Computer Programming for Engineers Spring 2023Lab 8 - Pointers.pptx

Pointers in C++
• Addresses in memory
• Programs can manipulate addresses
directly
&expr Evaluates to the address of the
location expr evaluates to
*expr Evaluates to the value stored in
the address expr evaluates to

Parameter Passing in C++
• Actual parameters are values
void swap (int a, int b) {
int tmp = b;
b = a;
a = tmp;
}
int main (void) {
int i = 3;
int j = 4;
swap (i, j);
…
}
The value of i (3) is passed, not its location!
swap does nothing

Parameter Passing Using Pointers
void swap (int *a, int *b) {
int tmp = *b;
*b = *a;
*a = tmp;
}
int main (void) {
int i = 3;
int j = 4;
swap (&i, &j);
cout << i << j << endl;
}
The value of &i is passed, which is the address of i

Pointers and Arrays
The name of the array variable is a pointer to
the first element of that array!
int arr[5] = {1, 2, 3, 4, 5}; // initialize array
int *ptr = arr; // set pointer to array
// You can now use ptr and arr interchangeably!
cout << ptr[2] << ‘n’; // What will this print?

Pointers and Arrays
The same relationship goes backwards as well!
int arr[5] = {1, 2, 3, 4, 5}; // initialize array
cout << arr << ‘n’; // what will this print?
// in fact, when we use square brackets, we actually do the following:
cout << arr[3] << ‘n’; // this is the same thing as:
cout << *(arr + 3) << ‘n’; // this works because array variable names are
// pointers
themselves!

Dynamic Memory Allocation
int *iPtr; // declaring a pointer variable
iPtr = new int(100); // dynamic allocation of a single intege
double *dPtr; // declaring a pointer variable
dPtr = new double[20]; // dynamic allocation of an array
delete iPtr; // dynamic de-allocation
delete[] dPtr; // dynamic de-allocation of an array

Pointers To Pointers
Graphically, the construct of a pointer to pointer can be depicted as
shown below: pointer_one is the first pointer, pointing to the second
pointer, pointer_two and finallypointer_two is pointing to a normal
variable num that hold integer 10.
char ch = ‘A’;
char **secondPtr, *firstPtr; // declare first and second pointers
firstPtr = &ch; // store address of ch in firstPtr
secondPtr = &firstPtr; // store the address of firstPtr in secondPtr
**secondPtr = ‘B’; // store ‘B’ in ch

Exercise 1
• Assuming that the statement int x, *ptr=&x; places the variable x into memory at
address 8688 and the ptr variable at 8684. Write down the output you expect for the
following code:
x=130;
cout << x << endl;
cout << ptr << endl;
x=131;
cout << x << endl;
cout << &ptr << endl;
x=132;
cout << &x << endl;
cout << ptr << endl;
x=133;
cout << &x << endl;
cout << *ptr << endl;
if(x != *ptr)
cout << “Hello1” << endl;
else
cout << “Hello2” << endl;

Exercise 2
Identify the errors:
#include <iostream>
void main()
{
int x, y=131, *ptr;
x=132;
y=*ptr;
cout << x << endl;
cout << y << endl;
return 0;
}
#include <iostream>
void main()
{
int x,y=131, *ptr;
x=132;
ptr=&x;
&y=ptr;
cout << x << endl;
cout << y << endl;
return 0;
}
#include <iostream>
void main()
{
int x=131,y, *ptr=y;
y=132;
*ptr=x;
cout << x << endl;
cout << y << endl;
return 0;
}

Exercise 2
Identify the errors:
#include <iostream>
void main()
{
int x[2]={1,2}, *ptr;
ptr=x;
ptr=&x[0];
ptr=x[1];
cout << x << endl;
cout << y << endl;
return 0;
}
#include <iostream>
void main()
{
int x=1,y=0,
*xPtr=&x, *yPtr=&y;
if(xPtr != NULL)
cout << (*xPtr / *yPtr) <<
endl;
return 0;
}

Example 3
Write a program that calls a function to swap
two characters and print them in the new
form using call by pointer.

Example 4
Write a function that receives a pointer to a
character string and a character. The
function should return the number of times
that the character occurred in the string.
Note that a string is an array of characters
terminated by the special character ‘0’.
Assume that the function has the following
prototype:
int char_count (char *ptr, char
c)

Example 5
Write a program that reads a data file that contains
the time (second) and coordinates of the position of
a robot that is roaming in unknown space. The first
row stores the number of readings. Each subsequent
line includes the time, the x coordinate, and the y
coordinate for the robot. The program should
determine the times when the robot comes inside a
circle centered at the origin with a radius R (entered
by the user). Use dynamic memory allocation to
create run-time arrays for storing the time, x
coordinate, and y coordinate.

Example 6
Write a program that dynamically resize an
array to a bigger size. The program creates an
array of an arbitrary size (specified by the
user). Then the user may extend the array to
a bigger size (this involves creating a
temporary array with larger size, copying the
values of the elements, and adding new values
into the extension cells). Print the new array
to the user to confirm the functionality.

Example 7
G O L F 0
H O C K E Y 0
F O O T B A L L 0
C R I C K E T 0
S H O O T I N G 0
Write a program that uses double pointer to create an array of
strings to store the names of sports as shown in the following
figure. Print these strings on the output screen on separate lines.
Make sure to deallocate the memory before terminating the
program.
sports

Computer Programming for Engineers Spring 2023Lab 8 - Pointers.pptx

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