Algo>Arrays

C++ Style Data Structures: Arrays(1)
• An ordered set (sequence) with a fixed
number of elements, all of the same
type,
where the basic operation is
direct access to each element in the
array so values can be retrieved from
or stored in this element.

C++ Style Data Structures: Arrays (2)
Properties:
Ordered so there is a first element, a second one, etc.
Fixed number of elements — fixed capacity
Elements must be the same type (and size);
∴ use arrays only for homogeneous data sets.
Direct access: Access an element by giving its location
The time to access each element is the same for all
elements, regardless of position.
in contrast to sequential access (where to access an
element, one must first access all those that precede it.)

Declaring arrays in C++
where
element_type is any type
array_name is the name of the array — any valid identifier
CAPACITY (a positive integer constant) is the number of elements
in the array
score[0]
score[1]
score[2]
score[3]
score[99]
.
.
.
.
.
.
element_type array_name[CAPACITY];
e.g., double score[100];
The elements (or positions) of the array are indexed 0, 1,
2, . . ., CAPACITY - 1.
Can't input the
capacity, Why?
The compiler reserves a block of “consecutive” memory
locations, enough to hold CAPACITY values of type
element_type.

indices numbered 0, 1, 2, . . ., CAPACITY - 1
How well does C/C++ implement an array ADT?
As an ADT In C++
ordered
fixed size
same type elements
direct access
element_type is the type of elements
CAPACITY specifies the capacity of the array
subscript operator []

an array literal
Array Initialization
Example:
double rate[5] = {0.11, 0.13, 0.16, 0.18, 0.21};
Note 1: If fewer values supplied than array's capacity, remaining elements
assigned 0.
double rate[5] = {0.11, 0.13, 0.16};
Note 2: It is an error if more values are supplied than the declared size of the array.
How this error is handled, however, will vary from one compiler to another.
rate
0 1 2 3 4
0.11 0.13 0.16 0 0
rate
0 1 2 3 4
0.11 0.13 0.16 0.18 0.21
In C++, arrays can be initialized when they are declared.
Numeric arrays:
element_type num_array[CAPACITY] = {list_of_initial_values};

Note 1: If fewer values are supplied than the declared size of the array,
the zeroes used to fill un-initialized elements are interpreted as
the null character '0' whose ASCII code is 0.
const int NAME_LENGTH = 10;
char collegeName[NAME_LENGTH]={'C', 'a', 'l', 'v', 'i', 'n'};
vowel
0 1 2 3 4
A E I O U
char vowel[5] = {'A', 'E', 'I', 'O', 'U'};
Character Arrays:
Character arrays may be initialized in the same manner as numeric arrays.
declares vowel to be an array of 5 characters and initializes it as follows:
collegeName
0 1 2 3 4 5 6 7 8 9
C a l v i n 0 0 0 0

Addresses
When an array is declared, the address of the first byte (or word) in the block of
memory associated with the array is called the base address of the array.
Each array reference must be translated into an offset from this base address.
For example, if each element of array score will be stored in 8 bytes and the base
address of score is 0x1396. A statement such ascout << score[3] << endl;
requires that array reference score[3]
be translated into a memory address:
0x1396 + 3 * sizeof (double)
= 0x1396 + 3 * 8
= 0x13ae
The contents of the memory word with this address
0x13ae can then be retrieved and displayed.
An address translation like this is carried out each time
an array element is accessed.
score[3] →
[0]
[1]
[2]
[3]
[99]
.
.
.
.
.
.
score → 0x1396
0x13ae
What will be the
time complexity

The value of array_name is actually the base address of array_name
array_name + index is the address of array_name[index].
An array reference array_name[index]
is equivalent to
For example, the following statements of pseudocode are equivalent:
print score[3]
print *(score + 3)
Note: No bounds checking of indices is done!
* is the dereferencing operator
*ref returns the contents of the memory location with address ref
*(array_name + index)
What will happen incase
of going overboard

Problems with Arrays
1. The capacity of Array can NOT change during program execution.
What is the problem?
Memory wastage
Out of range errors
2. Arrays are NOT self contained objects
What is the problem?
No way to find the last value stored.
Not a self contained object as per OOP principles.

C++ Style Multidimensional Arrays
Most high level languages support arrays with more than one dimension.
2D arrays are useful when data has to be arranged in tabular form.
Higher dimensional arrays appropriate when several characteristics associated
with data.
Test 1 Test 2 Test 3 Test 4
Student 1 99.0 93.5 89.0 91.0
Student 2 66.0 68.0 84.5 82.0
Student 3 88.5 78.5 70.0 65.0
: : : : :
: : : : :
Student-n 100.0 99.5 100.0 99.0
For storage and processing, use a two-dimensional array.
Example: A table of test scores for several different students on
several different tests.

Declaring Two-Dimensional Arrays
Standard form of declaration:
element_type array_name[NUM_ROWS][NUM_COLUMNS];
Example:
const int NUM_ROWS = 30,
NUM_COLUMNS = 4;
double scoresTable[NUM_ROWS][NUM_COLUMNS];
Initialization
♦ List the initial values in braces, row by row;
♦ May use internal braces for each row to improve readability.
Example:
double rates[][] = {{0.50, 0.55, 0.53}, // first row
{0.63, 0.58, 0.55}}; // second row
[0]
[1]
[2]
[3]
[29]
[0] [1] [2] [3]

Processing Two-Dimensional Arrays
♦ Remember: Rows (and) columns are numbered from zero!!
♦ Use doubly-indexed variables:
scoresTable[2][3] is the entry in row 2 and column 3
↑ ↑
row index column index
♦ Use nested loops to vary the two indices, most often in a rowwise manner.
Counting
from 0

Higher-Dimensional Arrays
The methods for 2D arrays extend in the obvious way to 3D arrays.
Example: To store and process a table of test scores for several different
students on several different tests for several different semesters
const int SEMS = 10, STUDENTS = 30, TESTS = 4;
typedef double ThreeDimArray[SEMS][STUDENTS][TESTS];
ThreeDimArray gradeBook;
gradeBook[4][2][3] is the score of 4th
semester for student 2 on test 3
// number of semesters, students and tests all counted from zero!!

Arrays of Arrays
double scoresTable[30][4];
Declares scoresTable to be a one-dimensional array containing
30 elements, each of which is a one-dimensional array of 4 real numbers; that is,
scoresTable is a one-dimensional array of rows , each of which has 4
real values. We could declare it as
typedef double RowOfTable[4];
RowOfTable scoresTable[30];
[0]
[1]
[2]
[3]
[29]
[0] [[1] [2] [3]
[0] [[1] [2] [3]
[0]
[1]
[2]
[3]
[29]

scoresTable[i] is the i-th row of the table
Address Translation:Address Translation:
The array-of-arrays structure of multidimensional arrays explains
address translation.
Suppose the base address of scoresTable is 0x12348:
scoresTable[10] 0x12348 + 10*(sizeof RowOfTable)
In general, an n-dimensional array can be viewed (recursively) as a
one-dimensional array whose elements are (n - 1)-dimensional arrays.
In any case:
scoresTable[i][j] should be thought of as (scoresTable[i])[j]
that is, as finding the j-th element of scoresTable[i].
→
scoresTable[10]
[3] → base(scoresTable[10]) + 3*(sizeof double)
scoresTable[10]
[4]
[3]
[0]
[1]
[9]
[10]
= 0x12348 + 10 * (4 * 8) + 3 * 8
= 0x124a0
= 0x12348 + 10 * (4 * 8)

Implementing Multidimensional Arrays
More complicated than one dimensional arrays.
Memory is organized as a sequence of memory locations, and is thus 1D
How to use a 1D structure to store a MD structure?
A B C D
E F G H
I J K L
A character requires a single byte
Compiler instructed to reserve 12 consecutive bytes
Two ways to store consecutively i.e. rowwise and columnwise.

Implementing Multidimensional Arrays
A B C D
E F G H
I J K L
RowWise
A
B
C
D
E
F
G
H
I
J
K
L
ColumnWise
A
E
I
B
F
J
C
G
K
D
H
L
A B C D
E F G H
I J K L
A B C D
E F G H
I J K L

Algo>Arrays

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