Array and Pointers

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Unit - 02
Array and Pointers
An Introduction
Unit Introduction
This unit covers the usage of pointers and
arrays in C++
Unit Objectives
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After covering this unit you will understand…
Arrays in C++
Pointers in C++
Using pointers
Pointer Arithmetic
Pointer constants, variables and strings
Function Pointers
Array and Pointer
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An Array is a collection of contiguous memory
blocks of same data type
An Array has a name, which is the address of
the first memory block
A Pointer is a variable, which holds the address
of another variable
Arrays
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Array index starts at zero (0)
int array[10];
// reserves space for 10 int types in memory from 0 to 9
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Accessing elements from an array
int y = array[0]; // Here, first element is retrieved
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Storing values in an array
array[4] = 5; // Here the 5th element is given value 5
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Arrays are extremely fast since they are indexed
An Array size cannot be changed at runtime
Example: Arrays
// This program stores numbers from 0 to 9 in an array
#include <stdlib.h>
void main()
{
int array[10];
for (int j=0; j<=9; j++)
{
array[j] = j;
}
}
Pointers
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Pointers point to a memory space
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Pointers are denoted by ‘*’ e.g.
int a = 47;
int* pointerToA = &a;
Example: Pointers
#include <stdlib.h>
#include <iostream.h>
void function(int* pointer)
{
cout << "p = " << *pointer << endl; // prints 4
*pointer = 6;
cout << "p = " << *pointer << endl; // prints 6
}
void main()
{
int x = 4;
function(&x);
// pass the address to the function
cout << "x = " << x << endl;
//prints 6
}
Pointer Arithmetic
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Pointer arithmetic is a special type of arithmetic
int* p;
p++; // adds 2-bytes to the pointer address, since type
// is int
double* pd;
pd++; // adds 4-bytes to the pointer address,since type is
// double
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Similar case for classes and structs
The compiler knows how many bytes to go
forward
Allowed Pointer Arithmetic
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Following are the allowed pointer operations:
Subtraction of pointers results in the number of
elements between the pointers
 Addition of integral values in pointers
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Add or subtract pointers with an integral value
Adding two pointers is not allowed
Example: Pointer Arithmetic
#include <iostream.h>
void main()
{
int* p1; // simple way of defining pointers
int* p2;
int* p3;
int x;
p2 = &x;
p1 = p2;
p2 = p2 + 5;
cout << (p2 - p1);// prints the number of elements
// between the two pointers, which is 5
// p3 = p2 + p1; is illegal operation
}
Constants and Variables
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Pointers are variables by default
Constant Pointers can be declared by using the
const keyword
int intVal = 10;
int* const intPointer = &intVal;
/* intPointer is a pointer, which is const, that points to
an int */
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In the above example you can change the value
in intVal but not the address
*intPointer = 20;
/* allowed because the pointer (i.e. the address) is constant,
while the value inside the pointer can be variable */
Example: Constants & Variables
#include <iostream.h>
void main()
{
int b = 25;
/* (shown below) an int pointer that is stored at a
constant memory address */
int* const a = &b;
b = 35;
*a = 45;
}
//
//
//
//
// ‘a’ holds the address of ‘b’
// which has a value of 25
‘a’ still points to ‘b’ which now has 35
allowed - since the value can change
inside the address that the pointer points
to
Array as arguments
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Arrays are passed by reference in functions
Arrays can’t be passed by value
Example: Array as arguments
#include <stdlib.h>
function(int* array)
{
// you can use int array[] as arg
// do something with the array
}
void main()
{
int array[3] = { 1, 2, 3 };
function(array); // any changes made in array inside the
}
// function are reflected in the array
Pointers and String in C++
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String declaration in C++
As an array
 As a char*
 As a string
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// char charArray[10];
// char* charString;
// string stringTypeArray;
Array of pointers declaration e.g.
char* arrayOfPointers[10]; // array of 10 pointers
for (int j=0; j<10; j++)
{
arrayOfPointers[j] = strcat(“String ”, j);
}
Function Pointers
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Pointer that can hold function addresses is called
Function Pointer
Steps required are
Define a function pointer
 Initialise the pointer with function address
 Call the function by using pointer
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While defining a function pointer, parenthesis
are required otherwise it becomes function
declaration
Example: Function Pointer
void func()
{
cout << "func() called..." << endl;
}
int main()
{
void (*fp)();
fp = func;
(*fp)();
}
// define a function pointer
// Initialise it, assign function address
// call function using function pointer
Complex Function Pointers
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A function pointer can hold address of another
pointer to a function, e.g.
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double ((*fp1)(int))(float));
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fp1 is a pointer to a function that takes an integer
argument and returns a pointer to another function that
takes a float and returns double
Array of Function Pointers
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Like array of simple pointers, there can also be
array of function pointers
Useful for table-driven code implementation
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Instead of using if-else or case statements use a state
variable
Example: Array of Function Pointers
// define functions a(), b() and c()
void func1()
{
cout << ”func1() called..." << endl;
}
void func2()
{
cout << ”func2() called..." << endl;
}
void func3()
{
cout << ”func3() called..." << endl;
}
// func_table is an array of function pointers to functions that
accept nothing and return void
// initialise array with function addresses
void (*func_table[])() = {func1, func2, func3 };
Example: Array of Function Pointers (contd.)
void main()
{
cout << ”Enter a number (1-3) to call a function: “;
int n;
cin >> n;
if (n < 1 || n > 3) {
cout << “Invalid number,should be 1-3” << endl;
}
// call function using function pointer
(*func_table[n-1]();
// n is a state variable
}
Unit Summary
In this unit you have covered …
 Arrays
 Pointers
 Uses of Pointers
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