5/3/01 Sudeshna Sarkar, CSE, IIT Kharagpur1 Structures Lecture 21 5.3.2001.

5/3/01 Sudeshna Sarkar, CSE, IIT Kharagpur

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Structures

Lecture 215.3.2001.


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Heterogeneous Structures

Collection of values of possibly different types.

Name the collection.Name the components.

Example : Student recordSinghal

name "V Singhal"rollno "00CS1001"classtest 14midterm 78final 73grade ‘B


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Structure : terminology A struct is a group of items (variables)

which may be of different types. Each item is identified by its own

identifier, each of which is known as a member or field of the structure.

A struct is sometimes called a record or structure.

Structs are the basis of classes in C++ and Java.


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Structure declaration

struct {char first[10];char midinit;char last[20];

} sname, ename;

This declaration createstwo structure variables,sname and ename, eachof which contains 3members.We can use sname.first,ename.midinit, etc.


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Members To access the members of a

structure, we use the member access operator “.”.strcpy (sname.first, “Sudeshna”);sname.midinit = ‘K’;strcpy (sname.last, “Sarkar”) ;


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Tagged structure

struct nametype {char first[10];char midinit;char last[20];

};struct nametype sname, ename;typedef struct nametype

NTYPE;NTYPE aname, bname;

This definition createsa structure tag nametypecontaining 3 members:first, midinit, last.Variables may be declaredof type struct <tagname>.

typedef is normally usedto give names to a struct type.


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typedeftypedef struct {

char first[10];char midinit;char last[20];

} NAMETYPE;NAMETYPE

sname,ename;


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Another example#define MAX_NAME 40typedef struct {

char name[MAX_NAME+1];char rollno[10];int classtest;int midterm;int final;char grade;

} StudentRecord;Defines a new data type called StudentRecord.

Does not declare a variable.


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Declaring struct variables

/* typedef structs go at top of program */. . .int .....float ....StudentRecord s1;StudentRecord singhal ;/* StudentRecord is a type; s1 and singhal are variables*/

struct nametype aname;/* struct nametype is a type; aname is a variable */


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Things you can and can't do

You canUse = to assign whole struct variables

You can Have a struct as a function return type

You cannot Use == to directly compare struct

variables; can compare fields directly You cannot

Directly scanf or printf structs; can read fields one by one.


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Struct initializers

/* typedef structs go on top */StudentRecord s1 = {"V Singhal",

"00CS1002", 15, 78, 73, 'B'};

Using components of struct variables

s1.classtest = 46;s1.midterm = 78;scanf ("%d", &s1.rollno) ;


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Assigning whole structs

s1 = singhal;is equivalent to

strcpy(s1.name, singhal.name) ; strcpy(s1.rollno, singhal.rollno; s1.classtest = singhal.classtest; s1.midterm = singhal.midterm; s1.final = singhal.final; s1.grade = singhal.grade;


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Within a given structure, the member names must be unique.

However, members in different structures may have the same name.

A member is always accessed through a structure identifier.struct fruit {

char name[20];int calories;

};struct vegetable { char name[30];

int calories;};

struct fruit mango;struct vegetable potato;It is clear that we can access mango.calories andpotato.calories withoutany ambiguity.


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Complicated structures A member of a structure can be an array or

another structure.struct grocerylist {

struct fruit flist[10];struct vegetable vlist[20];

} ; You can have an array of structures.

struct card {int pips;char suit;

} deck[52] ;


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A function using struct array

int fail (StudentRecord slist []) {int i, cnt=0;for (i=0; i<CLASS_SIZE; i++)

cnt += slist[i].grade == ‘F’;return cnt;

}


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Using structures with functions Structures can be passed as arguments to

functions. Structures can be returned from functions. Call by value is used if a structure is a function

parameter, meaning that a local copy is made for use in the body of the function. If a member of the structure is an array, then the array gets copied as well.

If the structure is large, passing the structure as an argument can be relatively inefficient. An address of th structure may be used as the parameter.


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Union A union is like a structure, except that the

members of a union share the same space in memory.union int_or_float {

int i;float f;

}; It is the programmer’s responsibility to know

which representation is currently stored in a union variable.


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Arrays of Structures A struct represents a single record. Typically structs are used to deal with

collections of such records Examples : student records, employee

records, book records, ... In each case we will hav multiple instances

of the struct type.Arrays of structs are the natural way to do

this.


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Arrays of structs : declaration & use

Each declaration below declares an array, where each array element is a structure:point corner_points[10] ;StudentRecord btech01[MAXS] ;

We access a field of a struct in an array by specifying the array element and then the field :btech01[i].namecorner_points[4].x


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Naming in struct Arrayspoint pentagon[5];

xy

xy

xy

xy

xy

pentagon : an array of points

pentagon[1] : a point structure

pentagon[4].x : a double


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Using Arrays of structs

StudentRecord class[MAXS];...for (i=0; i<nstudents; i++) {

scanf (“%d%d”, &class[i].midterm, &class[i].final);

class[i].grade = (double)(class[i].midterm+class[i].final)/50.0;

}


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struct Array elements as parameters

void draw_line (point p1, point p2) { ... }...point pentagon[5];...for (i=0;i<4;i++)

draw_line (pentagon[i], pentagon[i+1]);

draw_line (pentagon[4], pentagon[0]);


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structs as Parameters

A single struct is passed by value. all of its components are copied from

the argument (actual parameter) to initialize the (formal) parameter.

point set_midpt (point a, point b) { ... }int main (void) {

point p1, p2, m;...m = set_midpt(p1, p2);

}


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Passing Arrays of structs An array of structs is an array. When any array is an argument (actual parameter), it

is passed by reference, not copied [As for any array] The parameter is an alias of the actual array

argument.int avg (StudentRec class[MAX]) { ... }int main (void) {

StudentRec bt01[MAX];int average;...average = avg_midpt(bt01) ;

}

5/3/01 Sudeshna Sarkar, CSE, IIT Kharagpur1 Structures Lecture 21 5.3.2001.

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