Lecture 15 Arrays with C++ programming.ppt

Lecture Topics
 Array definition
 Array declaration
 Array assignment
 Array Input / Output
 Examples

What is an Array?
 It’s a collection of variables (the same type)
grouped into one name.
 More specifically, it’s a group of memory
locations for variables of the same type and
specified by the same name.
 It makes dealing with related variables much
easier.

Parts of Arrays
Elements
 Refers to the number of individual items
represented by the array
Index (or more formally, Subscript)
 Refers to one particular element in the array
 The first position in an array is represented by
an index, or subscript of 0 (zero). For example,
• arrStudentGrades[ 0 ]
 The second position is referred to by
• arrStudentGrades[ 1 ]

Arrays
 An array is an ordered list of values
0 1 2 3 4 5 6 7 8 9
79 87 94 82 67 98 87 81 74 91
An array of size N is indexed from zero to N-1
scores
The entire array
has a single name
Each value has a numeric index
This array holds 10 values that are indexed from 0 to 9

Arrays
 A particular value in an array is referenced using the
array name followed by the index in brackets
 For example, the expression
scores[2]
refers to the value 94 (the 3rd value in the array)
 That expression represents a place to store a single
integer and can be used wherever an integer variable
can be used

Arrays
 For example, an array element can be assigned
a value, printed, or used in a calculation:
scores[2] = 89;
scores[first] = scores[first] + 2;
mean = (scores[0] + scores[1])/2;
printf ("Top = %d”, scores[5]);

So how do we use arrays?
 Same concepts as other variables apply
 Must declare the array
 Must initialize the array
 Can use arrays in expressions and
functions, setting elements’ values or using
their values, similar to the use of ordinary
variables

Declaring an array
 The declaration is similar to the declaration
of other variables (except for the brackets
and number of elements):
int iMyFirstArray[ 15 ], iMySecondArray[ 20 ];
 You can use a #define constant to set the
size of the array
#define GRID_ROWS_MAX 8
int arrGridRows[ GRID_ROWS_MAX ] ;

Declaring Arrays
 The scores array could be declared as follows:
int scores[10] ;
 The type of the variable scores is an array of
integers
 The variable scores is set to a new blank
array that can hold 10 integers

Declaring Arrays
 Some examples of array declarations:
double prices[500] ;
int factor[12] , age[6];
char codes[30] ;

Initializing an Array
 You can initialize an array when you declare
it, as you do with other variables
 Syntax is slightly different, as you are now
initializing more than one element at a time
 One way at declaration, using initializers:
int iMyFirstArray[ 5 ] = { 0, 0, 0, 0, 0 } ;
 Note the braces around the initial zeroes
which themselves are separated by commas

Initializing an Array
(cont’d)
 If you specify fewer initializing values
than you have elements, all the rest are
initialized to a value of 0. For example:
int iMyFirstArray[ 5 ] = { 0 }; would set
all elements to 0
int iMyFirstArray[ 5 ] = { 4 }; would set
the zeroth element to 4 and the rest to 0!

Initializing an array
without specifying size
 You can also initialize and set the
number of elements with the same
step:
int iMyFirstArray[ ] = { 0, 0, 0, 0, 0 } ;
 Note: there is NO size specified; C
automatically makes it 5 elements since
you gave five initial values

Initializer Lists
 Examples:
int units[ ] = {147, 323, 89, 933, 540,
269, 97, 114, 298, 476};
char letterGrades[ ] = {'A', 'B', 'C', 'D', ’F'};
 Note that an initializer list can only be
used in the array declaration

Initializing array with a for
loop
 After declaring an array, you can initialize it in
the body of your program by using a for loop:
int i = 0, iMyFirstArray[ 5 ] ; /* size is 5*/
for ( i = 0 ; i <= 4 ; i++ )
{
iMyFirstArray[ i ] = 0 ;
} /* end for i */
 Note the upper bound is 4, not 5. That is, you
loop through 0 to 4 to initialize an array with
5 elements

Bounds Checking
 Once an array is created, it has a fixed
size
 An index used in an array reference must
specify a valid element
 That is, the index value must be in bounds
(0 to N-1)

Bounds Checking
 For example, if the array codes can hold
100 values, it can be indexed using only
the numbers 0 to 99
 If count has the value 100, then the
following reference will cause a problem:
for (int index=0; index <= 100; index++)
codes[index] = index*50 + epsilon;
problem

Array Input/ Output
 We typically use for loops for any kind of
array processing.
 To input an array, one by one:
for (i=0; i<10 ; i++ )
{
printf(“ Enter element %d : “, i );
scanf ( “ %d “, &scores[i] );
}

Array Output
 To display an array, one element per
line:
for (i=0; i<10 ; i++ )
{
printf(“ scores [%d] : %dn“, i ,
scores[i] );
}

#define SIZE 10
int main()
{
int myFirstArray[SIZE], i;
for (i=0; i<=SIZE-1; i++)
{
myFirstArray[i] = i * 2;
printf("myFirstArray with subscript of %d holds the
value %dn", i, myFirstArray[i]);
}
return 0;
}

Two-Dimensional Arrays
 A one-dimensional array stores a list of elements
 A two-dimensional array can be thought of as a
table of elements, with rows and columns
one
dimension
two
dimensions

Two Dimensional Arrays
 Declaration: int arr[5][5];
 Traversed using two nested for loops:
 for (i = 0; i<=4 ; i++)
 for (j = 0; j<=4; j++)
 arr[i][j] = i * j;

Multi dimensional arrays
 Arrays can be 3 dimensional, 4
dimensional, or of n dimensions.
 int arr[3][3][3];

Parallel Arrays
 These are independent arrays of the
same size, that have a meaningful
connection to each other.
 For example, one array with a students
gpa, and one with his letter grade.
GPA grade
3.8 A

Example
 Write a program that reads in an array
of 10 elements, gets their average, and
then display the deviation of each
element from that average.

Lecture 15 Arrays with C++ programming.ppt

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