INTRODUCTION TO C++ FUNCTIONS. Dept. of Electronic Engineering, NCHU. Original slides are from

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1 INTRODUCTION TO C++ FUNCTIONS Original slides are from Dept. of Electronic Engineering, NCHU

2 Outline 2 Functions: Program modules in C Function Definitions Function Prototypes Math Library Functions Header files Calling Functions: Call-by-value and Call-byreference Scope Rules Recursion

3 What is a function? 3 Top-down design: Divide and conquer Partition a problem into several small sub-problems Each sub-problem can be solved by a function (module) Function A module with a specific function name Given a set of inputs, return an output Two types of functions User-defined functions ANSI C standard library functions, such as print(), scanf(), pow()

4 Function call 4 Invoke functions Provide function name and arguments (inputs) Return results (output) Format: output = function_name(input1, input2, ) Example printf( There are 50 students\n ); Function name: printf Inputs: There are 50 students\n pow(5.0, 3.0) Function name: pow Inputs: 5.0, 3.0 Output: 125 (5 3 )

5 Advantages of functions 5 Reusability Readability Ease of maintenance Avoid code repetition Use ANCI C standard library to improve portability

6 Function Definition 6 return-data-type function-name (data-type arg1, data-type arg2, ) { declarations; statements; return output; Function-name Any valid identifier (follow the same rule with variables) Parameter list A set of inputs separated by comma (,) Specify a data type for each input argument

7 Function Definition 7 Return-data-type Data type of the result void: Indicate that the function returns nothing Need to return an output with the specified data type If return-data-type is void return; If something returned return output; // output needs to be the specified type Function body Declarations and statements Declarations: variables used inside the function block, which can not be used outside the function Functions can not be defined inside a function

8 Function Definition: example 8 double compute_avg(int a, int b, int c) { Starting point return (double) (a + b + c)/3; int main() { double avg; int a, b, c; scanf( %d %d %d, &a, &b, &c); avg = compute_avg(a, b, c); return 1; Function definition: Name: compute_avg Inputs: a, b, c (integer) Output: return a double value // call function A function needs to be defined before function call

9 Function Prototype 9 Declare function prototypes before function call if the function is defined after function call Put semicolon (;) at the end of function prototype Argument names can be omitted in function prototypes double compute_avg(int, int, int); // function prototype int main() { double avg; int a, b, c; scanf( %d %d %d, &a, &b, &c); avg = compute_avg(a, b, c); return 1; double compute_avg(int a, int b, int c) { return (double) (a + b + c)/3; // call function // function definition

10 Function Prototype 10 Parameter names are sometimes included in function prototypes for documentation purposes. The compiler ignores these names. double compute_avg(int a, int b, int c); int main() { double avg; int a, b, c; scanf( %d %d %d, &a, &b, &c); avg = compute_avg(a, b, c); return 1; double compute_avg(int a, int b, int c) { return (double) (a + b + c)/3; // function prototype // call function // function definition

11 Programming error 11 Variable name can not be the same with argument names void foo(int tmp) { /* tmp is the argument name, so it can not be used as a variable name */ int tmp = 1; return; int main() { foo(1); return 1; // call function

12 Programming error 12 Must return a result in any condition int foo(int tmp) { /* no returned value when tmp <= 0 */ if (tmp > 0) return tmp + 1; int main() { foo(-1); return 1; // call function

13 Programming error 13 The returned value can only be assigned to another variable with the same type double foo(int a, int b) { return (double) (a+b)/2; int main() { /* the returned data type of foo() is double, but the type of avg is int */ int avg; avg = foo(2, 3); // call function return 1; Can force it to transform the data type by avg = (int) foo(2, 3)

14 Programming error 14 A variable declared in the function can not be used outside the function double foo(int a, int b) { int sum = a + b; return (double) sum/2 int main() { /* assign the results of foo() to avg */ double avg = foo(2, 3); // call function /* sum is declared in foo(), so can not be used here */ printf( sum is %d, avg is %f\n, sum, avg); return 1;

15 Available usage 15 Function call A() can be the argument of another function call B() if the return type of A() is not void int max(int a, int b) { if (a > b) return a; else return b; int main() { int i, j; scanf( %d %d, &i, &j); /* print the output of max(i,j) use %d because max() returns an integer value */ printf( max is %d\n, max(i, j)); return 1;

16 Available usage 16 The output of function call can be used as a logical control /* return 1 if a > b; otherwise, 0 */ int test_larger (int a, int b) { if (a > b) return 1; else return 0; int main() { int i, j; for (i = 1; i <= 5; i++) { for (j = 1; j <= 5; j++) { if (test_larger(i, j)) printf( (%d,%d)\n, i, j); // print if i > j return 1; (2,1) (3,1) (3,2) (4,1) (4,2) (4,3) (5,1) (5,2) (5,3) (5,4) output

17 Available usage 17 Call another function in a function int square (int); int square_sum(int); int square (int num) { return num * num; int square_sum(int num) { int sum = 0; for(int i = 1; i <= num; i++) sum += square(i); return sum; Function definition of square () must be placed before that of square_sum() Otherwise, declare function prototypes in the beginning of the program int main() { printf( square sum = %d\n, square_sum(10)); return 1;

18 Math Library Functions 18 Math library functions perform common mathematical calculations #include <math.h> Reference Function Function definition Description Example cos() double cos(double x) cosine of an angle of x sin(0.0)=0.0 sin() double sin(double x) sine of an angle of x cos(0.0)=1.0 tan() double tan(double x) tangent of an angle of x tan(0.0)=0.0 acos() double acos(double x) arc cosine of x asin() double asin(double x) arc sine of x atan() double atan(double x) arc tangent of x

19 Math Library Functions 19 Function Function definition Description Example sqrt() double sqrt(double x) square root of x sqrt(9.0)=3.0 exp() double exp(double x) exponential function e x exp(1.0)= log() double log(double x) natural logarithm of x (base e) log10() double log10(double x) natural logarithm of x (base e) log( )=1.0 log10(100.0)=2.0 fabs() double fabs(double x) Absolute value of x abs(-2.3)=2.3 ceil() double ceil(double x) round x to the smallest integer not less than x floor() double floor(double x) round x to the largest integer not greater than x pow() double pow(double x, double y) x raised by power y (x y ) ceil(2.3)=3.0 ceil(-9.8)=-9.0 floor(2.7)=2.0 floor(-9.8)=-10 pow(2.0,3)=8.0 pow(4.0,0.5)=2.0

20 Outline 20 Functions: Program modules in C Function Definitions Function Prototypes Math Library Functions Headers Calling Functions: Call-by-value and Call-byreference Scope Rules Recursion

21 Headers 21 Standard library header files Contain function prototypes for library functions <stdlib.h>, <math.h>, etc Load with #include <filename> #include <math.h> Custom header files Create file with functions Save as filename.h Load in other files with #include "filename.h" Advantage: Reusability

22 Standard library header files 22 <assert.h> <ctype.h> Convert lowercase letters to uppercase letters and vice versa <errno.h> <float.h> <limits.h> <locale.h> <math.h> Math library functions <setjmp.h>

23 Standard library header files 23 <signal.h> <stdarg.h> <stddef.h> <stdio.h> Standard input/output library functions <stdlib.h> Convert number to text and text to number, memory allocation, random number, and other utility functions <string.h> String-processing functions <time.h> Time and date

24 Custom header files 24.c file #include myheader.h int main() { statements; void function1() { statements; return; myheader.h file #include <stdio.h> #include <stdlib.h> void function1(); int function2(int, int); int function2(int a, int b) { statements; return output;

25 Outline 25 Functions: Program modules in C Function Definitions Function Prototypes Math Library Functions Headers Calling Functions: Call-by-value and Call-byreference Scope Rules Recursion

26 Calling Functions 26 Call by value Passes copy of argument to function Changes in function do not effect original Use when function does not need to modify argument Avoids accidental changes Call by reference (or call by address) Passes original argument Changes in function effect original Only used with trusted functions

27 Example: Call by value 27 double foo (double); int main () { double area, r = 2.5; area = foo(r); printf( area: %f\n, area); return 0; double foo (double radius) { double a; a = * radius * radius; return a; Copy 2.5 to another memory block main() r 2.5 compute_area(r); foo() radius 2.5

28 Outline 28 Calling Functions: Call-by-value and Call-byreference Scope Rules Recursion

29 Scope Rules 29 Types of variables Local variable Can only be referenced inside a function body Global variable Identifier defined outside function, known in all functions Global variables, function definitions, function prototypes

30 Example: local variable 30 double foo (double); int main () { double area, r = 2.5; area = foo(r); printf( area: %f\n, area); return 0; double foo (double radius) { double a; a = * radius * radius; return a; Scope of area and r Scope of a and radius

31 Example: local variable 31 double foo (double); int main () { double area, r = 2.5; area = foo(r); printf( area: %f\n, area); return 0; double foo (double radius) { double a; a = * radius * radius; return a; main() area r compute_area(r); Copy Copy 2.5 foo() a radius

32 Example: local variable 32 double foo (double); int main () { double area, r = 2.5; area = foo(r); printf( area: %f r: %f\n, area, r); return 0; area: 7.07 r: 2.5 double foo (double r) { double area; r = 1.5; area = * r * r; return area; r in main() and foo() are two different variables occupying two memory blocks with the same name!! area 7.07 main() r 2.5 compute_area(r); Copy 7.07 area foo() r Copy 2.5 Modifying the value of r in foo() does not affect the value of r in main()

33 Example: global variable 33 double foo (); double r; int main () { r = 2.5; double area; area = foo(); printf( area: %f\n, area); printf( r:%f\n,r_; return 0; double foo () { double area; r = 1.5; area = * r * r; return area; Scope of area in main() Scope of r Scope of area in foo()

34 Example: global variable 34 double foo (); double r; int main () { r = 2.5; double area; area = foo(); printf( area: %f\n, area); printf( r: %f\n, r); return 0; area: 7.07 r: 1.5 double foo () { double area; r = 1.5; area = * r * r; return area; r main() area 7.07 compute_area(); Copy 7.07 foo() area 7.07

35 Example: global variable 35 double foo (); double r; int main () { r = 1; double area; area = foo(); printf( area: %f, r: %f\n, area, r); area = foo(); printf( area: %f, r: %f\n, area, r); return 0; double foo () { double area; area = * r * r; r++; return area; Copy 3.14 r 12 main() area 3.14 compute_area(); foo() area 3.14

36 Example: global variable 36 double foo (); double r; int main () { r = 1; double area; area = foo(); printf( area: %f, r: %f\n, area, r); area = foo(); printf( area: %f, r: %f\n, area, r); return 0; area: 3.14, r: 2 area: 12.57, r: 3 double foo () { double area; area = * r * r; r++; return area; r 32 main() area compute_area(); Copy foo() area 12.57

37 Calling Functions 37 Call by value Passes copy of argument to function Changes in function do not effect original Use when function does not need to modify argument Avoids accidental changes Call by reference Passes original argument Changes in function effect original Only used with trusted functions

38 Call by reference 38 The address of an argument is passed to a function Function definition function_name(data_type* arg_name) symbol *: pointer for getting the value stored in the specified address Calling function function_name(&variable_name); symbol &: get the address of a variable

39 Pointer 39 Address: *b 10 int a; b *b int *b; b = &a (assign the address of a to b) int main () { int a = 10; int * b = &a; printf( a: %d\n, a); printf( address of a: %d\n, b); printf( data pointed by b: %d\n, *b); return 0; output a: 10 address of a: data pointed by b: 10 *b (data pointed by b)

40 Example: Call by reference 40 double foo (double *); int main () { double area, r = 2.5; area = foo(&r); printf( area: %f r: %f\n, area, r); return 0; area main() r 2.5 compute_area(&r); Address Pass the address of r (&r = 21220) double foo (double * pr) { double area; *pr = 1.5; area = * (*pr) * (*pr); return area; area foo() pr 22110

41 Example: Call by reference 41 double foo (double); main() *pr r in main() is set to 1.5 int main () { double area, r = 2.5; area = foo(&r); printf( area: %f r: %f\n, area, r); return 0; area: 7.07 r: 1.5 double foo (double * pr) { double area; *pr = 1.5; area = * (*pr) * (*pr); return area; area 7.07 r compute_area(&r); Assign 1.5 to *pr (=r) area foo() pr * (*pr) * (*pr) = 3.14 * 1.5 * 1.5 Address 21220

42 Sample-05-4: swap 42 Write a function to swap the values of two variables Why the following code does not work? void swap (int a, int b) { int tmp; tmp = a; a = b; b = tmp; printf( swap: a = %d; b = %d\n, a, b); int main() { int a = 10, b = 5; printf( main; a = %d; b = %d\n, a, b); swap (a, b); printf( main; a = %d; b = %d\n, a, b); return 0; Local variables in swap() a b 10 5 tmp 1 a b a b 5 10 output main: a = 10; b = 5 swap: a = 5; b = 10 main: a = 10; b = 5

43 Sample-05-4: swap 43 *pa main() *pb void swap(int *, int *); int main() { int a = 10, b = 5; printf( main; a = %d; b = %d\n, a, b); swap (&a, &b); printf( main; a = %d; b = %d\n, a, b); return 0; void swap (int * pa, int * pb) { int tmp; tmp = *pa; *pa = *pb; *pb = tmp; a b swap(&a, &b) *pa *pb pa pb tmp 10 swap()

44 Outline 44 Calling Functions: Call-by-value and Call-byreference Scope Rules Recursion

45 Recursion 45 Functions that call themselves Example void foo () { foo(); Termination condition is required to ensure that the function will eventually end and return

46 Example: factorials 46 void fac (int num) { //if (num <= 1) // return 1; //else Must have a termination condition return (num * fac(number - 1)); int main() { int a, result; scanf( %d, &a); result = fac(a); printf( %d!=%d\n, a, result); return 0; Recursive Call

47 Example: factorials 47 5! = 5 * 4 * 3 * 2 * 1 = 5 * 4! = 4 * 3 * 2 * 1 = 4 * 3! = 3 * 2 * 1 = 3 * 2! 5! = 120 = 5 * 24 = 24 = 4 * 6 = 6 = 3 * 2 return factorial(5) =120 return factorial(4) =24 return factorial(3) =6 return factorial(2) =2 void factorial (int num) { if (num <= 1) return 1; else return (num * fac(num - 1)); = 2 * 1 = 2 * 1! 1 = 2 = 2 * 1 1 return factorial(1) =1

48 Sample-05-5: Fibonacci series 48 0, 1, 1, 2, 3, 5, 8... Each number is the sum of the previous two fib( n ) = fib( n - 1 ) + fib( n 2 ) Terminate when n = 0 or n = 1 if(n == 0 n == 1) return n; + + fib(n) 0 n= n=1 n=2 n=3 n=4 n=5 n=6

49 Sample-05-5: Fibonacci series 49 fib(3) return fib(2) + fib(1) return fib(1) + fib(0) return 1 return 1 return 0