CSO101: Computer Programming

Structures 1

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O.S.L. Bhavana

Composite Data Types

• How to store composite Name, Roll No, Marks, Section Name

of 158 students?

• char name[158][40];
• int rollNo[158];
• float marks[158];
• char section[158];

Maintaining many arrays and keeping track of their indices is combursome

Structures

• A structure is a user-defined data type
• A structure contains a number of different data types together

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Structures : Definition

• struct is a keyword;
• student is the name we gave to this structure
• In this example, the structure contains 4 variables of different data types
• Each variable in the structure is called a field of the structure
• The definition of structure must end with a semicolon

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Structures : Declaration of variables

• Syntax: struct structure_name variable_name;
• All variable naming rules apply for struct variables too.
• Declaration must come after definition of the structure
• Declaration along with definition is also possible.
• Declaration of a struct variable allocates space for all field of the structure in contiguous memory locations
• Ex2: on board

Structures: Memory Map

• Taught on board

Structure: Accessing Fields of a struct

• ‘ . ‘ operator is used to access fields of a structure
• a.marks refers to the marks field in the struct variable a
• a.marks can be treated as any float variable
• b.name refers to the character array name in the struct variable b
• b.name may be treated as any character array

Structures: Initialization (with list)

• List initialization can happen only in combination with declaration
• List initialization should not be separated from declaration
• In case of partial list initialization the uninitialized fields take value zero
• The whole struct variable may also be initialized to 0

Structures: Initialization after declaration

• After declaration, list initialization cannot be done and each field of the structure has to be initialized separately
• A uninitialized field may have garbage values

Arrays of Structures

• Array of a structure is a collection of structure variables of the same structure
• Declaration: struct struture_name array_name [array_size];
• Ex: struct student classBE[158];
• Accessing Fields of an array element: array_name[array_index].fieldname
• Ex: classBE[23].marks;
• classBE[i] refers to the i+1th element in the array of students

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Array of Structures

Memory Map of Array of Structures

• Taught on board

Assignment of Structures

• To copy the data of a structure variable into another variable of the same structure
• Each field may be copied individually
• The full structure may be copied at once

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Structures: Nesting

• Field of structure may itself be another structure

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Structures: sizeof

• Fields of a structure are stored in memory one after the other

in the order they are declared in the structure

• But they are not always in memory locations next to each other

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Structures: sizeof

• In the code sizeof the structure is printed to be 8 and not 5
• The address of x.b is 4bytes away from the address of x.a

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Structures: Data Alignment

• The memory is a sequence of bytes
• The CPU accesses memory in the form of memory words. A word is collection of 4bytes or 8 bytes depending on the machine
• The memory is actually organised as 4 (or 8) data banks. Suppose if a memory has 16bytes it can be organised in 4 data banks as follows

A word

Another word

Structures: Data Alignment

• Since each word can be accessed in just one memory access, a 4-byte integer is usually stored at addresses that are multiples of 4
• A integer stored in memory cells 8,9,10,11 can be accessed in one memory access by retreiving the word starting with address 8
• If an integer is stored in memory cells 6,7,8,9 it should be accessed by making 2 memory accesses – one to the word starting at 4 and one to the word starting at 8

Memory cells 6,7,8,9

do not form a word

Structures: Data alignment

• With the same reasoning a (built-in datatype) variable is stored at an address which is a multiple of the size of the data type
• char is stored at an address that is a multiple of sizeof(char)
• short int is stored at an address that is a multiple of sizeof(short int)
• Similarly for other data types
• In the previous example, if the char field is stored at address 4, the next field which is an integer (by default) will not be stored in cells 5 or 6 or 7, but it will be stored from the address 8.
• The total size of the struct is turns out 8 bytes

Structures: Data Alignment

8 bytes is the size of the above struct.

First byte for char;

2^nd byte left for alignment;

3,4 for short int

5-8 for int