Functions in C Programming Dr. Ahmed Telba

If else // if #include using namespace std; int main() { unsigned short dnum ; cout<< "Enter number of day(1-7): "; cin>> dnum; cout<< "\n"; if(dnum == 1) cout << "the day is Friday"; else if(dnum == 2) cout << "the day is Saturday"; else if(dnum == 3) cout << "the day is Sunday"; else if(dnum == 4) cout << "the day is Monday"; else if(dnum == 5) cout << "the day is Tuesday"; else if(dnum == 6) cout << "the day is Tuesday"; else if(dnum == 7) cout << "the day is Thursday"; else cout << "Sorry we're closed "; cout<<'\n'; system("pause"); return 0; }

Switch case --- // switch #include using namespace std; int main() { unsigned short dnum ; cout<< "Enter number of day(1-7): "; cin>> dnum; cout<< "\n"; switch(dnum){ case 1: // if dnum = 1 cout << "the day is Friday"; break; case 2: // if dnum = 2 cout << "the day is Saturday"; break; case 3: // if dnum = 3 cout << "the day is Sunday"; break; case 4: // if dnum = 4 cout << "the day is Monday"; break; case 5: // if dnum = 5 cout << "the day is Tuesday"; break; case 6: // if dnum = 6 cout << "the day is Wednesday"; break; case 7: // if dnum = 7 cout << "the day is Thursday"; break; default: // if dnum 7 cout << "Sorry we're closed "; break; } cout<< "\n"; system("pause"); return 0; }

Type of Variables Global Variables – Variables declared before main() function. – Global variables are defined to all parts in the program – If Global variable is changed in any part in the program, this change appears in all the program Local Variables – Variables defined inside different functions, including main() function – Local variables are defined only in their functions Note: DO NOT use a local variable of the same name as a global variable

Function

Example #include int I; // I is a global variable main() { int K; /* K is a local varaible defined in the main */ }

Functions

Return and Input Types

Function Declarations

Functions (I) Using functions we can structure our programs in a more modular way, accessing all the potential that structured programming can offer to us in C++. A function is a group of statements that is executed when it is called from some point of the program. The following is its format: type name ( parameter1, parameter2,...) { statements } where: type is the data type specified of the data returned by the function. name is the identifier by which it will be possible to call the function. parameters (as many as needed): Each parameter consists of a data type specified followed by an identifier, like any regular variable declaration (for example: int x) and which acts within the function as a regular local variable. They allow to pass arguments to the function when it is called. The different parameters are separated by commas. statements is the function's body. It is a block of statements surrounded by braces { }.

Example #include void print2number(int,int); main() { int x,y; x=3; y=4; print2number(x,y); } void print2number(int number1,int number2) { cout<<”number1,number2,number3; }

function example #include using namespace std; int addition (int a, int b) { int r; r=a+b; return (r); } int main () { int z; z = addition (5,3); cout << "The result is " << z; return 0; } The result is 8

Math Library Functions Revisited

Introduction Function Definition Void function Global Vs Local variables Random Number Generator Recursion Function Overloading Sample Code C++ PROGRAMMING SKILLS Part 3 User-Defined Functions

Functions in C++ Experience has shown that the best way to develop and maintain large programs is to construct it from smaller pieces(Modules) This technique Called “Divide and Conquer” main() { Return 0; } Bad Development Approach Easer To Design Build Debug Extend Modify Understand Reuse Better Organization Wise Development Approach main() { } function f1() { --- } function f2() { --- }

Functions in C++(Cont.) In FORTRAN Modules Known as Subprograms In Pascal Modules known as Procedures & Functions In C++ Modules Known as Functions & Classes Programs use new and “prepackaged” modules – New: programmer-defined functions and classes – Prepackaged: from the standard library

About Functions in C++ Functions invoked by a function–call-statement which consist of it’s name and information it needs (arguments) Boss To Worker Analogy  A Boss (the calling/caller function) asks a worker (the called function) to perform a task and return result when it is done. Main Boss Function AFunction B Function Z Worker Function B2Function B1 Worker Note: usual main( ) Calls other functions, but other functions can call each other

Function Calling Function Arguments can be: -Constant sqrt(9); -Variable sqrt(x); -Expression sqrt( x*9 + y) ; sqrt( sqrt(x) ) ; Functions called by writing functionName (argument); or functionName(argument1, argument2, …); Example cout << sqrt( ); sqrt (square root) function The preceding statement would print 30 All functions in math library return a double

Function Calling cout<< sqrt(9); Function Name argument 3 Output Parentheses used to enclose argument (s) Calling/invoking a function –sqrt(x); –Parentheses an operator used to call function Pass argument x Function gets its own copy of arguments –After finished, passes back result

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

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 Function Definition

Syntax format for function definition returned-value-type function-name (parameter-list) { Declarations of local variables 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)

Function Definition 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

#include using namespace std; // Creating and using a programmer-defined function. int square( int ); // function prototype int main() { // loop 10 times and calculate and output // square of x each time for ( int x = 1; x <= 10; x++ ) cout << square( x ) << " "; // function call cout << endl; system("pause"); return 0; // indicates successful termination } // end main // 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

#include using namespace std; int square(int); // prototype int cube(int); // prototype main() {int i; for (int i=1;i<=10;i++){ cout<< i<< "square=" << square(i) << endl; cout<< i<< "cube= " <<cube(i) << endl; } // end for system("pause"); return 0; } // end main function int square(int y) //function definition { return y*y; // returned Result } int cube(int y) //function definition { return y*y*y; // returned Result }

#include using namespace std; double maximum( double, double, double ); // function prototype int main() { double number1, number2; double number3; cout << "Enter three real numbers: "; cin >> number1 >> number2 >> number3; // number1, number2 and number3 are arguments to the maximum function call cout << "Maximum is: " << maximum( number1, number2, number3 ) << endl; system("pause"); return 0; } double maximum( double x, double y, double z ) { double max = x; // assume x is largest if ( y > max ) // if y is larger, max = y; // assign y to max if ( z > max ) // if z is larger, max = z; // assign z to max return max; // max is largest value } // end function maximum

Add two number #include using namespace std; void add2Nums(int,int); main() { int a, b; cout<<"enter tow Number:"; cin >>a >> b; add2Nums(a, b); system("pause"); return 0; } void add2Nums(int x, int y) { cout<< x<< "+" << y << "=" << x+y; }

function example #include using namespace std; int subtraction (int a, int b) { int r; r=a-b; return (r); } int main () { int x=5, y=3, z; z = subtraction (7,2); cout << "The first result is " << z << '\n'; cout << "The second result is " << subtraction (7,2) << '\n'; cout << "The third result is " << subtraction (x,y) << '\n'; z= 4 + subtraction (x,y); cout << "The fourth result is " << z << '\n'; return 0; }

#include using namespace std; void printmessage () { cout << "I'm a function!"; } int main () { printmessage (); return 0; }

x=2, y=6, z=14 /// passing parameters by reference #include using namespace std; void duplicate (int& a, int& b, int& c) { a*=2; b*=2; c*=2; } int main () { int x=1, y=3, z=7; duplicate (x, y, z); cout << "x=" << x << ", y=" << y << ", z=" << z; return 0; }

// more than one returning value #include using namespace std; void prevnext (int x, int& prev, int& next) { prev = x-1; next = x+1; } int main () { int x=100, y, z; prevnext (x, y, z); cout << "Previous=" << y << ", Next=" << z; return 0; } Previous=99, Next=101

// default values in functions #include using namespace std; int divide (int a, int b=2) { int r; r=a/b; return (r); } int main () { cout << divide (12); cout << endl; cout << divide (20,4); return 0; } 6 5

#include using namespace std; long factorial (long a) { if (a > 1) return (a * factorial (a-1)); else return (1); } int main () { long number; cout << "Please type a number: "; cin >> number; cout << number << "! = " << factorial (number); return 0; } Please type a number: 9 9! =

// declaring functions prototypes #include using namespace std; void odd (int a); void even (int a); int main () { int i; do { cout << "Type a number (0 to exit): "; cin >> i; odd (i); } while (i!=0); return 0; } void odd (int a) { if ((a%2)!=0) cout << "Number is odd.\n"; else even (a); } void even (int a) { if ((a%2)==0) cout << "Number is even.\n"; else odd (a); } Type a number (0 to exit): 9 Number is odd. Type a number (0 to exit): 6 Number is even. Type a number (0 to exit): 1030 Number is even. Type a number (0 to exit): 0 Nu

Example

Including Functions

Variable Scope

Example: Bessel Function Γ(n) = (n − 1)!

Example: Bessel Function doubleJ(int n,double xx) { doublexasymp=17.5; double tk,s; int NN=60,k,i; if(xx==0. && n==0) return 1.; if(xx<xasymp) { tk=pow((xx/2.),n)/fact(n); s=tk; for(k=0;k<=NN;k++) { tk=-tk*xx*xx/4./(k+1.)/(k+n+1.); s=s+tk; } else s=sqrt(2./Pi/xx)*cos(xx-Pi/4.-n*Pi/2.); return(s); }