Pointers and
Pass By Reference
8.1 and 8.2
4/29/16
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
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Addresses
• Before using pass-by-reference we need to
look at addresses in C programs.
3
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
Storing Addresses
• numAddr = &num;
• A variable that can store an address is known as a
pointer variable or pointer
5
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
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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.
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Using Addresses (continued)
8
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
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Declaring and Using Pointers
(continued)
10
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
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Passing Addresses to a Function
(continued)
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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
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Another Example
• Given the radius of the circle, have a function
find the area and circumference.
• circle.c is started
14
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.
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
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