Csc1100 lecture12 ch08_pt2

A First Book of C++A First Book of C++
Chapter 8 (Pt 2)Chapter 8 (Pt 2)
Arrays and PointersArrays and Pointers

∗ In this chapter, you will learn about:
∗ Introduction to Pointers
∗ Array Names as Pointers
∗ Pointer Arithmetic
∗ Passing Addresses
∗ Common Programming Errors
A First Book of C++ 4th Edition 2
Objectives

∗ If grade is a single-dimensional array containing five
integers, the fourth element is grade[3]
∗ C++ compiler computation of the address of grade[3]:
(assuming 4 bytes per integer)
&grade[3] = &grade[0] + (3 * 4)
col*byte size (offset)
∗ This statement reads as “the address of grade[3] equals
the address of grade[0] plus 12”
∗ Figure 8.12 illustrates the address computation used to locate
grade[3]
Array Names as Pointers

Array Names as Pointers (cont'd.)

∗ Offset: number of positions beyond first element in
array
∗ Using offset, we can simulate process used by
computer to access array elements
∗ Example (Figure 8.14 and Table 8.1):
∗ Store address of grade[0] in pointer gPtr
∗ Use offset to find location of grade[3]
∗ Expression *(gPtr + 3) references variable that is
three integers beyond variable pointed to by gPtr

//Program 8.6 – Method 1, assign starting address to pointer
#include <iostream>
using namespace std;
int main()
{
const int ARRAYSIZE = 5;
int *gptr; // declare a pointer to an int
int i, grade[ARRAYSIZE] = {98,87,92,79,85};
gptr = &grade[0]; // store starting of array address
for (i=0; i < ARRAYSIZE; i++)
cout << “nElement ” << i << “ is “ << *(gptr + i);
cout << endl;
return 0;
}
Accessing Array Elements using Pointers 1Accessing Array Elements using Pointers 1

//Program 8.7 – Method 2, use array itself as pointer
#include <iostream>
using namespace std;
int main()
{
const int ARRAYSIZE = 5;
int i, grade[ARRAYSIZE] = {98,87,92,79,85};
for (i=0; i < ARRAYSIZE; i++)
cout << “nElement ” << i << “ is “ << *(grade + i);
cout << endl;
return 0;
}
Accessing Array Elements Using Pointers 2Accessing Array Elements Using Pointers 2

∗ Static array allocation: as variables are defined,
storage is assigned from a memory pool
∗ Specific memory locations are fixed for life of a variable,
used or not
∗ Example: function requests storage for an array of
500 integers
∗ If application requires less than 500 integers, unused
storage is not released until array goes out of scope
∗ If more than 500 integers required, array size must be
increased and the function recompiled
Dynamic Array Allocation

∗ Dynamic allocation: storage allocated is determined
and adjusted as program is run
∗ Useful when dealing with lists
∗ Allows list to expand and contract as list items are
added and deleted
∗ Example: constructing list of grades
∗ Don’t know number of grades ultimately needed
∗ Need a mechanism to enlarge and shrink array
∗ new and delete operators: provide capability
Dynamic Array Allocation (cont’d.)

Dynamic Array Allocation (cont'd.)

∗ Dynamic storage requests for scalar variables or
arrays made in declaration or assignment statements
∗ Example 1 (assign value to pointer upon declaration):
int *num = new int;
∗ Reserves space for an integer variable
∗ Stores address of this variable into pointer num

∗ Example 2: same function as Example 1
(declare pointer first, assign value in next statement)
int *num;
num = new int;
∗ Heap: free storage area of a computer
∗ Consists of unallocated memory, can be allocated to a
running program
∗ In Examples 1 and 2, new storage comes from free
storage area (heap)

∗ Example of dynamic allocation of an array:
int *grade = new int[200];
∗ Statement reserves storage for 200 integers and places address
of first integer into pointer grade
∗ Same example with variable dimension
cout << "Enter the number of grades to be"
<< " processed: ";
cin >> numgrades;
int *grades = new int[numgrades];
∗ Size of array depends on user input
∗ Values accessed by array notation: grades[i]
∗ Values accessed by pointer notation: *(grades+i)

∗ By adding to and subtracting from pointers, we can
obtain different addresses
∗ Pointers can be compared using relational operators (
==, !=, <, >, etc.)
∗ Consider declarations:
int nums[100];
int *nPt;
∗ Set address of nums[0] into nPt using:
nPt = &nums[0];
nPt = nums;
Pointer Arithmetic
similar (an array is by default a pointer)

∗ After nPt is assigned a valid address, values can be
added or subtracted to produce new addresses
∗ Scaling: automatic adjustment of computed address,
ensures points to value of correct type
∗ Example: nPt = nPt + 4;
∗ Assuming an integer requires 4 bytes, the computer
multiplies 4 by 4 and adds 16 to the address in nPt
Pointer Arithmetic (cont'd.)
computing new address

Pointer Arithmetic (cont'd.)
+4 bytes +4 bytes +4 bytes +4 bytes

∗ Pointers can be initialized when declared
∗ Example: int *ptNum = &miles;
∗ Above initialization valid only if miles was declared as an integer
prior to above statement
∗ The following statements produce an error
int *ptNum = &miles;//miles not declared yet
int miles;
∗ Arrays can be initialized within declarations
double *zing = &prices[0];
∗ This statement is valid if prices has already been declared as
a double-precision array
Pointer Initialization

∗ Passing addresses to function using reference
variables was addressed in Chapter 6
∗ Implied use of addresses because the reference does
provide the function with an address
∗ The function call swap(num1, num2) does not tell
whether parameters are passed by value or reference
∗ Must look at function prototype or header line to
determine
Passing Addresses

∗ Explicit passing of an address to a function: place the
address operator (&) in front of variable being passed
∗ Example (Figure 8.19 and Program 8.11):
swap(&firstnum, &secnum);//in function call
∗ This function call passes the addresses of firstnum and
secnum to swap()
∗ Explicitly passing addresses using the address operator is
effectively a pass by reference
Passing Addresses (cont'd.)

∗ When array is passed to a function, address of first
location is the only item passed
∗ Program 8.13 passes an array to a function using
conventional array notation
Passing Arrays

//vals = &nums[0]

∗ Parameter vals in header line declaration for
findMax() in Program 8.13 actually receives the
address of array nums
∗ Thus, val is really a pointer
∗ Another suitable header line for findMax() is:
int findMax(int *vals, int numels)
// here vals is declared as a pointer
// to an integer
Passing Arrays (cont'd.)

∗ Access to multidimensional arrays can be made using
pointer notation
∗ Example: consider the declaration:
int nums[2][3] = { {16,18,20},
{25,26,27} };
∗ Creates an array of elements and a set of pointer
constants named nums, nums[0], and nums[1] (as
shown in Figure 8.26)
Advanced Pointer Notation

Advanced Pointer Notation (cont'd.)Advanced Pointer Notation (cont'd.)

∗ Two-dimensional array pointer constants allow for
accessing array elements in several ways
∗ Address of first element in first row of nums is
nums[0]
∗ Address of first element in second row is nums[1]
∗ Variable pointed to by nums[0] is num[0][0]
∗ Variable pointed to by nums[1] is num[1][0]
∗ Each element can be accessed by applying an offset
to the correct pointer
Advanced Pointer Notation (cont'd.)

Advanced Pointer Notation (cont'd.)
Pointer Notation Subscript Notation Values
*(*nums) nums[0][0] 16
*(*nums + 1) nums[0][1] 18
*(*nums + 2) nums[0][2] 20
*(*(nums + 1)) nums[1][0] 25
*(*(nums + 1) + 1) nums[1][1] 26
*(*(nums + 1) + 2) nums[1][2] 27

∗ Attempting to explicitly store an address in a variable that
has not been declared as a pointer
∗ Using a pointer to access nonexistent array elements
∗ Incorrectly applying address and indirection operators
∗ If pt is a pointer variable, the expressions:
pt = &45 //must be variable name, e.g:&num
pt = &(miles + 10)//&(miles + num)-> ok
are both invalid because they attempt to take
the address of a value
Common Programming Errors

∗ Taking addresses of pointer constants
∗ For example, given the declarations:
int nums[25];
int *pt;
the assignment pt = &nums; //INVALID
∗ nums is a pointer constant that is itself equivalent to an
address; the correct assignment is :
pt = nums OR pt = &nums[0]
∗ Taking addresses of a reference argument, reference
variable, or register variable
(cont'd.)

∗ Initializing pointer variables incorrectly
∗ Initialization int *pt = 5; // INVALID
∗ pt is a pointer to an integer; it must be initialized with a valid
address
∗ Becoming confused about whether a variable contains an
address or is an address
int &dist = miles; // contains an address or is an address???
int dist = &miles; // contains an address or is an address???
∗ Forgetting to use the bracket set, [ ], after the delete or
new operator when dynamically allocating or deallocating
memory
(cont'd.)

∗ Every variable has: data type, address, value
∗ A pointer is a variable that is used to store the address
of another variable
∗ An array name is a pointer constant
∗ Access to an array element using a subscript can
always be replaced using a pointer
∗ Arrays can be dynamically created as a program is
executing
Summary

∗ Arrays are passed to functions as addresses
∗ When a single-dimensional array is passed to a
function, the parameter declaration for the function
can be either an array declaration or a pointer
declaration
∗ Pointers can be incremented, decremented,
compared, and assigned
Summary (cont'd.)

Csc1100 lecture12 ch08_pt2

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Csc1100 lecture12 ch08_pt2