3.Arrays and Pointers

7/30/2019 3.Arrays and Pointers

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Chapter 6 - Arrays

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Outline6.1 Introduction6.2 Arrays6.3 Declaring Arrays6.4 Examples Using Arrays6.5 Passing Arrays to Functions

6.6 Sorting Arrays6.7 Case Study: Computing Mean, Median and Mode Using Arrays6.8 Searching Arrays6.9 Multiple-Subscripted Arrays


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6.1 Introduction

Arrays

Structures of related data items

Static entity

same size throughout program Dynamic data structures discussed in Chapter 12

2


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6.2 Arrays

Array Group of consecutive memory locations Same name and type

To refer to an element, specify Array name

Position number

Format:

arrayname[position number] First element at position 0 n element array named c:

c[ 0 ], c[ 1 ]...c[ n 1 ]

3

Name of array

(Note that all

elements of this

array have the

same name, c)

Position number

of the element

within array c

c[6]

-45

6

0

72

1543

-89

0

62

-3

1

6453

78

c[0]

c[1]

c[2]

c[3]

c[11]

c[10]

c[9]

c[8]

c[7]

c[5]

c[4]


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6.2 Arrays

Array elements are like normal variablesc[ 0 ] = 3;

printf( "%d", c[ 0 ] );

Perform operations in subscript. If x equals 3

c[ 5 - 2 ] == c[ 3 ] == c[ x ]

4


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Array Elements & IndicesEach member of an array is identified by unique index or

subscript assigned to it

The dimension of an array is determined by the number of

indices needed to uniquely identify each element.

An index is a positive integer enclosed in [ ]placed

immediately after the array name.

An index holds integer values starting with zero.

An array with 11 elements will look like -


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Defining an Array

An array has some particular characteristics and has to be

defined with them.

These characteristics include -

which indicates the location of the

first member of the array.

a constant evaluating to a +ve value


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Defining an Array - I

An array is defined in the same way as a variable is defined

except that the array name is followed by one or more

expressions, enclosed within square brackets [ ], specifying

the array dimension.


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Norms with Arrays

All elements of an array are of the same type.

Each element of an array can be used anywhere that a

variable is allowed or required.

Each element of an array can be referenced using a variable

or an integer expression.

Arrays can have their data types like int, char, float


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Array Handling In C


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Array Handling In CAn array is treated differently from a variable in C.

Two arrays, even if they are of the same type and size cannot

be tested for equality.

It is not possible to assign one array directly to another.

Values cannot be assigned to an array on the whole, instead

values are assigned to the elements of the array.


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Array Initialization

In the following example the array elements have been

assigned valued using the for loop.

In case of extern and static arrays, the elements are

automatically initialized to zero.

Each element of an Automatic array needs to be initialized

separately.


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Reading Unknown Number Of ElementsConsider the following case -

A program is written to calculate the average of given numbers

and print it. The total numbers to be input can range from 1 to

50. An array would naturally be considered the best way to

store the numbers. But a problem may arise in when fewer

numbers than the maximum length of the array is entered.

Solution -


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Strings / Character ArraysOutput -

The input for the above is of 4 characters and the 5 th

character is the null character.

The above output is for an input of 5 characters.


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String FunctionsC supports a wide range of string functions, which are

found in the standard header file

Some of them are listed below -


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Two Dimensional ArraysThe simplest and the most commonly used

multi - dimensional array is the two - dimensional array.

A two-dimensional array can be thought of as an array

of two single dimensional arrays.

A two - dimensional array looks like a school time-table

consisting of rows and columns.

A twodimensional array is declared as -


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Initialization of Multidimensional Arrays

The result of the above assignment will be as follows :


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Initialization of Multidimensional Arrays - I


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Initialization of Multidimensional Arrays - II

The result of the assignment will be as follows :

A two - dimensional string array is declared in the

following manner :


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6.3 Declaring Arrays

When declaring arrays, specify Name Type of array

Number of elementsarrayType arrayName[ numberOfElements ]; Examples:int c[ 10 ];float myArray[ 3284 ];

Declaring multiple arrays of same type Format similar to regular variables

Example:int b[ 100 ], x[ 27 ];

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6.4 Examples Using Arrays

Initializersint n[ 5 ] = { 1, 2, 3, 4, 5 };

If not enough initializers, rightmost elements become

0 int n[ 5 ] = { 0 } All elements 0

If too many a syntax error is produced syntax error

C arrays have no bounds checking

If size omitted, initializers determine itint n[ ] = { 1, 2, 3, 4, 5 };

5 initializers, therefore 5 element array

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6.4 Examples Using Arrays

Character arrays String first is really a static array of characters Character arrays can be initialized using string literals

char string1[] = "first";

Null character '\0' terminates strings string1 actually has 6 elements

It is equivalent tocharstring1[]={'f','i','r','s','t','\0'}; Can access individual charactersstring1[3] is character s

Array name is address of array, so & not needed forscanfscanf("%s",string2);

Reads characters until whitespace encountered Can write beyond end of array, be careful

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1 /* Fig. 6.10: fig06 10.c


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2000 Prentice Hall, Inc.

All rights reserved.

1. Initialize strings

2. Print strings

2.1 Define loop

2.2 Print characters

individually

2.3 Input string

3. Print string

Program Output 23

1 / Fig. 6.10: fig06_10.c

2 Treating character arrays as strings */

3 #include

4

5 int main()

6 {

7 char string1[ 20 ], string2[] = "string literal";

8 int i;

9

10 printf(" Enter a string: ");

11 scanf( "%s", string1 );

12 printf( "string1 is: %s\nstring2: is %s\n"

13 "string1 with spaces between characters is:\n",

14 string1, string2 );

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16 for ( i = 0; string1[ i ] != '\0'; i++ )

17 printf( "%c ", string1[ i ] );

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19 printf( "\n" );

20 return 0;

21 }

Enter a string: Hello therestring1 is: Hellostring2 is: string literalstring1 with spaces between characters is:

H e l l o


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6.5 Passing Arrays to Functions

Passing arrays To pass an array argument to a function, specify the name

of the array without any bracketsint myArray[24];myFunction(myArray,24);

Array size usually passed to function

Arrays passed call-by-reference

Name of array is address of first element

Function knows where the array is stored

Modifies original memory locations Passing array elements

Passed by call-by-value

Pass subscripted name (i.e.,myArray[3]) to function24


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6.5 Passing Arrays to Functions

Function prototypevoid modifyArray( int b[], intarraySize );

Parameter names optional in prototype

int b[] could be written int []

int arraySize could be simply int

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1 /* Fig. 6.13: fig06 13.c


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1. Function definitions

2. Pass array to afunction

2.1 Pass array elementto a function

3. Print

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g g _

2 Passing arrays and individual array elements to functions */

3 #include

4 #define SIZE 5

5

6 voidmodifyArray( int [], int ); /* appears strange */

7 voidmodifyElement( int );

89 int main()

10 {

11 int a[ SIZE ] = { 0, 1, 2, 3, 4 }, i;

12

13 printf( "Effects of passing entire array call "

14 "by reference:\n\nThe values of the "

15 "original array are:\n" );

16

17 for ( i = 0; i


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3.1 Functiondefinitions

Program Output

27

34 voidmodifyArray( int b[], int size )

35 {

36 int j;

37

38 for ( j = 0; j


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6.6 Sorting Arrays

Sorting data Important computing application Virtually every organization must sort some data

Bubble sort (sinking sort) Several passes through the array Successive pairs of elements are compared

If increasing order (or identical ), no change If decreasing order, elements exchanged

Repeat

Example: original: 3 4 2 6 7 pass 1: 3 2 4 6 7 pass 2: 2 3 4 6 7 Small elements "bubble" to the top

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6.8 Searching Arrays: Linear

Search and Binary Search

Search an array for a key value

Linear search

Simple Compare each element of array with key value

Useful for small and unsorted arrays

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6.8 Searching Arrays: Linear

Search and Binary Search

Binary search For sorted arrays

Comparesmiddle element with key If equal, match found Ifkey < middle, looks in first half of array

Ifkey > middle, looks in last half

Repeat

Very fast; at most n steps, where 2n > number ofelements

30 element array takes at most 5 steps

25 > 30 so at most 5 steps

30

5


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6.9 Multiple-Subscripted Arrays

Multiple subscripted arrays

Tables with rows and columns (mby n array)

Like matrices: specify row, then column

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Row 0

Row 1

Row 2

Column 0 Column 1 Column 2 Column 3

a[0][0]a[1][0]

a[2][0]

a[0][1]a[1][1]

a[2][1]

a[0][2]a[1][2]

a[2][2]

a[0][3]a[1][3]

a[2][3]

Row subscriptArray name

Column subscript


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6.9 Multiple-Subscripted Arrays

Initialization

int b[2][2]={{1,2},{3,4}}; Initializers grouped by row in braces

If not enough, unspecified elements set to zero

int b[2][2]={{1},{3,4}}; Referencing elements

Specify row, then column

printf("%d",b[0][1]);

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1 2

3 4

1 0

3 4


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Session 7


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What is a Pointer?A pointer is a variable, which contains the address of a memory

location of another variable.

If one variable contains the address of another variable, the first

variable is said to point to the second variable.

A pointer provides an indirect method of accessing the value of

a data item.

Pointers can point to variables of other fundamental data types

like int, char, or double or data aggregates like arrays or

structures.


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What are Pointers used for?Pointers are used in situations when passing actual values is

difficult or not desired. Some of the situations where pointers can be

used are -

To return more than one value from a function.

To pass arrays and strings more conveniently from onefunction to another.

To manipulate arrays easily by moving pointers to them

instead of moving the arrays itself.

To allocate memory and access it (Direct Memory

Allocation).

To create complex data structures, such as linked lists.


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Pointer VariablesIf a variable is to be used as a pointer, it must be declared.

A pointer declaration consists of a base type, a *, and a variable

name.

General declaration syntax is : -

For Example:


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Pointer OperatorsThere are 2 special operators which are used with pointers :

The & operator is a unary operator and it returns the memory address

of the operand.

The second operator * is the complement of &. It is a unary operator

and returns the value contained in the memory location pointed to by

the pointer variables value.


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Assigning Values To PointersValues can be assigned to pointers through the & operator.

The assignment statement will be -

Here the address of var is stored in the variable ptr_var.

It is also possible to assign values to pointers through another pointer

variable pointing to a data item of the same data type.


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Assigning Values To Pointers - IVariables can be assigned values through their pointers as well.

The above declaration will assign 10 to the variable var if ptr_var

points to var.


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Pointer ArithmeticAddition and subtraction are the only operations that can be

performed on pointers.

Take a look at the following example :

Let var be an integer type variable

having the value 500 and stored at the

address 1000.

Then ptr_var has the value 1000 stored in it. Since integers are 2

bytes long, after the expression ptr_var++; ptr_var will have the

value as 1002 and not 1001.


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Some More Examples

Each time a pointer is incremented, it points to the memory location

of the next element of its base type.

Each time it is decremented it points to the location of the previous

element.

All other pointers will increase or decrease depending on the length

of the data type they are pointing to.


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Pointer ComparisonsTwo pointers can be compared in a relational expression provided

both the pointers are pointing to variables of the same type.

Consider that ptr_a and ptr_b are 2 pointer variables, which point to

data elements a and b. In this case the following comparisons are

possible:


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Pointers as Function Arguments

The address of the data item is passed and thus the function can

freely access the contents of that address from within the function

When pointers are passed to a function:

The program routine can access the variables that lie within the

scope of the function.


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The function as well as the calling routine recognizes any change

made to the contents of the address.

In this way, function arguments permit data-items to be altered in the

calling routine and the function.

When the arguments are pointers or arrays, a call by reference is made

to the function as opposed to a call by value for the variable arguments.

Pointers as Function Arguments - I


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Function Definition

indicates that the arguments ptr_str points to type char and ptr_int

points to type int.

The function can be invoked by the statement,

where pstr is declared as a pointer and the address of the variable var

is passed.


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Assigning a value through,

Function Definition - I

in the function can now assign values to the variable var in the calling

routine, enabling a two way transfer to and from the function.


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Pointers and Single Dimensional Arrays

The address of an array element can be expressed in two ways :

By writing the actual array element preceded by the ampersand

sign.

By writing an expression in which the subscript is added to the

array name.

The following program shows the relationship between array

elements and their addresses.


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Pointers and Single Dimensional Arrays


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Pointers and Multi Dimensional Arrays

A two-dimensional array can be defined as a pointer to a group of

contiguous one-dimensional arrays.

A two-dimensional array declaration can be written as :

instead of


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Pointers & StringsUsing strings as pointers can be explained with the help of the


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Allocating MemoryThe following example uses a two-dimensional array to record

marks of 5 students for 10 subjects.

It uses pointers and malloc() function, to assign memory.

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3.Arrays and Pointers