Structures

Heterogeneous Structures Collection of values of possibly different types. Name the collection. Name the components. Example : Student record Singhal name "V Singhal" rollno "00CS1001" classtest 14 midterm 78 final 73 grade ‘B

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.

Structure declaration struct { char first[10]; char midinit; char last[20]; } sname, ename; This declaration creates two structure variables, sname and ename, each of which contains 3 members. We can use sname.first, ename.midinit, etc.

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”) ;

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 creates a structure tag nametype containing 3 members: first, midinit, last. Variables may be declared of type struct. typedef is normally used to give names to a struct type.

typedef typedef struct { char first[10]; char midinit; char last[20]; } NAMETYPE; NAMETYPE sname,ename;

Another example #define MAX_NAME 40 typedef 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.

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 */

Things you can and can't do You can Use = 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.

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) ;

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;

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 and potato.calories without any ambiguity.

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] ;

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; }

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.

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.

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.

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].name corner_points[4].x

Naming in struct Arrays point pentagon[5]; x y x y x y x y x y pentagon : an array of points pentagon[1] : a point structure pentagon[4].x : a double

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; }

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]);

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); }

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) ; }

Dynamic Memory Allocation, Structure pointers

Basic Idea Many a time we face situations where data is dynamic in nature. Amount of data cannot be predicted beforehand. Number of data item keeps changing during program execution. Such situations can be handled more easily and effectively using dynamic memory management techniques.

C language requires the number of elements in an array to be specified at compile time. Often leads to wastage or memory space or program failure. Dynamic Memory Allocation Memory space required can be specified at the time of execution. C supports allocating and freeing memory dynamically using library routines.

Memory Allocation Process in C Local variables Free memory Global variables Instructions Permanent storage area Stack Heap

The program instructions and the global variables are stored in a region known as permanent storage area. The local variables are stored in another area called stack. The memory space between these two areas is available for dynamic allocation during execution of the program. This free region is called the heap. The size of the heap keeps changing

Memory Allocation Functions malloc: Allocates requested number of bytes and returns a pointer to the first byte of the allocated space. calloc: Allocates space for an array of elements, initializes them to zero and then returns a pointer to the memory. free : Frees previously allocated space. realloc: Modifies the size of previously allocated space.

Dynamic Memory Allocation used to dynamically create space for arrays, structures, etc. int main () { int *a ; int n;.... a = (int *) calloc (n, sizeof(int));.... } a = malloc (n*sizeof(int));

Space that has been dynamically allocated with either calloc() or malloc() does not get returned to the function upon function exit. The programmer must use free() explicitly to return the space. ptr = malloc (...) ; free (ptr) ;

void read_array (int *a, int n) ; int sum_array (int *a, int n) ; void wrt_array (int *a, int n) ; int main () { int *a, n; printf (“Input n: “) ; scanf (“%d”, &n) ; a = calloc (n, sizeof (int)) ; read_array (a, n) ; wrt_array (a, n) ; printf (“Sum = %d\n”, sum_array(a, n); }

void read_array (int *a, int n) { int i; for (i=0; i<n; i++) scanf (“%d”, &a[i]) ; } void sum_array (int *a, int n) { int i, sum=0; for (i=0; i<n; i++) sum += a[i] ; return sum; } void wrt_array (int *a, int n) { int i; }

Arrays of Pointers Array elements can be of any type array of structures array of pointers

int main (void) { char word[MAXWORD]; char * w[N]; /* an array of pointers */ int i, n; /* n: no of words to sort */ for (i=0; scanf(“%s”, word) == 1); ++i) { w[i] = calloc (strlen(word)+1, sizeof(char)); if (w[i] == NULL) exit(0); strcpy (w[i], word) ; } n = i; sortwords (w, n) ; wrt_words (w, n); return 0; }

w Input : A is for apple or alphabet pie which all get a slice of come taste it and try A\0 is for apple try

void sort_words (char *w[], int n) { int i, j; for (i=0; i<n; ++i) for (j=i+1; j<n; ++j) if (strcmp(w[i], w[j]) > 0) swap (&w[i], &w[j]) ; } void swap (char **p, char **q) { char *tmp ; tmp = *p; *p = *q; *q = tmp; }

w w[i] for\0 apple Before swapping w[j]

w w[i] for\0 apple After swapping w[j]

Pointers to Structure

Pointers and Structures You may recall that the name of an array stands for the address of its zero-th element. Also true for the names of arrays of structure variables. Consider the declaration: struct stud { int roll; char dept_code[25]; float cgpa; } class[100], *ptr ;

The name class represents the address of the zero-th element of the structure array. ptr is a pointer to data objects of the type struct stud. The assignment ptr = class ; will assign the address of class[0] to ptr. When the pointer ptr is incremented by one (ptr++) : The value of ptr is actually increased by sizeof(stud). It is made to point to the next record.

Once ptr points to a structure variable, the members can be accessed as: ptr –> roll ; ptr –> dept_code ; ptr –> cgpa ; The symbol “–>” is called the arrow operator.

Warning When using structure pointers, we should take care of operator precedence. Member operator “.” has higher precedence than “*”. ptr –> roll and (*ptr).roll mean the same thing. *ptr.roll will lead to error. The operator “–>” enjoys the highest priority among operators. ++ptr –> roll will increment roll, not ptr. (++ptr) –> roll will do the intended thing.

Program to add two complex numbers using pointers typedef struct { float re; float im; } complex; main() { complex a, b, c; scanf (“%f %f”, &a.re, &a.im); scanf (“%f %f”, &b.re, &b.im); add (&a, &b, &c) ; printf (“\n %f %f”, c,re, c.im); }

void add (complex * x, complex * y, complex * t) { t->re = x->re + y->re ; t->im = x->im + y->im ; }

Structure and list processing

Dynamic allocation: review Variables in C are allocated in one of 3 spots: the run-time stack : variables declared local to functions are allocated during execution the global data section : Global variables are allocated here and are accessible by all parts of a program. the heap : Dynamically allocated data items malloc, calloc, realloc manage the heap region of the mmory. If the allocation is not successful a NULL value is returned.

Bad Pointers When a pointer is first allocated, it does not have a pointee. The pointer is uninitialized or bad. A dereference operation on a bad pointer is a serious runtime error. Each pointer must be assigned a pointee before it can support dereference operations. int * numPtr; Every pointer starts out with a bad value. Correct code overwrites the bad value.

Example pointer code. int * numPtr; int num = 42; numPtr = &num; *numPtr = 73; numPtr = malloc (sizeof (int)); *numPtr = 73;

int a=1, b=2, c=3; int *p, *q; 1 a 3 c 2 b xxx p q

p = &a ; q = &b ; 1 a 3 c 2 b p q

c = *p ; p = q ; *p = 13 ; 1 a 1 c 13 b p q

Bad pointer Example void BadPointer () { int *p; *p = 42; } int * Bad2 () { int num, *p; num = 42; p = &num; return p; } x x x p X

A function call malloc(size) allocates a block of mrmory in the heap and returns a pointer to the new block. size is the integer size of the block in bytes. Heap memory is not deallocated when the creating function exits. malloc generates a generic pointer to a generic data item (void *) or NULL if it cannot fulfill the request. Type cast the pointer returned by malloc to the type of variable we are assigning it to. free : takes as its parameter a pointer to an allocated region and de-allocates memory space.

Dynamic memory allocation: review typedef struct { int hiTemp; int loTemp; double precip; } WeatherData; main () { int numdays; WeatherData * days; scanf (“%d”, &numdays) ; days=(WeatherData *)malloc (sizeof(WeatherData)*numdays); if (days == NULL) printf (“Insufficient memory”);... free (days) ; }

Self-referential structures Dynamic data structures : Structures with pointer members that refer to the same structure. Arrays and other simple variables are allocated at block entry. But dynamic data structures require storage management routine to explicitly obtain and release memory.

Self-referential structures struct list { int data ; struct list * next ; } ; The pointer variable next is called a link. Each structure is linked to a succeeding structure by next.

Pictorial representation A structure of type struct list datanext The pointer variable next contains either an address of the location in memory of the successor list element or the special value NULL defined as 0. NULL is used to denote the end of the list.

struct list a, b, c; a.data = 1; b.data = 2; c.data = 3; a.next = b.next = c.next = NULL; 1NULL datanext a 2NULL datanext b 3NULL datanext c

Chaining these together a.next = &b; b.next = &c; 1 datanext a 2 datanext b 3 datanext c NULL What are the values of : a.next->data a.next->next->data 2323

Linear Linked Lists A head pointer addresses the first element of the list. Each element points at a successor element. The last element has a link value NULL.

Header file : list.h #include typedef char DATA; struct list { DATA d; struct list * next; }; typedef struct list ELEMENT; typedef ELEMENT * LINK;

Storage allocation LINK head ; head = malloc (sizeof(ELEMENT)); head->d = ‘n’; head->next = NULL; creates a single element list. nNULL head

Storage allocation head->next = malloc (sizeof(ELEMENT)); head->next->d = ‘e’; head->next->next = NULL; A second element is added. n head eNULL

Storage allocation head->next=>next = malloc (sizeof(ELEMENT)); head->next->next->d = ‘e’; head->next->next-> = NULL; We have a 3 element list pointed to by head. The list ends when next has the sentinel value NULL. n head ewNULL

List operations  Create a list  Count the elements  Look up an element  Concatenate two lists  Insert an element  Delete an element

Produce a list from a string (recursive version) #include “list.h” LINK StrToList (char s[]) { LINK head ; if (s[0] == ‘\0’) return NULL ; else { head = malloc (sizeof(ELEMENT)); head->d = s[0]; head->next = StrToList (s+1); return head; }

#include “list.h” LINK SToL (char s[]) { LINK head = NULL, tail; int i; if (s[0] != ‘\0’) { head = malloc (sizeof(ELEMENT)); head->d = s[0]; tail = head; for (i=1; s[i] != ‘\0’; i++) { tail->next = malloc(sizeof(ELEMENT)); tail = tail->next; tail->d = s[i]; } tail->next = NULL; } return head; } list from a string (iterative version)

?A head tail 1. A one-element list 2. A second element is attached A head tail ?? 3. Updating the tail A head tail ?B 4. after assigning NULL A head tail NULLB

/* Count a list recursively */ int count (LINK head) { if (head == NULL) return 0; return 1+count(head->next); } /* Count a list iteratively */ int count (LINK head) { int cnt = 0; for ( ; head != NULL; head=head->next) ++cnt; return cnt; }

/* Print a List */ void PrintList (LINK head) { if (head == NULL) printf (“NULL”) ; else { printf (“%c --> “, head->d) ; PrintList (head->next); }

/* Concatenate two Lists */ void concatenate (LINK ahead, LINK bhead) { if (ahead->next == NULL) ahead->next = bhead ; else concatenate (ahead->next, bhead); }

Insertion Insertion in a list takes a fixed amount of time once the position in the list is found. A C p2 p1 B q Before Insertion

Insertion /* Inserting an element in a linked list. */ void insert (LINK p1, LINK p2, LINK q) { p1->next = q; q->next = p2; } A C p2 p1 B q After Insertion

Deletion Before deletion 123 p p->next = p->next->next; After deletion 123 p garbage

Deletion Before deletion 123 p q = p->next; p->next = p->next->next; After deletion 123 p q free (q) ;

Delete a list and free memory /* Recursive deletion of a list */ void delete_list (LINK head) { if (head != NULL) { delete_list (head->next) ; free (head) ; /* Release storage */ }