CSO101: Computer Programming

Structures & Unions

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O.S.L. Bhavana

Structures

• Structures/unions may also be declared without explicit name, in which case its variables can only be declared with definition.

struct {int a; char b;} x,y;

union {int a; char b;} u,v;

int main(){scanf("%d", &u.a); printf("Hello World %d", u.a); return 0;}

• No more variables of the above structures can be declared after the definition

Passing arrays as structure fields

• When field of a structure is an array and the structure is being passed to a function then implicitly the array will also passed as a field of a structure
• Ex: struct FY{int a[10]; char b;} x,y={1,2,3,4,5,6,7,8,9,0,'l’};

void func(struct FY q){ printf("%d",q.a[9]); }

int main( ){ func(y); return 0;} // prints 0

• In the above example the structure and its fields are passed by value
• Similarly one can write a variant with pass by reference

typedef

• typedef is a keyword used to create synonyms for datatype names
• Ex1: The statement “ typedef int Length; ” makes Length a

synonym for the data type int

• Now variables of integer datatype may be declared in two ways
• int a,b,c; //(or alternatively)
• Length a,b,c;

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• Ex2: typedef struct student Stdnt; // lets to declare variables

of the structure student as

Stdnt a,b;

• Ex3: typedef int * intptr; // lets declare integer pointer variables

as “ intptr ptr1, ptr2; ”

Here both ptr1 and ptr2 will be declared as integer pointers

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typedef

• Syntax: typedef Existing-typename New-typename;
• Ex: typedef char* string; string a,b; // a,b are declared to be character pointers

typedef

• We can also merge a struct definition and typedef statement
• In this example cubd is defined as short form for struct cuboid

typedef

• typedef helps in writing a more readable code
• typedef too has scope restrictions

Unions

• Union is a user-defined data type in C
• Like Structures, unions also have fields/members of multiple datatypes .
• Unions allow to store variables of different datatypes at same memory location
• Unions are cleverly used to save memory
• Unlike in structures, all the fields of a union share the memory. Recall, for structures each field has a separate allocation in memory

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Unions

• Memory allocated to a union is the amount of memory required to store its largest field. Ex: sizeof(xyz) = 4
• Total 4 bytes of memory is allocated to an xyz variable

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Unions

• union is a keyword used to define unions
• Memory allocated by union is shared by the fields of the union
• Modifying value of any field will also effect the values of other fields
• In list initialization, only the first field of the union is initialized.

Ex: union xyz var1 = {‘A’,23,4.5} is valid. union xyz var1 = {2.3} is invalid

The type of the first member must match with the first value in list initialization

• Access to an field
• results in correct value if it is the last field to be updated
• results in incorrect/garbage values if it is not the last updated field
• Fields of unions can be accessed using ‘.’ and -> operators appropriately
• Direct assignment is also possible between two unions of same type

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Use of Pointers and Functions with Unions