Pointers & Arrays

CS2023 Winter 2004

Outcomes: Pointers & Arrays

� “C for Java Programmers”, Chapter 8, section 8.12, and Chapter 10, section 10.2

� Other textbooks on C on reserve

� After the conclusion of this section you should be able to

� Identify how pointers and array are similar and how they are different

� Use pointers to traverse C arrays (1D and 2D) and pointer subtraction to count how many elements have been traversed

� Use pointers to pass portions of C arrays to functions

Arrays and Pointers

� A single-dimensional array is a typed constant pointer initialized to point to a block of memory that can hold a number of objects.

��� � �� � � �

��� � ��� �� �

��

is an

� � �

pointer that points to a block of memory that can hold 1000 integer objects

� ��

is a pointer to

� � �.

�� � � �� �

� �� � �� �

Arrays and Pointers

� When an array is declared, compiler allocates enough contiguous space in memory to contain all the elements of array��� � � �� � � �� ��� �

� Base address of array is address of first element of array. Say bytes numbered 300, 304, 308, ..., 696 are alocated as addresses of a[0], a[1],... a[99]

� � � � is equivalent to � � � � � � �

Causes 300 to be assigned to p

0 1 2 3 4 5 ... 99

!

"

Arrays Assignment and Pointers

# Can access first element of an array using:

$ %& ' ( )+*

# or through pointer:

, ( $*

- , ( )*

# Can access other elements of a using pointer arithmetic

. 0 1 2 3 4 5 ... 99

/0

Pointer Arithmetic

Valid operations on pointers include:

- the sum of a pointer and an integer- the difference of a pointer and an integer- pointer comparison- the difference of two pointers.

Sum of Pointer and Integer

1 To access other objects in the block of memory pointed to 2, use

2 3 4

where 4 is an integer. This expression yields a pointer to the n-th object beyond the one that 2 currently points to.

1 The exact meaning of "object" is determined by the type of 2.

Sum of Pointer and Integer

5�6 7 8 9: ;< =�> ?

> @ 8 ?

= > @ A ?

= B > C A D @ E ?

= B > C E D @ : ?F G H

I

JI K L I K M

Sum of Pointer and Integer

N�O P Q RS T UV W�X V W�Y Z

X [ Q \ ] Z

W�X [ ^ Z

Y [ X \ ^ Z

W Y [ _ Z

X \ [ _ Z

W�X [ ` Z

a 0 1 2 3 4 5 6 7 8 9

bc

a d

0 1 2 3 4 5 6 7 8 9

bc

ea d

0 1 2 3 4 5 6 7 8 9

c f

e b

Sum of Pointer and Integer: ith object

g�h ij j kl l m n�o prq m n s t o u m v�w i x k xh yz

{| h j } ~

� ��� � v�

or

� � v�

� �� �

and

�� are equivalent.

Sum of Pointer and Integer: ith object

� ��� �� � � �� �� �

��� � �� � � � � � � � �� ��� �

� � � �

�� � � � �   � � ¡ �� � � � � ¢ ¢ £

� � ��¤ ¥ � � � �¦ §� �¦ � � ¢ � £ � �   £ � �� ¨ �

� © � ª � � «� ¬­

®¯ � °± ²� £ �

� � � ¢ �� � � £ � ³´ µ �

Difference of Pointer and Integer

Often, we need to access objects preceding the object pointed to by a pointer ¶·

¶ ¸ ¹

yields a pointer to the n-th object before the one that º currently points to.

Difference of Pointer and Integer

»�¼ ½ ¾ ¿À Á Âà Ä�Å Ã Ä�Æ Ç

Å È ¾ É Ê Ç

Ä�Å È Ë Ç

Æ È Å Ì Í Ç

Ä Æ È Î Ç

Å Ì È Î Ç

Ä�Å È Í Ç

0 1 2 3 4 5 6 7 8 9

Ï

Ð Ñ

Ò

0 1 2 3 4 5 6 7 8 9

Ó

Ô Õ

Ö× Ø

0 1 2 3 4 5 6 7 8 9

Ù Ú

ÛÜ

Example

Ý Þàß áâ ãß äå æç è

â ã é êìë í êìî í ï ð äå æç ñóò

ë ô ïò

õ ö ê â ã â é â ï ÷ â ø ß é ùàß ú ÷àû ü ý ê ö

ö ê û þ éë þ é â ã ÿ ß � ß ÿ� ß û ÿ Þàß ÿ ê ö

áû ÿ � â ô � í î ô ë � ä å æç � �ò â � ä å æç ò â � � �

ë ÿ â ã é á � á� ã í ê � î � â � �ò

� � è

�

��� � ��� ��

Careful!

Given a block of memory of

�� ��

objects, pointed to by �, you can set � to point to the last object in the block using:

� � �� �� !

rather than

" # $% &'

(‘off by one’ error).

Pointer Comparison

( Two pointers of the same type, ) and *, may be compared as long as both of them point to objects within a single memory block

+ Pointers may be compared using the <, >, <=, >=, == , !=

, When you are comparing two pointers, you are comparing the values of those pointers rather than the contents of memory locations pointed to by these pointers

Pointers and traversalsAssuming-/. 0 12 3 465 7 4 5 8:9

and ; pointing to an array of

<= >?

doubles:

@:A B C/D E F D G D B A H:I J K F LNM D E O D P QR ST M D E P P U

D B A H I J K V F VD EM

V D B E/W K X/Y J Z:[ Y B H/\ V

@:A B C/D E F D P QR ST ] LM D E ^ F D M D E ] ] U

D B E/W K @ C_ ` @ W _ G VD E U M

Block Traversal Idiom

ab c d e f d g h ij kl f d h h m

n o p pq p

Example: largest element

rs tvu wyx z{ w | u }�~ z� u � w u } u �u { w �{ ~ z z~ � � s r

��� x � } u � �{ �� }�~ z� u � w � �� x � } u � �� � ��{ w { �

�� � x � } u s �~ � � s � ���

�� z � �~ � � � � � � � � � � � � � � � � { � � � � � �

� � � s �~ � � s � � �

�~ � � � ���

z u w x z{ s �~ ��

�

Example: block copy

Copy the contents of a block (or array) pointed to by � of size

�� ��

to another block pointed to by �:

�/  ¡ ¢£ ¤ ¥6¦ §¨ ¥6© §:ª

�/  ¡ ¢£ ¤ ¦ «¬ ­® ¯ ° ¨ © «¬ ­ ® ¯ °

±  ² ³ © § ´ © ¨ ¦ § ´ ¦ ª © § µ © ¶¬ ­ ® ¯ª © § ¶ ¶ ¨¦ § ¶ ¶ ·¥6© § ´ ¥ ¦ §ª

Pointer Subtraction

Given two pointers, ¸ and ¹, such that:• both pointers are of the same type, • º > »

• both point to objects in a single memory block, the expression

¼ ½ ¾

yields the number of objects between ¿ and À

,

including the object pointed to by Á. The type of the result of pointer difference is  ÃyÄ ÅÆ Ç ÇÈ Ã

, defined in É Ê Ë Ë/Ì ÍÎ Ï

.

Pointer Subtraction

Ð/Ñ Ò Ó ÔÕ Ö ×Ø Ù6Ú Ø Ù6Û Ü

Ú Ý Ó Ü

Û Ý Ó Þ ß Ü

Ú à ÐÑ Ò áâ ãä å ãØ Û æ Ú ç ÜOutput: 3

0 1 2 3 4 5 6 7 8 9

èé

ê

Example

Find the first occurrence of the value

ë

in a block of integers (

ì íî

if not found):ïñð ò óô õ ï ò ïô ðö

ïñð ò ÷ óùø ÷ ó ïö

ú ÷ ó ïñð ï ò ïñû ü ïþý ÿ � ÷ ú

� � ��ô � � ó ï � óö ó ï � ó � � � ö ó ï � � �

ï � � ÷ ó ï � � �

� � ÿ û �ö

óô õ ï ò ïô ð � � ó ï � � ó � � � � � � �� � ó ï�� ó � �ö

Example

Find the first occurrence of the value

�

in a block of integers (

� ��

if not found):��� � � ! � � � � "

��� � # �%$ # � � "

& # � ��� � � ��' ( �*) + , # &

- - -. / 0 � � 1 � " � � 2 � 3 45 67 " � � 3 3 8

� . 0 # � � 1 1 9 8

: / +' ; "

� ! � � � � 1 0 � � 1 1 � 3 45 67 8 < 7 => ? � �A@ � 3 B "C D E

FG

F HIJK LNM L JO = 3

Arrays as Parameters

When arrays are used as function parameters, they are actually

treated as pointers. The following two declarations are equivalent:

PRQ S TU V PW XY Z[ \] ^ U _ _ `a b PRQ S c Ped ^ feg

PRQ S T U V Ph X Y Z[ \] ^ i U _ _ b PRQ S c Pd ^ fg

The second parameter is necessary to specify the size of the array.

jlk m no p qr n n ps t jlk m u vw x j k m y j{z o | }

j k m ~�� j{�

�{� n t � j � u� � j � � j � y j{z o � � j � � |

j � t y� pk � t� � q� x � j | � � � |

no m�� nk � �

n o m�� nk � �

�� � j q � n jlk m r n n ps t j k m u vw x jlk m y j z o | }

j k m ~�� j{�

�{� n t � j � u� � j � � j � y j{z o � � j � � |

� n jlk m � t � q� x ~�� j | ��

�� ��� � �� �� � � �� � � � �� �   � � � � ¡ � �   � �� � � � � � � ¡ � �

¢  � �� � � £¤ ¥¦ §¨

  �© ª �l�  « £¤ ¥¦ ¬{­

� �® � � �   � � � � ¡ ®  °¯ £ ¤ ¥¦ ± ² ² ¨ ±

³´ ´ µ´ ¶

·¸ ¹lº » ¼¸ ¸ ½¾ ¿À Á Âà ÄÅ Æ ¶

·¸ ¹lº » ¼¸ ¸ ½¾ ¿À Á Âà ÄÅ Ç È É Æ ¶ ÊË ·¸ Ì Í ¹{Î Ë Ê

·¸ ¹lº » ¼¸ ¸ ½¾ ¿À Ï Ð Á Âà ÄÅ Ç Ð Æ ¶ Ê Ë ÑÒ Í Í ¹Î Ë Ê

·¸ ¹lº » ¼¸ ¸ ½¾ ¿À Ï Ð Á Ó Æ ¶ Ê Ë Ñ ÌÔ Õ Ì º » Ë Ê

Are Arrays and Pointers Interchangeable?

Ö Array parameters are interchangeable with pointer parameters, but array variables are not the same as pointer variables.

× C compiler converts name of an array to a constant pointer when necessary

Ø sizeof(array) returns number of bytes occupied by array, whereas sizeof(pointer) returns number of bytes used to store pointer

Are Arrays and Pointers Interchangeable?

ÙRÚ Û ÙÜ ÝÞ ß ß ß àeá

ÙRÚ Û âäã å Ù Ú Ûçæ è é Ü á

ê ëeì íî ïð ëñ ò

is

ó ô ô ôõ ö ÷eø ùú ûü ÷Rý þ ÿ

� ��� �� �� � � � � �� ��

is the number of bytes used to

store a pointer to an

�� �.

Pointers & Multi-Dimensional Arrays

� � �� � ��� � � �� � ���

� example: ! " # $% & $' &�(

) Arrays stored in row-major order

0 1 2543

*+ , -*+ , .

/

0 12 30 14 3

0 1 2 543

56 7 8 56 7 9

Pointers & Multi-Dimensional Arrays

:<; = > ?@ AB C D EF G H ?@ AB C I EJ G HLK:<; = MN OQP R N SK

TN M U MN O V WK MN O X @ AB C D EF GK M N O Y Y Z

TN M U RN S V WK RN S X @ AB C I EJ GK RN S Y Y Z

> ? MN O H ? RN S H V WK

View [ as a one-dimensional array of integers:

\<] ^ _a` b

cLd e f ` g hji kl m kl m b ` n g hi ko pq r s tu vxw y m ko pq r z t{ v w y m b

` | | }

_a` g l b

Pointers & Multi-Dimensional Arrays

Point to row

~

in array �:

�<� � �a� �

� � �j� � �� �� � �

or:

�� � �� �L�

since � �� �

is the

�

th row of �

contrast with single-dimensional array where � �� �

equivalent to

� ��   ¡¢

Pointers & Multi-Dimensional Arrays

Clear row

£

of array ¤:

¥<¦ § ¥<¦ § ¨ ©ª «¬ ­ ® ¯° ± ² ©ª «¬ ­ ³ ¯´ ± ²¶µ ·a¸ µ ¥L¹

ºL» ¼ ½ ¸ ¾ ¨ © ¥ ² ¹ ¸ ¿ ¨ © ¥ ² À ª «¬ ­ ³ ¯´ ± ¹ ¸ À À Á

·a¸ ¾ Â ¹

Can pass à ÄÅ Æ

to a function that is expecting a one-dimensional array as its argument. Find largest element in row

Ç

of È:

É<Ê ËÌ ÍÎ Ï Ð ÑÒ Ó ÔÕ É<Ê ËÌ ÍÎ Ï Ö Ê × Ò Ø¶Ù ÚÛ ÜÕ ÝÞ ß à áLâ

Pointers & Multi-Dimensional Arrays

Caution:

• The base address of a two-dimensional array ã is äæå çè é çè é

, not ê. • The array name ë by itself is equivalent to

ìæí îï ð

Storage Mapping Function

ñò ó ô õö ÷ õø ÷�ù

ô õ ñ ÷ õú ÷

is equivalent to

û üý þ ÿ� � ÿ � � � � � � ��

• Note that column size is required in storage mapping function, which is why it must be specified in function declaration� � � � � � � � �� �� �� � �� � � �� � � �