Aggregate data structures: structs As well as more on arrays

1
Aggregate data structuresstructsAs well as
more on arrays

• COMP5212
• Week 4

Reference Hanly and Koffman, Ch 7, 8, 11.1-5
2
Post-conditions

• You will be able to
• use enums
• use structures
• typedefs
• arrays (strings)
• use pointers with them
• draw pictures of these in memory

3

• Memory review so far
• enums
• Heterogenous data structures - struct
• typedef
• Sizes in memory
• Homogenous data structures - arrays

4
The picture so far simple types

• simple data types
• int, char, float.....
• pointers to simple data types
• int , char , float

5
Simple data types
6
char
6
int
6.0
double
666666
char
666666
int
666666
double
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The picture so far array types

• Arrays of elements of the same simple type
• int, char, int, char .

7
Array of char
With arrays of char, each of the boxes in the
diagram below is really 1 byte. Adding 1 to
the pointer adds 1 byte.
ptr
A
2
7
m
l

y
(ptr1)
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Array of int
With arrays of int, each of the boxes in the
diagram below is really 4 bytes. Adding 1 to
the pointer adds 4 bytes.
ptr
-1
234
17
12
53
92
725
(ptr1)
9

• Memory review so far
• enums
• Heterogenous data structures - struct
• typedef
• Sizes in memory
• Homogenous data structures - arrays

10
How to represent this?

• One, two, three, four, five, six, seven, eight,
  nine, ten, eleven, twelve, thirteen, fourteen,
  fifteen, sixteen, seventeen, nineteen, twenty,
  twenty_one, twenty_two, twenty_three,
  twenty_four, twenty_five, twenty_six,
  twenty_seven, twenty_eight, twenty_nine, thirty,
  thirty_one, thirty_two, thirty_three,
  thirty_four, thirty_five, thirty_six, thirty_se

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Enumerated types

• Another simple type
• Maps to int
• Associates names with values
• You saw it in the lecture on style
• It is here for completeness

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Enumerated types

• enum day_name
• Sun, Mon, Tue, Wed, Thu, Fri, Sat, day_undef
• Maps to integers, 0 .. 7
• Sun
• So very close to int

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Enumerated types

• enum month_name
• Jan, Feb, Mar, Apr, May, Jun,
• Jul, Aug, Sep, Oct, Nov, Dec,
• month_undef

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Recall array of int
With arrays of int, each of the boxes in the
diagram below is really 4 bytes. Adding 1 to
the pointer adds 4 bytes.
ptr
-1
234
17
12
53
92
725
(ptr1)
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Now - array of day_name
With arrays of int-enum, each of the boxes in the
diagram below is really 4 bytes. Adding 1 to
the pointer adds 4 bytes.
ptr
Sun
Sat
Sun
Mon
Wed
Sat
Thu
(ptr1)
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Now - array of (int) day_name
With arrays of enum, each of the boxes in the
diagram below is really 4 bytes. Adding 1 to
the pointer adds 4 bytes.
ptr
0
6
0
1
3
6
5
(ptr1)
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Memory review so far enums Heterogenous data
structures - struct typedef Sizes in
memory Homogenous data structures - arrays
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Structure definition

• struct date
• enum day_name day
• int day_num
• enum month_name month
• int year

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Structure variables

• struct date
• enum day_name day
• int day_num
• enum month_name month
• int year
• struct date moonlanding
• struct date deadline day_undef, 1, Jan,
  2000
• struct date completion

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• struct date
• enum day_name day
• int day_num
• enum month_name month
• int year
• struct date moonlanding
• struct date deadline day_undef, 1,
  Jan, 2000
• struct date completion

Structure definition
Structure initialisation
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Another structure definition

• struct car_desc
• enum car_cols colour
• enum car_make make
• int year

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General form

• struct tag
• member-declarations
• identifier-list
• Once tag is defined, can declare structs with
• struct tag identifier-list

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• Memory review so far
• enums
• Heterogenous data structures - struct
• typedef
• Sizes in memory
• Homogenous data structures - arrays

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record/structure

• typedef struct date
• enum day_name day
• int day_num
• enum month_name month
• int year
• Date

members/fields of the record
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• typedef struct date
• enum day_name day
• int day_num
• enum month_name month
• int year
• Date
• Date Big_day Mon, 7, Jan, 1980
• Date moonlanding
• Date dopday day_undef, 1, Jan, 2000
• Date completion

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Accessing struct members/fields

• Use . (the dot operator)
• E.g.
• printf(month of landing d\n,moonlanding.month
  )
• If it is a pointer struct variable, use the
  indirection operators (-gt) or ( ).
• completion-gtmonth
• (completion).month

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• Memory review so far
• enums
• Heterogenous data structures - struct
• typedef
• Sizes in memory
• Homogenous data structures - arrays

28
Array of char
With arrays of char, each of the boxes in the
diagram below is really 1 byte. Adding 1 to
the pointer adds 1 byte.
ptr
A
2
7
m
l

y
(ptr1)
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Array of int
With arrays of int, each of the boxes in the
diagram below is really 4 bytes. Adding 1 to
the pointer adds 4 bytes.
ptr
-1
234
17
12
53
92
725
(ptr1)
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Array of THING
With arrays of THING, each of the boxes in the
diagram below is sizeof(THING) bytes. Adding 1
to the pointer adds sizeof(THING) bytes.
str
jkjkjjjjjkjkj
jkjkjjjjjkjkj
jkjkjjjjjkjkj
jkjkjjjjjkjkj
jkjkjjjjjkjkj
jkjkjjjjjkjkj
jkjkjjjjjkjkj
(str1)
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Remember Pointer Scalars
This works because of a special (hidden) value
associated with a pointer at the time of
declaration - the pointers scalar value. It
is used in arithmetic operations to ensure that
the expected behaviour occurs.
Typical scalar value is 1
char msg int ptr short fred
Typical scalar value is 4
Typical scalar value is 2
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When you need to know the size of things
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Accessing the size of data elements

• sizeof operator
• sizeof(int)
• sizeof(char)
• sizeof(Date)
• Why would you want to know sizeof?
• soon to be used with malloc

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Exercise
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Exercise

• Given the declarations
• struct A
• int a
• int b
• int c10
• struct A x
• struct A p
• What does this look like in memory?
• What do these do?

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• struct A
• int a
• int b
• int c10
• struct A x
• struct A p
• What do these do?
• x.a
• (p).a
• (p-gtb)
• p -gt c0
• x p

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• Memory review so far
• Enums
• Heterogenous data structures - struct
• Typedef
• Sizes in memory
• Homogenous data structures - arrays

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Examples of initialisations

• int A10 1, 1, 1, 1, 1, 1, 1, 1, 1, 1
• int B10 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
• int B10 0
• int B10
• int C 1, 2, 3, 4, 5, 6
• int C_size sizeof C / sizeof (int)

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General form

• type array-namesize
• value, ...

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Remember array-pointeryi equivalent to (yi)

• Consider these declarations.
• int AN
• int pa A0
• struct date BN
• struct date pb B0
• Draw the memory model.

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• int AN
• int pa A0
• struct date BN
• struct date pb B0
• A0
• pa
• A
• Aj
• (pa j)
• (A j)
• Bj
• (pb j)
• (B j)

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On out of bounds arrays

• Defensive programming.

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On out of bounds arrays

• No checking!
• No help from the compiler!
• int A100
• ...
• An 77
• ...

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• / contains relevant macro definition
• /
• include ltassert.hgt
• ...
• int A100
• ...
• assert((n gt 0) (n lt 100))
• An 77
• ...

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Multi-dimensional arrays

• Freedom to see things flexibly

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• int A35
• 1, 0, 0, 0, 1 ,
• 1, 1, 1, 1, 1 ,
• 0, 0, 1, 0, 1

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• void sum()
• int i
• int j
• for (i 0 i lt 3 i)
• for (j 0 j lt 5 j)
• APLUSBij Aij Bij

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Post-conditions (so far)

• You will be able to
• use enums
• use structures
• typedefs
• arrays (strings)
• use pointers with them
• draw pictures of these in memory

49
Sources

• All images (in order) from
• Microsoft Word clipart
• http//www.10-7.com/humor/photopages/
• sign.jpg
• keepright.jpg
• one ways.jpg
• killerchurch.jpg
• marquee.jpg
• they are assholes.jpg
• wendys.jpg
• santa plane.jpg
• Lecture prepared by Judy Kay Selvakennedy