.99

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Function Prototypes • Function prototype contains – Function name – Parameters (number and data type) – Return type (void if returns nothing)

• Prototype must match function definition – Function prototype double maximum( double, double, double );

– Function definition double maximum( double x, double y, double z ) { … }

• Function signature – Part of prototype with name and parameters • double maximum( double, double, double );  2003 Prentice Hall, Inc. All rights reserved.

Function signature

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Function Prototypes Data types

Size in bytes

long double                                           2 bytes  double                                         8 bytes float                                           4 bytes  unsigned long int 4 bytes long int 4 bytes long double 10 bytes unsigned int 2 bytes int 2 bytes unsigned short int 2 bytes short int 2 bytes unsigned char 1 byte char 1 byte bool 1 byte Fig. 3.5 Promotion hierarchy for built-in data types.

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Math Library Functions • Perform common mathematical calculations – Include the header file

• Functions called by writing – functionName (argument); or – functionName(argument1, argument2, …);

• Example cout << sqrt( 900.0 ); – sqrt (square root) function The preceding statement would print 30

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Math Library Functions • Function arguments can be – Constants • sqrt( 4 );

– Variables • sqrt( x );

– Expressions • sqrt( sqrt( x ) ) ; • sqrt( 3 - 6x );

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Method ceil( x )

Desc ription Example rounds x to the smallest integer ceil( 9.2 ) is 10.0 not less than x ceil( -9.8 ) is -9.0 cos( x ) trigonometric cosine of x cos( 0.0 ) is 1.0 (x in radians) exp( x ) exponential function ex exp( 1.0 ) is 2.71828 exp( 2.0 ) is 7.38906 fabs( x ) absolute value of x fabs( 5.1 ) is 5.1 fabs( 0.0 ) is 0.0 fabs( -8.76 ) is 8.76 floor( x ) rounds x to the largest integer floor( 9.2 ) is 9.0 not greater than x floor( -9.8 ) is -10.0 fmod( x, y ) remainder of x/y as a floating- fmod( 13.657, 2.333 ) is 1.992 point number log( x ) natural logarithm of x (base e) log( 2.718282 ) is 1.0 log( 7.389056 ) is 2.0 log10( x ) logarithm of x (base 10) log10( 10.0 ) is 1.0 log10( 100.0 ) is 2.0 pow( x, y ) x raised to power y (xy) pow( 2, 7 ) is 128 pow( 9, .5 ) is 3 sin( x ) trigonometric sine of x sin( 0.0 ) is 0 (x in radians) sqrt( x ) square root of x sqrt( 900.0 ) is 30.0 sqrt( 9.0 ) is 3.0 tan( x ) trigonometric tangent of x tan( 0.0 ) is 0 (x in radians) Fig. 3.2 Math library func tions.  2003 Prentice Hall, Inc. All rights reserved.

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Storage Classes • Variables have attributes – name, type, size, value – Storage class • How long variable exists in memory

– Scope • Where variable is visible in program

– Lifetime • The existence of variable for entire program or during call to the function or execution of block

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Storage Classes • Automatic storage class – Variable created when program enters its block – Variable destroyed when program leaves block – The complier starts off with an arbitrary value, (i.e. 0 or something else), or you can initialize explicitly – Lifetime of automatic variables is up to the time when function in which it is defined is executing – The scope is used to describe the visibility of a variable, meaning they can only accessed within that part of the program (function) in which it is defined – Only local variables of functions can be automatic • Automatic by default • keyword auto explicitly declares automatic – Not initialized

• Register storage class – register keyword • Hint: To place variable in high-speed register • Good for often-used items (loop counters) – Specify either register or auto, not both • register int counter = 1;  2003 Prentice Hall, Inc. All rights reserved.

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Automatic variables • •

Using auto keyword is same as not using that keyword For example:

void somefunc( ) { int somevar; // variables defined within float othervar; // the function body // other statements } void somefunc( ) { auto int somevar; //same as int somevar auto float othervar; //same as float othervar // other statements }

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Auto variables: visibility •

Automatic variables are visible or accessed only within the function in which they are defined

•

For example: void somefunc ( ) { int somevar; float othervar; somevar = 10; othervar = 11; nextvar = 12; } void otherfunc ( ) { int nextvar; somevar = 20; othervar = 21; nextvar = 22; }

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// automatic variable

// ok // ok // illegal: not visible in somefunc()

// automatic variable // illegal: not visible in otherfunc() // illegal: not visible in otherfunc() // ok

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Storage Classes (con’t) •

Static storage class – Variables exist for entire program – Static variables remember or preserve the value between function calls – Once initialized for the entire program – Visibility , inside the function in which it is defined, but it remains in existence for the life of the program – Lifetime of static variables is up to the existence for life of the program – Initialized automatically with 0 • static keyword – Local variables in function – Keeps value between function calls – Only known in own function  2003 Prentice Hall, Inc. All rights reserved.

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Storage Classes (con’t) •

External storage class – Declared outside to any function – Visible to all the functions – Lifetime of extern variables is up to the existence for life of the program – Initialized automatically with 0 • extern keyword – Default for global variables/functions • Defined outside of a function block – Also called global variables – Known in any function that comes after it

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Scope Rules • Scope – Portion of program where identifier can be used

• File scope (extern variables) – Defined outside a function, known in all functions – E.g. Global variables, function definitions and prototypes

• Function scope – Can only be referenced (visible) inside defining function

• Block scope (static variables) – Begins at declaration, ends at right brace } – Local variables, function parameters – static variables still have block scope • Storage class separate from scope

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// Fig. 3.12: fig03_12.cpp // A scoping example. #include

Outline fig03_12.cpp (1 of 5)

using std::cout; using std::endl; void useLocal( void ); // function prototype Declared outside of function; void useStaticLocal( void ); // function prototype global variable with file void useGlobal( void ); // function prototype

scope.

int x = 1;

// global variable

int main() { int x = 5;

// local variable to main

Local variable with function scope.

cout << "local x in main's outer scope is block, " << xgiving << endl; Create a new x { // start new scope

block scope. When the block ends, this x is destroyed.

int x = 7; cout << "local x in main's inner scope is " << x << endl; } // end new scope

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cout << "local x in main's outer scope is " << x << endl; useLocal(); useStaticLocal(); useGlobal(); useLocal(); useStaticLocal(); useGlobal();

// // // // // //

useLocal has local x useStaticLocal has static local x useGlobal uses global x useLocal reinitializes its local x static local x retains its prior value global x also retains its value

Outline fig03_12.cpp (2 of 5)

cout << "\nlocal x in main is " << x << endl; return 0;

// indicates successful termination

} // end main

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// useLocal reinitializes local variable x during each call void useLocal( void ) { auto int x = 25; // initialized each time useLocal is called cout << << ++x; cout << <<

endl << "local x isAutomatic " << x variable (local variable function). This " on entering useLocal" << of endl;

Outline fig03_12.cpp (3 of 5)

is destroyed when the function "local x is " << x exits, and reinitialized when " on exiting useLocal" << endl; the function begins.

} // end function useLocal

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// useStaticLocal initializes static local variable x only the // first time the function is called; value of x is saved // between calls to this function void useStaticLocal( void ) { // initialized only first time useStaticLocal is called static int x = 50; cout << << ++x; cout << <<

Outline fig03_12.cpp (4 of 5)

endl << "local static x is " << x " on entering useStaticLocal" << endl;

local variable of "local static x is " << Static x function; it is initialized " on exiting useStaticLocal" << endl;

} // end function useStaticLocal

only once, and retains its value between function calls.

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// useGlobal modifies global variable x during each call void useGlobal( void ) { cout << endl << "global x is " << x This function does not declarefig03_12.cpp << " on entering useGlobal" << endl; any variables. It uses the (5 of 5) x *= 10; global x declared in the cout << "global x is " << x beginning of the program. fig03_12.cpp << " on exiting useGlobal" << endl;

Outline

output (1 of 2)

} // end function useGlobal

local x in main's outer scope is 5 local x in main's inner scope is 7 local x in main's outer scope is 5 local x is 25 on entering useLocal local x is 26 on exiting useLocal local static x is 50 on entering useStaticLocal local static x is 51 on exiting useStaticLocal global x is 1 on entering useGlobal global x is 10 on exiting useGlobal local x is 25 on entering useLocal local x is 26 on exiting useLocal local static x is 51 on entering useStaticLocal local static x is 52 on exiting useStaticLocal global x is 10 on entering useGlobal global x is 100 on exiting useGlobal local x in main is 5

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Storage types automatic

static

external

Visibility

function

function

file

Lifetime

function

program

program

Initialized  value

Not initialized

0

0

Storage

Stack segment

Data segment

Data segment

Purpose

Variable used by a  single function

Same as auto, but  Variables used  must retain value  by several  when function  functions terminates

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Recursion • Recursive functions – Functions that call themselves – Can only solve a base case

• If not base case – Break problem into smaller problem(s) – Perform recursive call/recursive step • Slowly converges towards base case • Function makes call to itself inside the return statement

– Eventually base case gets solved • Solves entire problem

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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)

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// Fig. 3.14: fig03_14.cpp // Recursive factorial function. #include

Outline fig03_14.cpp (1 of 2)

using std::cout; using std::endl; #include using std::setw;

// header fileunsigned for “setw” Data type

long can hold an integer from 0 to 4 billion.

unsigned long factorial( unsigned long ); // function prototype int main() { Set width // Loop 10 times. During each iteration, calculate Set the field width to allow // factorial( i ) and display result. insertion for ( int i = 0; i <= 10; i++ next ) cout << setw( 2 ) << i << "! = " << factorial( i ) << endl; return 0;

// indicates successful termination

} // end main

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// recursive definition of function factorial base) case occurs when unsigned long factorial( unsigned long The number have 0! or 1!. All other { cases must be split up // base case if ( number <= 1 ) (recursive step). return 1; // recursive step else return number * factorial( number - 1 );

Outline we fig03_14.cpp (2 of 2) fig03_14.cpp output (1 of 1)

} // end function factorial = = = = = = = = = = =

1 1 2 6 24 120 720 5040 40320 362880 3628800

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Functions with Empty Parameter Lists • Empty parameter lists – void or leave parameter list empty – Indicates function takes no arguments – Function print takes no arguments and returns no value • void print(); • void print( void );

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// Fig. 3.18: fig03_18.cpp // Functions that take no arguments. #include

Outline fig03_18.cpp (1 of 2)

using std::cout; using std::endl; void function1(); void function2( void );

// function prototype // function prototype

int main() { function1(); function2();

// call function1 with no arguments // call function2 with no arguments

return 0;

// indicates successful termination

} // end main

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// function1 uses an empty parameter list to specify that // the function receives no arguments void function1() { cout << "function1 takes no arguments" << endl; } // end function1 // function2 uses a void parameter list to specify that // the function receives no arguments void function2( void ) { cout << "function2 also takes no arguments" << endl;

Outline fig03_18.cpp (2 of 2) fig03_18.cpp output (1 of 1)

} // end function2

function1 takes no arguments function2 also takes no arguments

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References and Reference Parameters • Call by value – Copy of data passed to function – Changes to copy do not change original – Prevent unwanted side effects

• Call by reference – – – –

Function can directly access data No new copy of data passed to function Only address of the variable is passed to function Original variable is accessed in a function with reference to its second name or alias – Both variables use the same memory location – Changes affect original  2003 Prentice Hall, Inc. All rights reserved.

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// Fig. 5.6: fig05_06.cpp // Cube a variable using pass-by-value. #include

Outline fig05_06.cpp (1 of 2)

using std::cout; using std::endl; int cubeByValue( int );

// prototype

int main() { int number = 5;

number cout << "The original value of number is " Pass << number; // pass number by value to cubeByValue number = cubeByValue( number );

by value; result returned by cubeByValue

cout << "\nThe new value of number is " << number << endl; return 0;

// indicates successful termination

} // end main

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// calculate and return cube of integer argument int cubeByValue( int n ) { cubeByValue receives return n * n * n; // cube local variable n and return result

parameter passed-by-value

} // end function cubeByValue

The original value of number is 5 The new value of number is 125

Cubes and returns local variable n

Outline fig05_06.cpp (2 of 2) fig05_06.cpp output (1 of 1)

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Calling Functions by Reference • Pass-by-reference with pointer arguments – Simulate pass-by-reference • Use pointers and indirection operator

– Pass address of argument using & operator – Arrays not passed with & because array name already pointer – * operator used as alias/nickname for variable inside of function

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// Fig. 3.20: fig03_20.cpp // Comparing pass-by-value and pass-by-reference // with references. #include using std::cout; using std::endl; int squareByValue( int ); void squareByReference( int & );

Notice the & operator, indicating pass-by-reference.

Outline fig03_20.cpp (1 of 2)

// function prototype // function prototype

int main() { int x = 2; int z = 4; // demonstrate squareByValue cout << "x = " << x << " before squareByValue\n"; cout << "Value returned by squareByValue: " << squareByValue( x ) << endl; cout << "x = " << x << " after squareByValue\n" << endl;

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// demonstrate squareByReference cout << "z = " << z << " before squareByReference" << endl; squareByReference( z ); cout << "z = " << z << " after squareByReference" << endl; return 0; // indicates successful termination } // end main

Outline fig03_20.cpp (2 of 2)

Changes number, but original parameter (x) is not squareByValue multiplies number by itself, stores the modified. result in number and returns the new value of number

// // int squareByValue( int number ) { return number *= number; // caller's argument not modified } // end function squareByValue

Changes numberRef, an squareByReference multiplies numberRef by itself alias and for the original stores the result in the variable to which numberRef parameter. Thus, z is refers in function main changed.

// // // void squareByReference( int &numberRef ) { numberRef *= numberRef; // caller's argument modified } // end function squareByReference

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x = 2 before squareByValue Value returned by squareByValue: 4 x = 2 after squareByValue z = 4 before squareByReference z = 16 after squareByReference

Outline fig03_20.cpp output (1 of 1)

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// Fig. 5.7: fig05_07.cpp // Cube a variable using pass-by-reference // with a pointer argument. #include using std::cout; using std::endl; void cubeByReference( int * );

Outline Prototype indicates parameter is pointer to int

fig05_07.cpp (1 of 2)

// prototype

int main() { int number = 5;

Apply address operator & to pass address of number to cubeByReference cubeByReference

cout << "The original value of number is " << number; // pass address of number to cubeByReference( &number );

cout << "\nThe new value of number is " << number << endl; return 0; } // end main

// indicates successful termination

cubeByReference modified variable number

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// calculate cube of *nPtr; modifies variable number in main void cubeByReference( int *nPtr ) { *nPtr = *nPtr * *nPtr * *nPtr; // cube *nPtr } // end function cubeByReference

The original value of number is 5 The new value of number is 125

cubeByReference receives address of int variable, i.e., pointer to an int

Outline fig05_07.cpp (2 of 2) fig05_07.cpp output (1 of 1)

Modify and access int variable using indirection operator *

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Default Arguments in function prototype • Default arguments – Arguments in which data is initialized during function declaration – If values of the arguments are specified in the function declaration then the arguments of the function in the function call can be omitted – If the arguments are omitted in function call then the default values are automatically passed to the function definition

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// Fig. 3.18: fig03_18.cpp // Default arguments. #include using std::cout; using std::endl; void main() { void temp(char []=“Pakistan”, int = 2); clrscr(); temp( ); temp(“islamabad”, 6); cout<<“Ok”; } void temp(char x[15], int y) { for (int c=1;c<=y;c++) cout<<x<<endl; return 0;

Outline fig03_18.cpp (1 of 2) // function prototype

// indicates successful termination

} // end main

Pakistan Pakistan islamabad Islamabad Islamabad Islamabad Islamabad Islamabad

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Default Arguments in function prototype • Function call with omitted parameters – If not enough parameters, rightmost go to their defaults – Default values • Can be constants or global variables

• Set defaults in function prototype int myFunction( int x = 1, int y = 2, int z = 3 );

– myFunction(3) • x = 3, y and z get defaults (i.e. y=2 and z=3)

– myFunction(3, 5) • x = 3, y = 5 and z gets default (i.e. z=3)

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// Fig. 3.23: fig03_23.cpp // Using default arguments. #include using std::cout; using std::endl;

Outline Set defaults in function prototype.

fig03_23.cpp (1 of 2)

// function prototype that specifies default arguments int boxVolume( int length = 1, int width = 1, int height = 1 ); int main() { // no arguments--use default values for all dimensions cout << "The default box volume is: " << boxVolume(); // specify length; default width and height cout << "\n\nThe volume of a box with length 10,\n" << "width 1 and height 1 is: " << boxVolume( 10 );

Function calls with some parameters missing – the rightmost parameters get their defaults.

// specify length and width; default height cout << "\n\nThe volume of a box with length 10,\n" << "width 5 and height 1 is: " << boxVolume( 10, 5 );

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// specify all arguments cout << "\n\nThe volume of a box with length 10,\n" << "width 5 and height 2 is: " << boxVolume( 10, 5, 2 ) << endl; return 0;

// indicates successful termination

} // end main

Outline fig03_23.cpp (2 of 2) fig03_23.cpp output (1 of 1)

// function boxVolume calculates the volume of a box int boxVolume( int length, int width, int height ) { return length * width * height; } // end function boxVolume

The default box volume is: 1 The volume of a box with length 10, width 1 and height 1 is: 10 The volume of a box with length 10, width 5 and height 1 is: 50 The volume of a box with length 10, width 5 and height 2 is: 100

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Function Overloading • Function overloading – Functions with same name and different parameters – Should perform similar tasks • i.e. function to square ints and function to square floats int square( int x) {return x * x;} float square(float x) { return x * x; }

• Overloaded functions distinguished by signature – Based on name and parameter types (order matters)

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// Fig. 3.25: fig03_25.cpp // Using overloaded functions. #include using std::cout; using std::endl;

Outline Overloaded functions have the same name, but the different parameters distinguish them.

fig03_25.cpp (1 of 2)

// function square for int values int square( int x ) { cout << "Called square with int argument: " << x << endl; return x * x; } // end int version of function square // function square for double values double square( double y ) { cout << "Called square with double argument: " << y << endl; return y * y; } // end double version of function square

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int main() { int intResult = square( 7 ); // calls int version double doubleResult = square( 7.5 ); // calls double version cout << "\nThe square of integer 7 is " << intResult The proper function is called << "\nThe square of double 7.5 is " << doubleResult based upon the argument << endl;

(int or double).

return 0;

Outline fig03_25.cpp (2 of 2) fig03_25.cpp output (1 of 1)

// indicates successful termination

} // end main

Called square with int argument: 7 Called square with double argument: 7.5 The square of integer 7 is 49 The square of double 7.5 is 56.25

 2003 Prentice Hall, Inc. All rights reserved.

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