Chapter 7 - Functions

Functions u Code group that performs single task u Specification refers to what goes into and out of function u Design refers to function’s task(s) u Two groups –Library –Programmer-defined Lesson 7.1

Function Categories u Return no value u Return a single value u Use “pass by reference” Lesson 7.1

Function Basics u Need three things to use function –Function declaration (prototype) t Indicates function exists and describes it t Must be done before use of function –Function call (invocation) t Causes control to transfer from function to function –Function definition t Header line followed by function body enclosed in braces Lesson 7.1

Function Declaration u Indicates function name, return type and types of values passed to function u General form ftype fname (atype, atype); u ftype is function return type u fname is function name u atype is argument type (may be different) –multiple argument separated by commas Lesson 7.1

Function Call u Transfers program control to function u General form fname (exp1, exp2); u fname is function name u exp1 and exp2 are arguments –Variables or constants –Called argument list Lesson 7.1

Function Definition u Includes function’s executable statements u General form (example with two arguments) Lesson 7.1 Function return type Function argument declarations Function body ftype fname (atype aname, atype aname) { … }

Argument Lists u Lists in function call and function header must coordinate –number –order –type u Example: Lesson 7.1 kinetic_energy (mass, velocity); void kinetic_energy (int m, double v) Function call in main Function header 2 arguments in each mass should be declared an integer and velocity a double in program

Function Arrangement u Not necessary for first function to be main u Function definition must be located outside body of any other function u Can write function declaration outside body of all functions or within function –if within, can only be called from function where declared u Function declaration MUST appear before call Lesson 7.1

Function Storage u When function called, memory allocated for –Variables in argument list –Variables declared in function body u When completes execution, memory freed and variable values lost –Can prevent lost and maintain u Called multiple times, allocated and freed repeatedly Lesson 7.1

Common Errors u Argument order not matching between function call and header u Mismatching data types u Pass more information than function needs to complete task Lesson 7.1

Returning a Value u Two items needed –appropriate return type for function –return statement in function u Function declaration and definition list return data type int fact(double); int fact(double arg) u return statement form return expression; Lesson 7.2 Can put expression in ( ).

Return Statement u Considered to be jump statement u Can appear anywhere in function body u More than one return statement can appear in function Lesson 7.2 if (expression) { return (a); } else { return (b); }

Returning Value From main u void type function do not return values –control transfers back to calling function automatically int main ( ) return (0); u Returns value 0 to operating system indicating normal termination Lesson 7.2

Recap: Pass By Value u Default u Function called –Memory set aside and value copied into new memory location –Calculations done inside function using value –When function finished executing, memory location destroyed Lesson 7.2

Pass By Reference u Use argument list to allow function to directly use calling function's values u Reference symbol & required in function declaration and header –Indicate arguments that will have values modified –Create aliases for original variable names Lesson 7.3

Pass By Reference u Example: Lesson 7.3 void func_name (int, double, double&, int&); func_name (x, y, z, a); void func_name ( int b, double c, double& d, int& e) Declaration Call Header x y z a b c d e

Scope u Refers to region in which declaration is active u Three kinds of scope –Block t Variable valid with block of code enclosed in braces –Function t Variable valid only within function where defined –File t Variable valid throughout all modules in file u Determined by identifier's declaration location Lesson 7.4

Scope Lesson 7.4 File containing functions main ( ) and calc ( ) int days; Function calc ( ) int y; for (int n=0;n<8;n++) { } Function main ( ) int x; for (int j=0;j<8;j++) { } days x x y y nj

Storage Classes u Allows manual modification to scope and storage rules u Stated in the declaration u Four specifiers –register : store in register –auto : memory freed after function executes –static : memory persists after function executes –extern : global variables among files Lesson 7.5

static u Variable maintains storage space and value after function finishes executing u Memory reserved and initialized only once –First time function called u General form: static type variable; u Can be initialized in declaration –Done once, does not initialize on other calls Lesson 7.5

Comparing global and static Variables u Similarity –Permanent storage created for both u Difference –Scope t Static variables accessed only from function in which declared t Global variables accessed from any function –With extern from any file Lesson 7.5

extern u Multiple files help manage large programs u Each compiled separately then linked to create executable file u Share variable –Variable declared as global without specifier in one file –Other files extern type variable; Lesson 7.5

Default Arguments u Argument assigned particular value when argument omitted in function call u Values specified in function declaration u Must have ordinary argument listed prior to default arguments Lesson 7.6 type function_name ( ordinary arguments, default arguments);

Using Default Arguments u Calling function –Must be called with at least the number of ordinary arguments void commute (double, double = 15.0, int = 8); commute ( 40.0 ); Call uses 40.0 for first argument, then default values: 15.0 for second argument and 8 for third Cannot specify first and third and use default for second – must have all defaults last! Lesson 7.6

Function Overloading u Defining two or more functions with same name u Each function takes different number of arguments –C++ knows which function is being called by counting number of arguments used –Should not use default arguments since ambiguity could result Lesson 7.7

Generating Random Numbers u Need three functions –rand ( ) t Returns pseudorandom integer in range 0 to –srand ( ) t Operates with rand( ) using global variable invisible to programmer –time ( ) t Returns number of seconds from midnight u Need mod (%) operator to restrict range Lesson 7.8

Generating a Random Number u rand( ) and srand ( ) work together –srand ( ) automatically "seeds" function rand ( ) –Functions linked through global variable u time ( ) used to get true random effect srand (time (0)); rand ( ) returns single integer time of day in seconds Call to create random number Lesson 7.8

Random Number in Specific Range u Use mod operator u Example: n = rand ( ); roll = (n % 6) + 1; Simulate roll of die so result should be integer from 1 to 6 Lesson 7.8

Summary u Define and call functions u Determine the scope of a variable u Pass values by reference u Overload functions u Create random numbers Chapter 7 Learned how to: