Introduction to Computing Lecture 08: Functions (Part I)

Transcription

1 Introduction to Computing Lecture 08: Functions (Part I) Assist.Prof.Dr. Nükhet ÖZBEK Ege University Department of Electrical & Electronics Engineering

2 Topics Uses of functions Functions Parameters Return values

3 Uses of functions Top-Down design or divide-and-conquer It is easier to solve multiple smaller problems instead of one big problem

4 Top-down design Suppose, we want to list parts of an automobile Automobile self starter Bench seat Brakes Bumper Buzzer Car battery Car doors Stop lamp Clutch Dashboard Exhaust pipe Fuel pump Grille Headlight Muffler Odometer

5 Top-down design It is easier to first divide the problem into sub problems: Chassis Motor parts Electronic parts and then, list the parts under sub categories

6 Uses of functions Top-Down design or divide-and-conquer It is easier to solve multiple smaller problems instead of one big problem It is easier to debug sub problems In case of an error, it is easy to isolate the problem to a specific section of code (a specific function)

7 Uses of functions Top-Down design or divide-and-conquer It is easier to solve multiple smaller problems instead of one big problem It is easier to debug sub problems A team of programmers can work on different sub problems and join sub problems later to form the solution

8 Uses of functions Top-Down design or divide-and-conquer It is easier to solve multiple smaller problems instead of one big problem It is easier to debug sub problems A team of programmers can work on different sub problems and join sub problems later to form the solution Code reuse

9 Code reuse Suppose we want to find a b and c d power1 = 1; for (count = b ; count > 0 ; count--) power1 *= a; power2 = 1; for (count = d ; count > 0 ; count--) power2 *= c;

10 Code reuse Suppose we want to find a b and c d power1 = power(a,b); power2 = power(c,d); No need to write the same code each time if we use functions We can also use this function in another program, either simply by copy-paste or adding it to a user defined library as we will see later

11 User-Defined Functions Can create your own functions, similar to printf() or sqrt() Implements the subroutine/ procedure/ module/ function definitions of an algorithm

12 Writing User-defined Functions Need to specify: the name of the function its parameters (input arguments) what it returns (output arguments) block of statements to be carried out when the function is called The block of statements is called the function body

13 Writing User-defined Functions type-specifier function_name(parameter list) body of function type-specifier is the type of value the function returns using the return statement Can be any valid type If no type specified, assumes int Can be void -> does not return anything

14 Example: hello1.c Prints a simple greeting. procedure sayhello output Hello World! Main Program do procedure sayhello

15 Example: hello1.c Prints a simple greeting. procedure sayhello output Hello World! #include <stdio.h> /* * Print a simple greeting. */ void sayhello ( void ) printf( Hello World!\n ); /* * Call a function which prints a * simple greeting. */ Main Program do procedure sayhello int main() sayhello(); return 0;

16 Example: hello1.c #include <stdio.h> /* * Print a simple greeting. */ Function definition void sayhello ( void ) printf( Hello World!\n ); /* * Call a function which prints a * simple greeting. */ int main() Function call sayhello(); return 0;

17 Example: hello1.c Function name Function body #include <stdio.h> /* * Print a simple greeting. */ void sayhello ( void ) printf( Hello World!\n ); /* * Call a function which prints a * simple greeting. */ int main() sayhello(); return 0;

18 Example: hello1.c Return type #include <stdio.h> /* * Print a simple greeting. */ void sayhello ( void ) printf( Hello World!\n ); Formal Parameter List /* * Call a function which prints a * simple greeting. */ int main() sayhello(); return 0;

19 Parameters Information passed to a function Formal parameters are local variables declared in the function declaration Actual parameters are variables or values passed to the function when it is called

20 Example: badsort.c /* Print two numbers in order. */ void badsort ( int a, int b ) if ( a > b ) printf("%d %d\n", b, a); else printf("%d %d\n", a, b); Parameters (aka Arguments)

21 Example: badsort2.c /* Print two numbers in order. */ void badsort ( a, b ) int a; int b; Old style. if ( a > b ) printf("%d %d\n", b, a); else printf("%d %d\n", a, b);

22 Example: badsort.c /* Print two numbers in order. */ void badsort ( int a, int b ) if ( a > b ) printf("%d %d\n", b, a); else printf("%d %d\n", a, b);

23 Example: badsort.c Formal parameters /* Print two numbers in order. */ void badsort ( int a, int b ) if ( a > b ) printf("%d %d\n", b, a); else printf("%d %d\n", a, b); int main() int x = 3, y = 5; Actual parameters badsort ( 10, 9 ); badsort ( y, x + 4 ); return 0;

24 Parameters (cont.) Parameters are passed by copying the value of the actual parameters to the formal parameters Changes to formal parameters do not affect the value of the actual parameters This type of function call is called call-byvalue

25 Example: badswap.c /* Swap the values of two variables. */ void badswap ( int a, int b ) int temp; temp = a; a = b; b = temp; int main() int a = 3, b = 5; printf("%d %d\n", a, b); badswap ( a, b ); printf("%d %d\n", a, b); return 0; printf("%d %d\n", a, b);

26 Example: badswap.c /* Swap the values of two variables. */ void badswap ( int a, int b ) int temp; temp = a; a = b; b = temp; int main() int a = 3, b = 5; printf("%d %d\n", a, b); badswap ( a, b ); printf("%d %d\n", a, b); return 0; printf("%d %d\n", a, b); Output: 3 5

27 Example: badswap.c /* Swap the values of two variables. */ void badswap ( int a, int b ) int temp; temp = a; a = b; b = temp; int main() int a = 3, b = 5; printf("%d %d\n", a, b); badswap ( a, b ); printf("%d %d\n", a, b); return 0; printf("%d %d\n", a, b); Output:

28 Example: badswap.c /* Swap the values of two variables. */ void badswap ( int a, int b ) int temp; temp = a; a = b; b = temp; int main() int a = 3, b = 5; printf("%d %d\n", a, b); badswap ( a, b ); printf("%d %d\n", a, b); return 0; printf("%d %d\n", a, b); Output:

29 Example: badswap.c /* Swap the values of two variables. */ void badswap ( int a, int b ) int temp; temp = a; a = b; b = temp; int main() int a = 3, b = 5; printf("%d %d\n", a, b); badswap ( a, b ); printf("%d %d\n", a, b); return 0; printf("%d %d\n", a, b); Called function s environment: a: 5 b: 3 Calling function s environment: a: 3 b: 5

30 Parameters (cont.) If a function does not take parameters, declare its formal argument list void or do not write anything. Declaration: void sayhello ( void ) printf( Hello World!\n ); Also valid: Function call: void sayhello () printf( Hello World!\n ); sayhello();

31 Parameters (cont.) The number of actual arguments used in a call to a function must be the same as the number of formal parameters listed in the function definition The order of arguments in the lists determines correspondence. The first actual argument corresponds to the first formal parameter, and so on.

32 Parameters (cont.) Each actual argument must be of a data type that can be assigned to the corresponding formal parameter

33 Functions Do s and Don t s DO use a function name that describes the purpose of the function DON'T pass values to a function that it doesn't need DON'T try to pass fewer (or more) arguments to a function than there are parameters. In C programs, the number of arguments passed must match the number of parameters

34 Return from a Function When the last statement is executed void sayhello ( void ) printf( Hello World!\n ); When a return statement is encountered int max (int a, int b) if(a>b) return a; return b; Notice there can be multiple return statements

35 Return Values Values are returned by copying a value specified after the return keyword All functions, except those of type void, return a value

36 Example: maxmin.c Return type /* Returns the larger of two numbers. */ int max (int a, int b) int result; if (a > b) result = a; else result = b; return result;

37 Example: maxmin.c /* Returns the larger of two numbers. */ int max (int a, int b) int result; if (a > b) result = a; else result = b; return result; For example: The value of the expression max(7,5) is the integer 7.

38 Example: maxmin.c This style okay. /* Returns the larger of two numbers. */ int max (int a, int b) int result; if (a > b) result = a; else result = b; return result;

39 Example: maxmin.c Version 2: /* Returns the larger of two numbers. */ int max (int a, int b) if (a > b) return a; else return b;

40 Example: maxmin.c Version 3: /* Returns the larger of two numbers. */ int max (int a, int b) if (a > b) b = a; return b; Note: Changing the value of b does not affect the actual parameter in the function call.

41 Example: maxmin.c #include <stdio.h> /* Returns the larger of two numbers. */ int max (int a, int b) if (a > b) return a; else return b; /* Returns the smaller of two numbers. */ int min (int a, int b) if (a < b) return a; else return b; int main() int maxofmins = max(min(1,2), min(3,4)); Or you can write printf("%d\n", maxofmins); max(min(1,2), min(3,4))); return 0;

42 Return Values (cont.) If a function does not return a value, declare its return type as void Declaration: void sayhello ( void ) printf( Hello World!\n ); Function call: sayhello();

43 Return Values (cont.) As long as a function is not declared as void, it can be used as an operand in any valid C expression x = power(y); if(max(x,y) > 100) printf( greater );

44 Return Values (cont.) However, a function can not be the target of an assignment swap(x,y) = 100; /* incorrect statement */ A function declared as void can not be used in any expression void f();... t = f(); /* incorrect */

45 Return Values (cont.) If you do not assign the return value of a function it is lost int multiply(int a, int b); int main() x = multiply(5,4); printf( %d, multiply(2,3)); multiply(8,3); /* lost */

46 Functions Do s and Don t s DON'T try to return a value that has a type different from the function's type DON'T let functions get too long. If a function starts getting long, try to break it into separate, smaller tasks DO limit each function to a single task DON'T have multiple return statements if they aren't needed. You should try to have one return when possible; however, sometimes having multiple return statements is easier and clearer