Lecture#6 functions in c++
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The document discusses functions in C++. It defines functions as modular pieces that divide programs into more manageable components. It describes function components like modules, functions, classes, and function calls. It provides examples of math library functions and how to define, call, and prototype functions. It also covers function parameters, return types, and scope rules for local variables and storage classes.
Lecture#6 functions in c++
- 1. EC-102 Computer System & Programming Instructor: Jahan Zeb Department of Computer Engineering (DCE) College of E&ME
- 2. Functions Divide and conquer – Construct a program from smaller pieces or components – Each piece more manageable than the original program
- 3. Program Components in C++ Modules: functions and classes Programs use new and “prepackaged” modules – New: programmer-defined functions, – Prepackaged: from the standard library Functions invoked by function call – Function name and information (arguments) it needs Function definitions – Only written once – Hidden from other functions
- 4. Program Components in C++ Boss to worker analogy – 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.
- 5. Math Library Functions Perform common mathematical calculations – Include the header file <cmath> 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 – All functions in math library return a double
- 6. Math Library Functions Function arguments can be – Constants • sqrt( 4 ); – Variables • sqrt( x ); – Expressions • sqrt( sqrt( x ) ) ; • sqrt( 3 - 6x );
- 7. Method Description Example ceil( x ) rounds x to the smallest integer not less than x ceil( 9.2 ) is 10.0 ceil( -9.8 ) is -9.0 cos( x ) trigonometric cosine of x (x in radians) cos( 0.0 ) is 1.0 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 not greater than x floor( 9.2 ) is 9.0 floor( -9.8 ) is -10.0 fmod( x, y ) remainder of x/y as a floating-point number fmod( 13.657, 2.333 ) is 1.992 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 (x in radians) sin( 0.0 ) is 0 sqrt( x ) square root of x sqrt( 900.0 ) is 30.0 sqrt( 9.0 ) is 3.0 tan( x ) trigonometric tangent of x (x in radians) tan( 0.0 ) is 0
- 8. 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
- 9. Function Definitions Function prototype – Tells compiler argument type and return type of function – int square( int ); • Function takes an int and returns an int 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
- 10. 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)
- 11. 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
- 12. 1 2 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 int square( int ); // function prototype 9 10 int main() 11 { 12 // loop 10 times and calculate and output 13 // square of x each time 14 for ( int x = 1; x <= 10; x++ ) 15 cout << square( x ) << " "; // function call 16 17 cout << endl; 18 19 return 0; // indicates successful termination 20 21 } // end main 22 Parentheses () cause function to be called. When done, it returns the result. Function prototype: specifies data types of arguments and return values. square expects an int, and returns an int.
- 13. 23 // square function definition returns square of an integer 24 int square( int y ) // y is a copy of argument to function 25 { 26 return y * y; // returns square of y as an int 27 28 } // end function square 1 4 9 16 25 36 49 64 81 100 Definition of square. y is a copy of the argument passed. Returns y * y, or y squared.
- 14. 1 2 // Finding the maximum of three floating-point numbers. 3 #include <iostream> 4 5 using std::cout; 6 using std::cin; 7 using std::endl; 8 9 double maximum( double, double, double ); // function prototype 10 11 int main() 12 { 13 double number1; 14 double number2; 15 double number3; 16 17 cout << "Enter three floating-point numbers: "; 18 cin >> number1 >> number2 >> number3; 19 20 // number1, number2 and number3 are arguments to 21 // the maximum function call 22 cout << "Maximum is: " 23 << maximum( number1, number2, number3 ) << endl; 24 25 return 0; // indicates successful termination Function maximum takes 3 arguments (all double) and returns a double.
- 15. 26 27 } // end main 28 29 // function maximum definition; 30 // x, y and z are parameters 31 double maximum( double x, double y, double z ) 32 { 33 double max = x; // assume x is largest 34 35 if ( y > max ) // if y is larger, 36 max = y; // assign y to max 37 38 if ( z > max ) // if z is larger, 39 max = z; // assign z to max 40 41 return max; // max is largest value 42 43 } // 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 Comma separated list for multiple parameters.
- 16. Function Prototypes Function prototype contains – Function name – Parameters (number and data type) – Return type (void if returns nothing) – Only needed if function definition after function call Prototype must match function definition – Function prototype double maximum( double, double, double ); – Definition double maximum( double x, double y, double z ) { … }
- 17. Function Prototypes Function signature – Part of prototype with name and parameters • double maximum( double, double, double ); Argument Coercion – Force arguments to be of proper type • Converting int (4) to double (4.0) cout << sqrt(4) – Conversion rules • Arguments usually converted automatically • Changing from double to int can truncate data – 3.4 to 3 Function signature
- 18. break and continue Statements break statement – Immediate exit from while, for, do/while, switch – Program continues with first statement after structure Common uses – Escape early from a loop – Skip the remainder of switch
- 19. 1 2 // Using the break statement in a for structure. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 // function main begins program execution 9 int main() 10 { 11 12 int x; // x declared here so it can be used after the loop 13 14 // loop 10 times 15 for ( x = 1; x <= 10; x++ ) { 16 17 // if x is 5, terminate loop 18 if ( x == 5 ) 19 break; // break loop only if x is 5 20 21 cout << x << " "; // display value of x 22 23 } // end for 24 25 cout << "nBroke out of loop when x became " << x << endl; Exits for structure when break executed.
- 20. 26 27 return 0; // indicate successful termination 28 29 } // end function main 1 2 3 4 Broke out of loop when x became 5
- 21. break and continue Statements continue statement – Used in while, for, do/while – Skips remainder of loop body – Proceeds with next iteration of loop while and do/while structure – Loop-continuation test evaluated immediately after the continue statement for structure – Increment expression executed – Next, loop-continuation test evaluated
- 22. 1 2 // Using the continue statement in a for structure. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 // function main begins program execution 9 int main() 10 { 11 // loop 10 times 12 for ( int x = 1; x <= 10; x++ ) { 13 14 // if x is 5, continue with next iteration of loop 15 if ( x == 5 ) 16 continue; // skip remaining code in loop body 17 18 cout << x << " "; // display value of x 19 20 } // end for structure 21 22 cout << "nUsed continue to skip printing the value 5" 23 << endl; 24 25 return 0; // indicate successful termination Skips to next iteration of the loop.
- 23. 26 27 } // end function main 1 2 3 4 6 7 8 9 10 Used continue to skip printing the value 5
- 24. 1 2 // Shifted, scaled integers produced by 1 + rand() % 6. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 #include <iomanip> 9 10 using std::setw; 11 12 #include <cstdlib> // contains function prototype for rand 13 14 int main() 15 { 16 // loop 20 times 17 for ( int counter = 1; counter <= 20; counter++ ) { 18 19 // pick random number from 1 to 6 and output it 20 cout << setw( 10 ) << ( 1 + rand() % 6 ); 21 22 // if counter divisible by 5, begin new line of output 23 if ( counter % 5 == 0 ) 24 cout << endl; 25 26 } // end for structure Output of rand() scaled and shifted to be a number between 1 and 6.
- 25. 27 28 return 0; // indicates successful termination 29 30 } // end main 6 6 5 5 6 5 1 1 5 3 6 6 2 4 2 6 2 3 4 1
- 26. Storage Classes Variables have attributes – Have seen name, type, size, value – Storage class • How long variable exists in memory – Scope • Where variable can be referenced in program – Linkage • For multiple-file program
- 27. Storage Classes Automatic storage class – Variable created when program enters its block – Variable destroyed when program leaves block – Only local variables of functions can be automatic • Automatic by default • keyword auto explicitly declares automatic – register keyword • Hint to place variable in high-speed register • Good for often-used items (loop counters) • Often unnecessary, compiler optimizes – Specify either register or auto, not both • register int counter = 1;
- 28. Storage Classes Static storage class – Variables exist for entire program – May not be accessible, scope rules still apply (more later) static keyword – Local variables in function – Keeps value between function calls – Only known in own function extern keyword – Default for global variables/functions • Globals: defined outside of a function block – Known in any function that comes after it
- 29. Scope Rules Scope – Portion of program where identifier can be used File scope – Defined outside a function, known in all functions – Global variables, function definitions and prototypes Function scope – Can only be referenced inside defining function
- 30. Scope Rules Block scope – Begins at declaration, ends at right brace } • Can only be referenced in this range – Local variables, function parameters – static variables still have block scope • Storage duration does not affect scope Function-prototype scope – Parameter list of prototype – Names in prototype optional • Compiler ignores – In a single prototype, name can be used once
- 31. 1 2 // A scoping example. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 void useLocal( void ); // function prototype 9 void useStaticLocal( void ); // function prototype 10 void useGlobal( void ); // function prototype 11 12 int x = 1; // global variable 13 14 int main() 15 { 16 int x = 5; // local variable to main 17 18 cout << "local x in main's outer scope is " << x << endl; 19 20 { // start new scope 21 22 int x = 7; 23 24 cout << "local x in main's inner scope is " << x << endl; 25 26 } // end new scope Declared outside of function; global variable with file scope. Local variable with function scope. Create a new block, giving x block scope. When the block ends, this x is destroyed.
- 32. 27 28 cout << "local x in main's outer scope is " << x << endl; 29 30 useLocal(); // useLocal has local x 31 useStaticLocal(); // useStaticLocal has static local x 32 useGlobal(); // useGlobal uses global x 33 useLocal(); // useLocal reinitializes its local x 34 useStaticLocal(); // static local x retains its prior value 35 useGlobal(); // global x also retains its value 36 37 cout << "nlocal x in main is " << x << endl; 38 39 return 0; // indicates successful termination 40 41 } // end main 42
- 33. 43 // useLocal reinitializes local variable x during each call 44 void useLocal( void ) 45 { 46 int x = 25; // initialized each time useLocal is called 47 48 cout << endl << "local x is " << x 49 << " on entering useLocal" << endl; 50 ++x; 51 cout << "local x is " << x 52 << " on exiting useLocal" << endl; 53 54 } // end function useLocal 55 Automatic variable (local variable of function). This is destroyed when the function exits, and reinitialized when the function begins.
- 34. 56 // useStaticLocal initializes static local variable x only the 57 // first time the function is called; value of x is saved 58 // between calls to this function 59 void useStaticLocal( void ) 60 { 61 // initialized only first time useStaticLocal is called 62 static int x = 50; 63 64 cout << endl << "local static x is " << x 65 << " on entering useStaticLocal" << endl; 66 ++x; 67 cout << "local static x is " << x 68 << " on exiting useStaticLocal" << endl; 69 70 } // end function useStaticLocal 71 Static local variable of function; it is initialized only once, and retains its value between function calls.
- 35. 72 // useGlobal modifies global variable x during each call 73 void useGlobal( void ) 74 { 75 cout << endl << "global x is " << x 76 << " on entering useGlobal" << endl; 77 x *= 10; 78 cout << "global x is " << x 79 << " on exiting useGlobal" << endl; 80 81 } // 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 This function does not declare any variables. It uses the global x declared in the beginning of the program.
- 36. 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
- 37. 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 );
- 38. 1 2 // Functions that take no arguments. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 void function1(); // function prototype 9 void function2( void ); // function prototype 10 11 int main() 12 { 13 function1(); // call function1 with no arguments 14 function2(); // call function2 with no arguments 15 16 return 0; // indicates successful termination 17 18 } // end main 19
- 39. 20 // function1 uses an empty parameter list to specify that 21 // the function receives no arguments 22 void function1() 23 { 24 cout << "function1 takes no arguments" << endl; 25 26 } // end function1 27 28 // function2 uses a void parameter list to specify that 29 // the function receives no arguments 30 void function2( void ) 31 { 32 cout << "function2 also takes no arguments" << endl; 33 34 } // end function2 function1 takes no arguments function2 also takes no arguments