Computer Programming 6th Week Functions (Function definition, function calls),

Transcription

1 Computer Programming 6th Week Functions (Function definition, function calls), Hazırlayan Asst. Prof. Dr. Tansu Filik

2 Computer Programming Previously on Bil-200 loops (do-while, for), Arrays, array operations, C libraries

3 Computer Programming C Libraries C Liabraries "math.h", "stdlib.h", "string.h"

4 Computer Programming C Libraries The main C language only consists of expressions and grammar rules. C, itself, has NO built in function! Most C compilers come with the standard libraries containing several useful functions. In order to be able to use these functions, you must use #include directive to include the associated library containing the useful function.

5 Computer Programming C Libraries Function Libraries <stdio.h> - printf(), fprintf(), scanf(), fscanf(), fopen(), putchar(), getchar(),... <math.h> - pow(), sqrt(), fabs()... <ctype.h> - toupper(), tolower(), isalpha(), isdigit(),... <stdlib.h> - rand(), srand(), exit(),...

6 Introduction to Programming Languages C Libraries Function Libraries : math.h Many mathematical function (prototypes) exist here. fabs (x) : absolute value of float x sqrt (x) : square root of x exp (x) : e to the power x (e = ) log (x) : ln x log10 (x) : base-10 logarithm pow(x, y) : x to the power y sin (x) : sine (radians) cos (x) : cosine (radians) tan (x) : tangent fmod (a, b) : remainder of a/b ceil (x) : least integer >= x floor (x) : greatest integer <= x

7 Introduction to Programming Languages C Libraries Function Libraries : stdlib.h exit(val) : used for terminating program. if (fp == NULL) exit(1); /* if file cannot open, quit*/ rand(void) : generates a random number for(i=0; i < 100; ++i) /* 100 random numbers*/ printf("%d ", rand()); srand(value) : seed for the randomizer srand(100); /* starting point of random sequence*/

8 Introduction to Programming Languages C Libraries Function Libraries : time.h time(null): Numbe of seconds passed since 01/01/1970. int secs; secs = time(null);

9 Introduction to Programming Languages C Libraries Example: Approximate pi value Pi is a very commonly used constant in mathematical expressions. This number has infinite decimal points. Special applications require as many of these decimals as possible. Simple applications may be OK with 22/7. In the following example, we will see how near to pi the value 22/7 is. In C, asin(1) is approximately equal to pi.

10 Approximate value of pi #include <stdio.h> #include <math.h> int main(void) double pi, yak, hata; asin, is the arc-sine function. pi = 2 * asin(1); /* realistic pi value*/ yak = (double) 22 / 7; /* approximate pi */ hata = fabs(yak pi) / pi; /* find error */ printf("pi = %.15f\n", pi); /* write result */ printf("yaklaşık = %.15f\n", yak); printf("hata = %.5f", hata * 100); return 0;

11 pi = yaklaşık = hata = Press any key to continue

12 time(null) Gives number of seconds since 01/01/1970 At 13:40 on 17/11/2015, its value was. zaman: #include<stdio.h> #include<time.h> void main() int t; t = time(null); printf("zaman: %d \n", t);

13 time(null) Before using you should write: #include <time.h> Usage: int t; t = time(null); Since each time instant is different, it is a good randomization seed.

14 Clock: tik-tak #include <stdio.h> #include <time.h> void main(void) int i, t; for(i=0; i <= 10; ++i) /* just for 10 sec */ t = time(null) + 1; /* t next sec */ while (time(null) < t); /* wait */ printf("tik\n"); /* write tik */ t = time(null) + 1; /* wait 1 sac */ while(time(null) < t); /* wait */ printf("tak\n"); /* write tak */

15 Introduction to Programming Languages Functions Functions The modules that are written by the programmer

16 Introduction to Programming Languages Functions Function Calls Calling a function means returning the value of a named list of commands that do a specific task. We may call functions that we write ourselves. We may call functions that already exist in the standard C library. Ex: printf, scanf, sqrt

17 Introduction to Programming Languages Functions Example Write a program which obtains and prints out the square-root of an entered number. #include <stdio.h> #include <math.h> /* Ekrandan girilen bir sayının karekökünü yazar. */ int main() float x, sonuc; printf("bir sayi giriniz"); scanf("%f", &x); sonuc = sqrt(x); printf("sonuc = %f", sonuc); return 0;

18 Introduction to Programming Languages Functions Until now, we wrote our program using a single function: main(). The sytax was: void main ( ) /* definitions */ /* expressions */ The term void indicates that this function returns NOTHING.

19 Introduction to Programming Languages Functions The () parantheses indicate that main is a function. Inside these parantheses, you send parameters to the function. For this particular case, we do not send anything to the function, so the parantheses are empty. Normally, functions require a list of parameters to be sent to them for processing and generating a result. That result may be returned to the function calling place.

20 Introduction to Programming Languages Functions A function can be written is three steps: 1. Function prototype declaration 2. The function itself (declaration + commands inside) 3. The expression that calls the function.

21 Introduction to Programming Languages Functions 1. Function prototypes Compilers trace the program from top to bottom. If you write a program which tries to use your functions, the compiler needs to know how the function is called. The prototypes are listed at the top to show the parametric interface of functions. void, char, int, float, double semicolon

22 Introduction to Programming Languages Functions 1. Function prototypes: Parameters Parameter list is a list of types that will enter to the function. float toplam(int a,int b,int c) ;

23 Introduction to Programming Languages Functions 2. Function definition In the real declaration of function, do not put ; at the end of first line. float toplam(int a,int b, int c) Here, the function toplam takes three variables: a, b and c. The return type is float. a, b and c values are sent by the place where the function is called.

24 Introduction to Programming Languages Functions 2. Function definition Function definition consists of a declaration line similar to the prototype, followed by a list of commands bounded by curly parantheses: float toplam( int a, int b, int c) float t; t = a + b + c ; return t; Note: The function returns a value using the return keyword. If the return type is void, you do not need to use a return line.

25 Introduction to Programming Languages Functions 2. Function definition float toplam( int a, int b, int c) float t; t = a + b + c ; return t; The variables a, b and c are valid only inside the function toplam. The variable t is also valid only inside the function, and its memory location is freed when the function finished.

26 Introduction to Programming Languages Functions 3. Function calls While calling a function, it must be compatible with the prototype and the return type. The sent parameters must also be compatible with the types. You may send proper variables, expressions, or values to a function. For this case: deger = toplam( i, j, k ) ; or deger = toplam( 5, x, 2.7 ) ;

27 Introduction to Programming Languages Functions 3. Function calls Inside the main() function, the function toplam() is called. In that case, the parameter values are copied into the parameter list variables of the function.

28 Introduction to Programming Languages Functions 3. Function calls main() data memory i 12 j 25 copied copied toplam() data memory a b 45 k copied c 45

29 #include<stdio.h> void func1(void); int func2(int n, char ch); void main(void) int i=1,j=2; char ch= 0 ; printf( %d %d\n,i,j); func1(); j=func2(i,ch); printf( %d %d\n,i,j); void func1(void) printf( selam!\n );... FUNCTIONS- review Prototype declarations. Before using in main or other functions, declare their names like this. Their bodies will be written later. Calling a void-type function. Result is not assigned to anything. There is no input (void). Function generates an int type output. Therefore, its output can be assigned to some variable. Inputs are an int and a char. One of the functions whose prototype was declared. It is fully defined here. The I/O style of this and its prototype are compatible.

30 ... int func2(int x, char y) int z; printf( Basta: x=%d, y=%c\n,x,y); z=x*y; printf( Simdi: z=%d\n,z); return z; Since function generates an int......an int variable is declared inside......and a value is assigned to it......and variable is returned (function value produced). /* As a result, this function adopts a value at the place where it is called. */

31 Introduction to Programming Languages Functions Prototypes are necessary if you plant to write functions at the end of the program. In this way, you can safely use functions within functions. If you write functions before main, or before other functions which call them, you do not need to use prototypes. In this case, if a function (a) is used in function (b), then (a) must be written on top of (b).

32 Introduction to Programming Languages Functions General function syntax

33 void face(void) printf( :-) ); void next_line(void) printf( \n ); void faces(int n) if(i<=0) return; else for(i=1;i<=n;i++) face(); next_line(); void main(void) faces(3); faces(2); faces(1); :-):-):-) :-):-) :-)

34 Introduction to Programming Languages Functions Exercise: function factorial() Write a function to evaluate the factorial of a number. For parameters less than zero, the return value will be 0. For zero and one, the return value is 1, for larger numbers, use a loop to evaluate. In the main function, write a loop to evaluate factorials of numbers from 0 to 10, and print on the screen. Do not forget to write the prototype.

35 Introduction to Programming Languages Functions Exercise: factorial program #include <stdio.h> int factorial( int x); for (i = 0 ; i < 10 ; i++ ) printf("%d! = %d\n", i, factorial(i)); void main() int i;

36 Introduction to Programming Languages Functions Exercise: factorial program int factorial ( int x ) int f = 1, k; for ( k = 2 ; k <= x ; k++ ) f *= k ; return f ; if ( x < 0 ) return 0; else if ( x==0 x==1 ) return 1; /* Return 1 for 0 or 1 */

37 Introduction to Programming Languages Functions Exercise: Write a function which takes two parameters and returns the factorial of the difference. Function must start as: int factorial_diff(int a, int b) This function may call the function factorial() which was previously implemented. Do not forget to write the prototypes.

38 Introduction to Programming Languages Functions Class Exercise: Main Program #include <...> printf("fark= %d\n", factorial_diff(a,b)); /* prototype */ int factorial_diff(int a, int b); void main() int a,b; printf("farkın alinacagi sayilari girin: "); scanf("%d %d", &a, &b);

39 Introduction to Programming Languages Functions - Phylosophy For early programming languages, procedural structure was a great improvement... Instead of a single big and nasty program; Intelligible and easily written blocks are preferable. Then, which parts should be made a function? How do we decide on the structure? How do functions communicate with each other and with main? There must be an interface standard.

40 Introduction to Programming Languages Recursion Some mathematical problems and graph theoretical questions are inherently recursive : n! = n * (n-1)!... (n-1)! =(n-1) * (n-2)! Fibonacci series: 0, 1, 1, 2, 3, 5, 8... fib( n ) = fib( n - 1 ) + fib( n 2 ) In this case, the function must call itself! (n-2)! = (n-2)*(n-3)!

41 long fibonacci( long n ) if (n == 0 n == 1) /* en alt */ return n; else return fibonacci( n - 1) + fibonacci( n 2 ); f( 3 ) return f( 2 ) + f( 1 ) return f( 1 ) + f( 0 ) return 1 return 1 return 0

42 long fibonacci( long n ) printf("%d\n",n); if (n == 0 n == 1) /* en alt */ return n; else return fibonacci( n - 1) + fibonacci( n 2 ); f( 3 ) return f( 2 ) + f( 1 ) return f( 1 ) + f( 0 ) return return 1 return 0

43 int factorial(int a) if(a<0) fprintf(stderr, Gecersiz sayi\n ); return; else if ((a==0) (a==1)) return 1; else return (a*factorial(a-1));

44 Exercise: tanh 1 x x x x x Evaluate the above expression until the added numbers become smaller than a determined threshold.

45 void a(void) int x=10; printf( %d\n,x); x++; printf( %d\n,x); void b(void) static int x=10; printf( %d\n,x); x++; printf( %d\n,x); The value of this variable is assigned to 10 for the first call of function. Then, each time the function is called, the previous value is remembered. void main(void) a(); a(); a(); a(); b(); b(); b(); b(); a: b:

46 Exercises: What's wrong with the following listing? #include <stdio.h> void print_msg( void ); main() print_msg( "This is a message to print" ); return 0; void print_msg( void ) puts( "This is a message to print" ); return 0; What's wrong with the following function definition? int twice(int y); return (2 * y);

47 The values of local variables are unknown to outside the function. Different functions may have local variables with same names. Their values have nothing to do with other functions variables. The variables (parameters) that are sent to the function can change their values inside the function. BUT!!! The variable which is inside the function call does not change. The function makes a copy of the variable, and uses it. It does not use the original variable.

48 !! Example!! Write a function which swaps the values of its two parameters: int a=6, b=4; printf("%d %d\n", a, b); degistir(a, b); printf("%d %d\n", a, b); The first printf should write "6 4", the next should "4 6".

49 void degistir ( int a, int b ) int gecici; Solution (?) gecici = a; a = b; b = gecici; a and b are local variables, specific to the function only. The function is unable to change the values.

50 Why doesn t it work? Because you may send parameters to functions in two ways: by value by reference By value: While calling function, a copy of the variable value is sent. The original variable is not sent. By reference: While calling function, the real address of the variable is sent to the function. Inside the function, the values at that address can be changed. So the variable value also changes! (Remember scanf())

51 Function calls Remember function: toplam(). When it is called, the loal variables work, and when the function finishes, the local variables are lost. So how can we send parameters by reference? By address-pointer syntax. main() memory area i 12 j 25? copied copied toplam() memory area a 12 b 25

52 Two-way communication with functions #include <stdio.h> void swap(int *a,int *b); void main() int i = 1, j = 2; &i and &j, are memory locations of i and j. We send addresses to the swap() function. swap(&i, &j); void swap(int *a, int *b) int t; a and b are address (pointer) variables. t = *a; *a = *b; *b = t; By writing *a and *b, we can rach to the values in the given address.

53 Sending addresses to functions main() memory area &i toplam() memory area a 12 i 100 &j 100 b j This is the memory location of variable j

54 We already use such functions Remember scanf(); fscanf(); fopen(); etc... Think of function scanf(). What is critical here? Why do we send addresses of variables: scanf( %d, &i); Because we want to change the value of variable i. Why do we use scanf( %s, str);? We have not seen yet, but str is already an address!

55 scanf(&num) illustration &num &num 2 byte Read from keyboard: (Assume 10 is entered) 0 A

56 If you send the variable with & (address) sign - just like doing in scanf() we can change its value inside the function by writing thing to its memory location. Since the memory location corresponds to the container of the original variable, its value automatically changes.. So, what is the syntax (format) of a function that accepts addresses? We use *variable notation.

57 void my_func(int *num) This part is just like other functions. The argument listing part is different. int *num; is a variable whose address is num. We call the above function as: my_func(&i);

58 Example: If we want to print out the value of the parameter which was sent: void my_func(int *num) printf("\n%d", num); printf("\n%d\n", *num); void main(void) int *num; int i = 1; my_func(&i);

59 void my_func(int *num) printf( %d\n,*num); *num = 2; printf( %d\n,*num); void main(void) int a=3; printf( %d\n,a); my_func(&a); printf( %d\n,a); This is the int variable; num is not int! If you call the function like this (with address)...then the function argument must be declared with * (a pointer) indicator.

60 void degis(int *n, int *m) int k; printf( Degismeden once, n=%d, m=%d,,*n,*m); k=*n; *n=*m; *m=*k; printf( Degistikten sonra, n=%d, m=%d,,*n,*m); void main(void) int n1=10,n2=20; printf( Fonksiyon oncesi n1=%d, n2=%d,n1,n2); degis(&n1,&n2); printf( Fonksiyon sonrasi n1=%d, n2=%d,n1,n2);

61 = &n1 = n *n=10 &n2 m *m=

62 If you want to use address/pointer convention, but keep the value unchanged, then use const int *const num; /* for instance... */

63 Class Exercise Write a function which reads three float variables from the user, and returns them. The name would be: read_return(). The main program must be like: #include <stdio.h> main() float a, b, c; read_return(&a, &b, &c); printf( The numbers: %f %f %f\n",a,b,c);

64 Alternative uses void getnum(int *num) int k; printf( bir sayi girin: ); scanf( %d,&k); num=k; void getnum(int *num) printf( bir sayi girin: ); scanf( %d,num); void main(void) int k; getnum(&k); scanf( %d,k);