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CSCI101 An Introduction to Programming using C

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An array variable is a special kind of object that can take many values - all of ... int result1; int result2; int result3; and so on . . . 11/8/09. Anne Dawson c 2006 ... – PowerPoint PPT presentation

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Title: CSCI101 An Introduction to Programming using C

1
CSCI101 An Introduction to Programming using
C
2

Arrays

3
Arrays

1 Introduction to Arrays
2 Arrays in Functions
3 Programming with Arrays

4
Arrays

1 Introduction to Arrays
2 Arrays in Functions
3 Programming with Arrays

A variable (or simple variable) can take a
single value.
An array variable is a special kind of object
that can take many values - all of the same type.

Say you want to store the exam results of 30
students. You could create 30 variables
int result1
int result2
int result3 and so on . . .

If you only had a few results to score, this
method would be acceptable. But what if you had
to store the results of each student in the
college, not just one class?

Storing 2000 results in this way would be a
very tedious and error-prone process. You would
have to type 2000 declaration statements and 2000
assignments.

Fortunately, by using an array variable, we
get around this problem . . .

10
What is an array variable?

First consider a simple variable, the type we
have used do far
int result

After this statement C reserves an area of
memory which is referred to by the name result
which is big enough to hold one integer number,
for example the number 90. Imagine this area of
memory as a mail box
result

90
12
An Array Variable

Consider an array variable as a line of these
boxes, each box is referred to by a number
0 1 2 3 4 5
6 7
result

13
An Array Variable

The line of boxes is known as an array
variable. An array variable is given a name,
just like the simple variable, for example
result, and each box is referred to by its
number result0, result1 and so on...
0 1 2 3 4 5
6 7
result

14
Important

The number of the box is known as the
subscript or index of the array element (box).
The variables that make up the array are known as
indexed variables, subscripted variables or
elements.

15
Important

Note In C the index of an array always
starts at 0.

16
Declaring an Array Variable

int result8
The number 8 is said to be the size of the
array. int is said to be the base type of the
array. The base type can be any type, in
particular it can be a class type, i.e. an object.

17
Declaring an Array Variable

int result8
// note use of square brackets
// after declaring an array, until you assign
values to the elements, the elements contain
garbage (denoted by )
0 1 2 3 4
5 6 7
result

18
Assigning Values to Arrays

result0 75
result1 90
result4 72
0 1 2 3 4 5
6 7

75
90

72

19
Assigning Values to Arrays at Declaration

int result8 75,90,0,0,72,0,0,0
int result 75,90,0,0,72,0,0,0
0 1 2 3 4 5
6 7

75
90
0
0
0
72
0
0
20
Initializing Arrays

int children3 2, 12, 1
The above declaration is equivalent to the
following code
int children3
children0 2
children1 12
children2 1

21
Initializing an Array

int b 5, 12, 11
is equivalent to
int b3 5, 12, 11

22
How big are the Elements?

An element is one of the boxes making up the
array. The size of the box in bytes depends on
the data type of what you put in there. . .

23
An Array Variable

Consider an array variable as a line of these
boxes. The boxes are all the same size.
0 1 2 3 4 5
6 7
result

24
Data Types

Each data type takes up a fixed amount of
memory. . .

If you declare result as an array of
integers, each element occupies 4 bytes of
memory.
If you declare result as an array of double,
then each element occupies 8 bytes of memory

26
Integer Data Types

Type Name Memory
short (int) 2 bytes
int 4 bytes
long (int) 8 bytes

27
Floating point Data Types

Type Name Memory
float 4 bytes
double 8 bytes
long double 10 bytes

28
A character

A character occupies a single byte, and its
byte value is the decimal value given by its
ASCII code.
A is decimal 65 or binary 01000001

29
A Program Using an Array array-01.cpp

http//www.annedawson.com/array-01.cpp

30
Indexed variable as an argumentarray-03.cpp

http//www.annedawson.com/array-03.cpp

31
adjust_days(vacationn)
32

for (n 0 n lt NUMBER_OF_EMPLOYEES n)
vacationn adjust_days(vacationn)
//
int adjust_days(int old_days)
return (old_days 5)

1.
2.
33

An entire array can be passed to a function. .
.

34
Arrays

1 Introduction to Arrays
2 Arrays in Functions
3 Programming with Arrays

The name of any array is the address of its
first element.

36
Array Parameters

A formal parameter for an entire array is
neither call-by-value or call-by-reference. It
is a new kind of formal parameter referred to as
an array parameter.

37
The formal parameter (in the function header) for
an entire array is, for exampleint arrayname
38
An example function headervoid fill_up(int
a, int size)
39

void fill_up(int a, int size) // prototype
void fill_up(int a, int size) // definition
cout ltlt "Enter " ltlt size ltlt "numbers\n"
for (int i 0 i lt size i)
cin gtgt ai
size-- // reduces size by 1
cout ltlt "The last array index used is "
ltlt size ltlt endl

An example function call
int score5
int number_of_scores 5
fill_up(score, number_of_scores)
the function call array integer

An array parameter behaves very much like a
call-by-reference parameter. This is because,
when you send an entire array to a function, you
pass across to the function the array name. The
name of any array is the address of its first
element.

An array parameter behaves very much like a
call-by-reference parameter. This is because,
when you send an entire array to a function, you
pass across to the function the array name. The
name of any array is the address of its first
element.

43
Using an entire array as a parameter to a function

You are actually passing the address of the
array variable (the start address). . .

Changes made to an array passed to a method
are permanent changes, as the function is working
on the actual array, not on a copy.

Call-by-reference parameters also work by
passing the address of the variable, rather than
a copy of the value of the variable as in
call-by-value. Think of the word 'reference' as
'referring to an address'.

Call-by-reference parameters also work by
passing the address of the variable, rather than
a copy of the value of the variable as in
call-by-value. Think of the word 'reference' as
'referring to an address'.

47
Why is an array parameter different to a
call-by-reference parameter?

Call-by-reference is for simple variables
(int, char etc), and the compiler knows the size
(in bytes) of all the simple variables, as the
size is fixed for each data type.

48
Integer Data Types

Type Name Memory
short (int) 2 bytes
int 4 bytes
long (int) 8 bytes

49
Floating point Data Types

Type Name Memory
float 4 bytes
double 8 bytes
long double 10 bytes

50
Using a simple variable as a call-by-reference
parameter to a function

You are actually passing the address of the
variable when you use call-by-reference. And the
compiler 'knows' the size in bytes of the
variable, because you also have to specify the
data type in the prototype.

51
Using an entire array as a parameter to a function

Again, you are actually passing the address of
the array variable (the start address). . .
but the compiler does not know the size in
bytes of the entire array, only the size in bytes
of a single element.

52
When you pass an entire array to a function

The compiler does not know the size in bytes
of the entire array, only the size in bytes of a
single element.
This is why when you pass an array to a
function, you should also pass an integer which
is the number of elements in the array.

int score5
int number_of_scores 5
fill_up(score, number_of_scores)
the function call array array size

Changes made to an array passed to a function
are permanent changes, as the function is working
on the actual array, not on a copy.

55
The const parameter modifier

If you don't want a function to change an
array passed to it you should then pass the array
using the const parameter modifier. The array
parameter is then known as a constant array
parameter

funct1(const int a,int size_of_a) // the
function prototype
56
The const parameter modifier

. . . can be used with any type of parameter,
but it is usually used only with array parameters
and call-by-reference parameters for classes.

57
Arrays

1 Introduction to Arrays
2 Arrays in Functions
3 Programming with Arrays

58
Programming with Arrays

Most programs involving arrays commonly sort and
search the elements of the array. . .

59
Sorting an Array

Sorting an array is the process of converting
an unordered array into an ordered array.

An unordered numerical array
0 1 2 3 4 5 6
7
An ordered numerical array
0 1 2 3 4 5 6
7

34
21
43
12
99
72
32
2
2
12
21
32
99
34
43
72
61

An unordered character array
0 1 2 3 4 5 6
7
An ordered character array
0 1 2 3 4 5 6
7

'Z'
'B'
'O'
'Y'
'C'
'P'
'A'
'L'
'A'
'B'
'C'
'L'
'Z'
'O'
'P'
'Y'
62

Searching Arrays....

63
Searching an Array

A very common task is to search an array for a
given value.
For example, an array contains student numbers
for students enrolled in a course. . .
you may want to search the array to see if a
particular student number is contained in it.

64
Linear Search Animation