Introduction to Systems Programming - COMP 1002, 1402

Transcription

1 FUNCTION AND PROCEDURES Introduction to Systems Programming - COMP 1002, 1402

2 Outline Why Function? What is function? How to use functions? Libraries Function call Variables Scope

3 Developing Large Programs Difficult to manage large code Teams of people may be involved Easy to forget what the project is all about People may leave the company Result: Code should be well organized well commented and documented Organized in a modular form (small coherent parts)

4 Top Down Design Begin with the main problem/idea Divide it into smaller problem/ideas Further divide each small problem/idea into smaller more manageable parts. The process is termed: Divide and Conquer When to stop? When the sub-problem/idea is understood and manageable Coding rule of thumb a module is 5-30 lines (one page)

5 Top-Down Module Example

6 Functions as Modules of Code Functions are the basic modules in C Every program has a main function The main function uses other functions Each function may use other functions Functions help organize and develop code

7 Functions : Organized Development 1. Allow organized development 2. Develop and test small pieces of code 3. Functions assignable to team members 4. Permit code reusability

8 Function and Organization

9 What is a Function? A function is a module (piece of code) aimed at accomplishing a single task Note: the scope of the task or its objective may large (recall breakdown of a problem/ideas)

10 What is a Function?

11 Function Definition return_type function_name (parameter list) { local variables code statements/expressions return expression

12 Function Definition Example: The Actual function to hypotenuse. /*Function Hypotenuse */ /*Input: Two sides of a Right triangle */ /*Output: The Hypotenuse */ double hypot(double x, double y) { double hypotenuse = sqrt( x*x + y*y); return (hypotenuse);

13 Function Definition Example: The Actual function to hypotenuse. /*Function Hypotenuse */ /*Input: Two sides of a Right triangle */ /*Output: The Hypotenuse */ double hypot(double x, double y) { return (sqrt( x*x + y*y));

14 Functions main() is a function. You can declare other functions with other names. Output of the function is the return value. Inputs to the functions are the parameters. On declaration, the type of the output and inputs must be declared. int square(int x) { printf( square ); return x * x; float sum(float x, float y) { return x + y;

15 Output of functions The output (return value) of functions can be used. They can also be used as inputs to other functions. int i, j; float f; j = 2; i = 5 + square(j); j = square(square(j)); f = sum(square(3),2.0); square(2) = 4; // error i = square(j)++; // error

16 Exercise What does this program print? #include <stdio.h> int check(int m) { printf( checking\n ); if (m>50) { return 0; else { return 1; int main() { int i; for (i=2;check(i);i = square(i)) { printf( in loop %d\n, i); printf( done\n ); int square(int x) { printf( square\n ); return x * x;

17 Declaration of functions Like variables, functions have to be declared first before you can use them. You can either declare first and define later, or define first. // define the function int func1(int m) { return m+2; int main() { return func1(2); // declare the function int func1(int m); int main() { return func1(2); // define the function int func1(int m) { return m+2;

18 Parameters and Arguments The names (symbols) of the inputs to a function are called parameters. The actual values that the parameters have at run-time are called arguments. However, sometimes people use these two terms interchangeably. int func1(int m) { return m+2; int main() { return func1(2); func1 has one parameter of type int called m Within func1, can refer to the name m Call func1 with actual argument 2

19 Evaluation of arguments Arguments are always evaluated before passing them into the functions. int square(int i) { return i*i; int func1(int m) { return m+2; int main() { return func1(square(2)); 1. Entry point: main() 2. Call square with argument 2 3. square returns 4 4. Call func1 with argument 4 5. func1 returns 6 6. main returns 6 int j; void func2(int i) { i = i + 5; int main() { j = 0; func2(j); return 0; What is the value of j after calling func2?

20 Predefined/coded functions Library We have already used a few: printf ( ) scanf( ) Somebody has already written these. Note that we need not know how they work. We do need to know what the functions do and how to use them documentation

21 Libraries of Code Often languages come with libraries of code Provided free of charge Beware of Copyright issues... Use library code whenever possible software is expensive (design, coding, testing, documentation, etc).

22 Using a Standard C Library

23 Three Important C Libraries stdio - Standard IO library File manipulation, input, output Definitions and manipulations of stdin stdout and stderr. math Math Library Defines most mathematical functions needed. stdlib Standard library Memory and miscellaneous (some math here).

24 math & stdlib Math #include<math.h> sometimes requires lm as compiler option Stdlib #include<stdlib.h>

25 Some Standard Library Functions Name Parameter Returns Example sqrt double double sqrt(25.0) pow double double pow(2.0,3.0) abs int int abs(-10) fabs double double fabs(6.8) ceil double double ceil(1.7) cos double double cos(3.2) sin double double sin(3.2) log double double log(127.5)

26 Examples : ceil() and floor()

27 Examples : rand()

28 Function : Random Numbers Get next pseudo-random number int x = rand( ); /* returns value in 0 to RAND_MAX */ Note rand() always generates the same sequence of random numbers If one wants a different sequence must initialize the random functions. Initialize a new seed for the random number generator: unsigned int seed = 2990; srand(seed);

29 Nesting Function Calls int i=10; double y; y = cos( pow( (double) i, 2))); Convert i to a double (unnecessary) Compute i squared Compute cosine of i square

30 User Defined Functions Three places where a function name is used: 1. Declaration (Prototype) 2. Call (Invocation) 3. Definition (Specification)

31 Declarations (Prototypes) Inform the calling program of the function Otherwise the compiler uses implicit declaration you may get a warning implicit declaration return_type function_name (parameter list); Example: int main(void); Void is a special reserved word meaning nothing - no parameter list

32 Example of Fn. Declarations double area(double length, double width); double perimeter(int radius); Identical to: double area(double, double); double perimeter(int)

33 Functions : Calling and Data Calling function passes zero or more parameters to called function Called function returns only ONE item main passes parameters to function1 function1 passes parameters to functiona

34 Function Declarations, Calls & Definitions #include <stdio.h> void greeting (void); int main(void) { greeting( ); return 0; /* Function Prototype*/ /* Function Call*/ /* Function Definition*/ void greeting(void) { printf ("\n Hello World \n");

35 Declarations, Calls & Definitions

36 Example of Fn. Declarations Example: A function to calculate hypotenuse. #include <stdio.h> #include <math.h> /* Function Prototype */ double hypot (double x, double y); int main () { double side1, side2; printf( Enter 2 sides of Right triangle\n,); scanf( %lf%lf, &side1, &side2); printf( The hypotenuse is: %f\n, hypot(side1,side2)); return 0;

37 Using the function s declaration the compiler compares the declaration to each occurrence of the function in the program. a. Number of parameters must be the same b. Type of parameters must be the same hypotenuse = hypot(side1,side2); double hypot(double x, double y)

38 The return statement A return statement should be used even if there is nothing to return. void myfunction(.) /*ends with the statement */ return;

39 Implicit Declaration If a function definition is not available then C creates an implicit function definition function parameters and return value are an int.

40 Example of Fn. Declarations Example: A function to calculate hypotenuse. #include <stdio.h> #include <math.h> int main () { double side1, side2; printf( Enter 2 sides of Right triangle\n,); scanf( %lf%lf, &side1, &side2); printf( The hypotenuse is: %f\n, hypot(side1,side2)); return 0; C implicit declaration has the form of int hypot (int x, int y);

41 Limitation: C-No Polymorphism! C is unlike C++ and JAVA Can only declare/define a function name once per program No overloading - parameters and return type are irrelevant double hypot(int x, int y); double hypot(double x, double y); Name functions carefully (e.g. fabs, abs, labs)

42 Header Files.h files usually contain function and constant declarations Single include gives many declarations Important if functions are in another file

43 Functions : Definition

44 Function : Return Type

45 Function : Calling (Invoking)

46 Function : Calling (Invoking)

47 Functions : Parameters Parameters are always passed by value Can be viewed as passed in two ways: 1. Pass by value send a copy of the value 2. Pass by reference (address) send a copy of the address of the variable

48 Parameters : Pass by Value Copy the value: From the caller Into the formal parameter of the called This is a one-way communication.. hypotenuse = hypot(side1,side2); double hypot(double x, double y)

49 hypoteneuse = hypot(side1,side2); double hypot(double x, double y) x is a copy of side1 y is a copy of side 2 So the function can change x or y and side1 and side2 are unaffected.

50 Function Param.: Pass by Value

51 Function Parameters Copy of the value is placed on the stack Transfer of control goes to the function Function can pick the parameters off the stack Original variables are untouched by function...

52 Function : Pass by Address Instead of copying a value: Copy the memory address!!! Then work with that address. Variables containing addresses are: pointers...

53 Function: Pass by Address

54 Address and Dereference & is the address operator int a = 5; &a /* is the address to an int */ /* type is (int *) */ * Is the dereference operator int *x; /* x is a pointer to an int */ x = &a; /* use the address of variable a */ *x = 12; /* contents of x is set to 12 */

55 Address and Dereference What s wrong with the following code? #include <stdio.h> void swap(int,int ); /* swapping */ int main(void) /*calls swap function*/ { int x; int y; printf("\n enter x and y: "); scanf( %d%d,&x,&y); swap(x,y); printf("new x: %d new y: %d\n",x, y); return 0;

56 Address and Dereference Here s the function swap... void swap (int a, int b) { int temp; temp = a; a = b; b = temp; Answer: It does not swap the values because of pass by value. Solution: Use a different dangerous passing mechanism. Don t pass the values but pass the addresses.

57 Address and Dereference Why is this dangerous Modify Calling Variables #include <stdio.h> void swap (int *, int *); int main( void ) /*calls swap*/ { int x; int y; printf("\n enter x and y: "); scanf( %d%d, &x, &y); swap ( &x, &y); printf("new x: %d new y: %d\n", x, y); return 0;

58 Address and Dereference Here is the real swap It works!! void swap (int *a, int *b) { int temp; temp = *a; *a = *b; *b = temp;

59 Call by value is useful Call by value is useful double fact(int n) { double total = 1; while (n >=1) { total *=n; n--; return (total); double fact(int n) { double total = 1; for(;n >=1;n--) { total *=n; return(total);

60 Call by value is useful #include <stdio.h> double fact(int n); int main(void) { int n = 10; printf( factorial(%d)=%f \n, n, fact(n));

61 Another function We want a function to print a square? XXXXXXX XXXXXXX XXXX XXXXXXX XXX XXXX XXXXXXX XXX XXXX XXXXXXX XXX XXXX XXXXXXX XXXXXXX

62 How do we do it? Nested loops.loops inside loops int i,j; char point ='X'; int size=4; for (i=1; i<= size; i++) { for (j=1; j<= size ;j++) { printf("%c", point); printf("\n");

63 Now we have to make it a function size nothing returned drawsquare

64 /* a function that draws a square of size "howbig" */ void drawsquare (int howbig) { int i; int j; printf("\n\n"); char point ='X'; for (i=1;i<= howbig;i++) { for (j=1;j<= howbig;j++) { printf("%c", point); printf("\n"); printf("\n\n"); return;

65 void drawsquare (int size); /*function prototype */ int main(void){ int sizeofsquare; printf("type an integer size of the square\n"); scanf("%d", &sizeofsquare); drawsquare(sizeofsquare); return 0;

66 Scope and Duration Global Scope Anything defined is visible from its definition to the end of the program. Local Scope Good from their definition to the end of their block (normally a function)

67 Scope and Duration Global Variables Placed outside the functions that use them Available to all functions Global variables should be AVOIDED...

68 Local Variables Scope is extent of their block Placed on the stack Destroyed on exit from function

69

70 Static Variable Variables which are declared static are visible only within their code segment (file or local block) Local variable is modified with static then: Variable exists from program beginning to end Only available in the function or file Initialized with 0 Functions can only be declared static visibility is only within the file

71 Example Write a program that calculates the factorial of an input integer number

72 QUESTIONS? 72