1. Pointers and Pass By Reference
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2. Pass by Reference Pass-by-value
Values from call copied into parameters
Only the value returned communicates back to the main, or calling function.
byValue.c
Pass-by-reference
Passes the address of the argument variable.
The called function can reference, or access, the variable using the passed address.
Also referred to as a call by reference 2 2

3. Addresses
Before using pass-by-reference we need to look at addresses in C programs. 3 3

4. Using Addresses in a Program
#include <stdio.h>
int main() {
int a = 5;
printf("a is %d\n", a);
printf("address of a is %p\n",&a );
return 0; {
Output is:
a is 5
address of a is 0x7fffaa63e41c
Address is hexadecimal, or base 16 4 4

5. Storing Addresses
numAddr = &num;
A variable that can store an address is known as a pointer variable or pointer 5 5

6. Declaring and Using Pointers
In declaring a pointer variable, C requires that we also specify the type of variable that is pointed to
int *numAddr;
This declaration can be read in a number of ways: as the variable pointed to by numAddr is an integer, or as numAddr points to an integer 6 6

7. Using Addresses Dereferencing operator: *
*numAddr means the variable whose address is stored in numAddr
Or, the variable pointed to by numAddr
When using a pointer, the value obtained is always found by first going to the pointer for an address. 7 7

8. Using Addresses (continued)‏8 8

9. Declaring and Using Pointers
#include <stdio.h>
int main() {
int a = 5;
int *p = NULL; //declare p as a pointer
//p points at nothing
p = &a; //Give p the address of a
//pointing p at a
*p = 8; //derefence p to access a
printf("Now a is %d\n", a);
return 0; }
OUTPUT:
Now a is 8 9 9

10. Declaring and Using Pointers (continued)‏10 10

11. Passing without Pointers
#include <stdio.h>
#include <stdlib.h>
void ConvFeetInches(int totDist,
int inFeet, int inInches) {
inFeet = totDist / 12;
inInches = totDist % 12;
return; }
int main(void) {
int initMeasure = 45;
int resFeet = 0;
int resIn = 0;
ConvFeetInches(initMeasure, resFeet, resIn);
printf("%d feet %d inches\n", resFeet, resIn);
return 0;
//ch8/feetInch1.c 11 11

12. Passing Addresses to a Function (continued)‏12 12

13. Passing Addresses to a Function ‏
#include <stdio.h>
#include <stdlib.h>
void ConvFeetInches(int totDist,
int* inFeet, int* inInches) {
*inFeet = totDist / 12;
*inInches = totDist % 12;
return; }
int main(void) {
int initMeasure = 45;
int resFeet = 0;
int resIn = 0;
ConvFeetInches(initMeasure, &resFeet, &resIn);
printf("%d feet %d inches\n", resFeet, resIn);
return 0;
//ch8/feetInch2.c 13 13

14. Another Example
Given the radius of the circle, have a function find the area and circumference.
circle.c is started 14 14

15. Participation
Write a C function the has reference parameters for the length and width of a rectangle. The function should enlarge the size of the rectangle up by a factor of 4. It should multiply the length and width being pointed to by the parameters by 4.

16. Summary
A pointer is a variable or parameter that is used to store the address of another variable
If a parameter or variable is a pointer, then the dereferencing operator, *, must be used to access the variable whose address is stored in the pointer
The address of a variable can be passed to a function
When a called function receives an address, it has the capability of directly accessing the respective calling function’s variable 16 16