Chapter 4. Section 4.4 Passing Parameters to Functions. CS 50 Hathairat Rattanasook

Transcription

1 Chapter 4 Section 4.4 Passing Parameters to Functions CS 50 Hathairat Rattanasook

2 Passing parameters Pass by Value: The variable is copied during the function call and a local copy of the variable exists in the function. Operating on the variable does not change the original parameter. Pass by Reference*: A reference or address to the variable is passed to the function. Operating on the local variable inside the function will also change the original parameter. *Note, that C does not have true pass by reference because even pointer parameters are pass by value since the address of the pointer is copied. CS 50 - Hathairat Rattanasook 1

3 Pass by Value The function below takes two int variables as parameters, adds them, and returns the value. #include <stdio.h> void add(int, int); int main() { int x = 2, y = 3; int result = add(x, y); printf( x: %d, y: %d, x, y); int add(int a, int b) { return a + b; Variable Address Memory... x y a b result The output of the program is: x: 2, y: 3 CS 50 - Hathairat Rattanasook 2

4 Pass by Reference The function below takes two pointers-to-int as parameters and adds the value of the second parameter to the value of the first parameters. #include <stdio.h> void add(int*, int*); int main() { int x = 2, y = 3; add(&x, &y); printf( x: %d, y: %d, x, y); void add(int* a, int* b) { *a = a + b; Variable Address Memory... x y a b The output of the program is: x: 5, y: 3 CS 50 - Hathairat Rattanasook 3

5 Pointers vs Variable Parameters Pointers are used as pass by reference. The address of the pointer is passed and can be used to operate on the original parameter. Variables are used as pass by value. The value of the variable is copied and operating on it will not change the original parameter. CS 50 - Hathairat Rattanasook 4

6 Example (1 / 2) #include <stdio.h> void getinput(double*, double*); double calculate(double, double); void display(double); int main() { double length, width, area; getinput( &length, &width ); // pass by reference area = calculate( length, width ); // pass by value display( area ); // pass by value void getinput(double* lengthptr, double* widthptr) { printf("enter the length: "); scanf("%lf", lengthptr); printf("enter the width: "); scanf("%lf", widthptr); // calculate(..) and display(..) methods are on next slide CS 50 - Hathairat Rattanasook 5

7 Example (2 / 2) double calculate(double length, double width) { return length * width; void display(double area) { printf("the area is %5.2f\n", area); This example asks the user for length and width, calculates the area, and displays the result. The method getinput(double*, double*) uses pass-by-reference to assign the user input to the original variables. The method calculate(double, double) uses pass-by-value to calculate the area and returns the result. It does not change the original variables. The method display(double) uses pass-by-value to display the area. It does not change the original variables. CS 50 - Hathairat Rattanasook 6

8 Questions? 1. Which of the two methods changes the original variable? a) pass-by-value b) pass-by-reference 2. What parameter type do you use for pass-by-reference? 3. Can you use pass-by-value and pass-by-reference in the same method? CS 50 - Hathairat Rattanasook 7

9 Answers 1. Which of the two methods changes the original parameter? a) pass-by-value b) pass-by-reference 2. What parameter types do you use for pass-by-reference? Pointers, you can then use the dereference operator (*) to write operate on the value of the original variable 3. Can you use pass-by-value and pass-by-reference in the same method? Yes, for example: void add(int* aptr, int b); CS 50 - Hathairat Rattanasook 8

10 Challenge? Can you declare a method updatepointer() that changes the pointer's value (address) of the original pointer to the value of another pointer? CS 50 - Hathairat Rattanasook 9

11 Answer Can you declare a method updatepointer() that changes the pointer's value (address) of the original pointer to the value of another pointer? #include <stdio.h> void updateaddress(int** intptr, int* anotherptr); int main() { int a = 2, b = 3; int *p1 = &a, *p2 = &b; // p1 points to a, p2 points to b updateaddress(&p1, p2); // pass address of p1 and value of p2 printf("a: %d, b: %d\n", a, b); // prints a: 2, b: 3 printf("p1: %d, p2: %d\n", *p1, *p2); // prints p1: 3, p2: 3 // intptr is a pointer to a pointer void updateaddress(int** intptr, int* anotherptr) { /* updates the value of the original pointer (p1 above) to point to anotherptr (p2 above)*/ *intptr = anotherptr; CS 50 - Hathairat Rattanasook 10