Structures_and_Files[1] - Read-Only.pptx

Data Structures

1. ANITS ENGINEERING COLLEGE (AUTONOMOUS | VISAKHAPATNAM) AR23 I B.Tech I Semester PSPC UNIT-V 1

2. D y nam i c Memory Al l oc a tion: The memory allocation that we have done till now was static memory allocation. The memory that could be used by the program was fixed i.e. we could not increase or decrease the size of memory during the execution of program. In many applications it is not possible to predict how much memory would be needed by the program at run time. For example if we declare an array of integers int emp_no[200]; In an array, it is must to specify the size of array while declaring, so the size of this array will be fixed during runtime.

3. Now two types of problems may occur. The first case is that the number of values to be stored is less than the size of array and hence there is wastage of memory. For example if we have to store only 50 values in the above array, then space for 150 values( 300 bytes) is wasted. In second case our program fails if we want to store more values than the size of array. For example, if there is need to store 205 values in the above array. To overcome these problems we should be able to allocate memory at run time. The process of allocating memory at the time of execution is called dynamic memory allocation. The allocation and release of this memory space can be done with the help of some built-in-functions whose prototypes are found in alloc.h and stdlib.h header files.

4. These functions take memory from a memory area called heap and release this memory 'whenever not required. Pointers play an important role in dynamic memory allocation because we can access the dynamically allocated memory only' through pointers. 1) malloc( ): Syntax: void *malloc(size_t size); This function is used to allocate memory dynamically. The argument size specifies the number of bytes. to be allocated. malloc( ) returns a pointer to the first byte of allocated memory. The returned pointer is of type void, which can be type cast to appropriate type of pointer. It is generally used as ptr = (datatype *) malloc (specified size);

5. For ex, int *ptr; ptr = ( int *) malloc (l0); This allocates 10 contiguous bytes of memory space and the address of first byte is stored in the pointer variable ptr. This space can hold 5 integers. The allocated memory contains garbage value. We can use sizeof operator to make the program portable and more readable. ptr = (int *) malloc ( 5 * sizeof (int) ); This allocates the memory space to hold five integer values.

6. If there is not sufficient memory available in heap then malloc( ) returns NULL. So we should always check the value returned by malloc( ). ptr = (float *) malloc(10 * sizeof(float) ); if ( ptr == NULL ) printf("Sufficient memory not available");

7. Ex: Program to calculate the sum of n numbers entered by the user using malloc #include <stdio.h> #in c l ud e < st d l ib . h > int main() { int n, i, *ptr, sum = 0; printf("Enter number of elements: "); scanf("%d", &n); ptr= (int *) malloc (n* sizeof (int) ); if(ptr == NULL) { printf("Error! memory not allocated."); exit(0); } printf("Enter elements: "); for(i = 0; i < n; i++) { scanf("%d", ptr + i); sum += *(ptr + i); } printf("Sum = %d", sum); free(ptr); return 0; }

8. 2) calloc(): Syntax: void *calloc(size_t n, size_t size); The calloc( ) function is used to allocate multiple blocks of memory. It is somewhat similar to malloc() function except for two differences. The first one is that it takes two arguments. The first argument specifies the number of blocks and the second one specifies the size of each block. For example ptr = (int *) calloc (5, sizeof(int)); This allocates 5 blocks of memory, each block contains 2 bytes and the starting address is stored in the pointer variable ptr, which is of type int. An equivalent malloc( ) call would be ptr = ( int * ) malloc ( 5 * sizeof(int) ); Here we have to do the calculation ourselves by multiplying, but calloc( ) function does the calculation for us.

9. Ex: Program to calculate the sum of n numbers entered by the user using calloc #include <stdio.h> #in c l ud e < st d l ib . h > int main() { int n, i, *ptr, sum = 0; printf("Enter number of elements: "); scanf("%d", &n); ptr = (int*) calloc(n, sizeof(int)); if(ptr == NULL) { printf("Error! memory not allocated."); exit(0); } printf("Enter elements: "); for(i = 0; i < n; i++) { scanf("%d", ptr + i); sum += *(ptr + i); } printf("Sum = %d", sum); free(ptr); return 0; }

10. 3) realloc(): Syntax: void *realloc( void *ptr, size_t newsize) We may want to increase or decrease the memory .allocated by malloc( ) or calloc( ). The function realloc( ) is used to change the size of the memory block. It alters the size of the memory block without losing the old data. This is known as reallocation of memory. This function takes two arguments, first is a pointer to the block of memory that was previously allocated by malloc( ) or calloc( ) and second one is the new size for that block. For example ptr = (int *) malloc (size); This statement allocates the memory of the specified size and thy starting address of this memory block is stored in the pointer variable ptr. If we want to change the size of this memory block, then we can use realloc( ) as ptr = (int *) realloc (ptr, newsize);

11. The newsize may be smaller or larger than the old size. If the newsize is larger, then the old data is not lost, otherwise old data will lost. #include <stdio.h> #include <stdlib.h> int main() { int n, i, *ptr, sum = 0, oldsize=3, newsize=5; ptr= (int *) malloc (oldsize*sizeof (int) ); if(ptr == NULL) { printf("Error! memory not allocated."); exit(0); } printf("Enter 3 elements: "); for(i = 0; i < 3; i++) { scanf("%d", ptr + i); sum += *(ptr + i); } printf("Before memory reallocation Sum = %d\n", sum); ptr= (int *) realloc (ptr, newsize*sizeof(int)); if (ptr==NULL) { printf ("Memory not available\n"); exit(1); } printf ("Enter 2 more integers:\n"); for(i=3; i<5; i++) { scanf("%d", ptr+i); sum += *(ptr + i); } printf("After memory reallocation Sum = %d", sum); free(ptr); return 0; }

12. 4) free(): Syntax: void free(void *p) The dynamically allocated memory is not automatically released; it will exist till the end of program. If we have finished working with the memory allocated dynamically, it is our responsibility to release that memory so that it can be reused. The function free( ) is used to release the memory space allocated dynamically. The memory released by free( ) is made available to the heap again and can be used for some other purpose. For example free( ptr ); Here ptr is a pointer variable that contains the base address of a memory block created by malloc( ) or calloc().

13. When the program terminates all the memory is released automatically by the operating system but it is a good practice to free whatever has been allocated dynamically. We won't get any errors if we don’t free the dynamically allocated memory, but this would lead to memory leak i.e. memory is slowly leaking away and can be reused only after the termination of program.

14. Example-1: Program for “how De-allocation of memory causes Dangling pointer” #include <stdio.h> #include <stdlib.h> int main() { int *ptr = (int *)malloc(sizeof(int)); *ptr = 100; free(ptr); printf("%d", *ptr); // it prints garbage value return 0; }

15. Exa m pl e - 2 : Prog r am for “how functi o n call causes Dangling pointer” #include <stdio.h> int *danglingPointer() { int temp = 10; return &temp; } int main() { int *ptr = danglingPointer(); printf("%d", *ptr); return 0; }

16. Structu r es: Array is a collection of same type of elements but in many real life applications we may need to group different types of logically related data. For example if we want to create a record of a person, contains name, age and height of that person, then we can't use array because all the three data elements are of different types. T o s t o r e t h e s e r e l a t e d f i e ld s o f d i f fe r e n t d a t a t y p e s w e c a n u s e a structure, which is capable of store heterogeneous data.

17. Struc t u re i s a c o l l e c t io n o f l o gi c a lly re l a t e d d a t a i t e m s o f d i f fere n t datatypes under a single name. Structure is a user-defined datatype. User-defined data type also called derived data type why because we derived it from the primary/basic data types. But, an array holds a similar datatype record, when the structure holds different datatypes records.

18. Defining a Structure: data type memberl; datatype member2; …… datatype memberN; } ; Here struct is a keyword, which tells the compiler that a structure is being defined. memberl, member2, ...memberN are known as members of the structure and are declared inside curlybraces. There should be a semicolon at the end of the curly braces. These members can be of any data type like int, char, float, array, pointers or another structure type. tagname is the name of the structure, it is used further in the program to declare variables of this structure type. tagname is name of user-defined/custom type struct tagname { Member declaration

19. Definition of a structure provides one more data type in addition to the built in data types. We can declare variables of this new data type that will have the format of the defined structure. It is important to note that definition of a structure template does not reserve any space in memory for the members; memory space is reserved only when actual variables of this structure type are declared.

20. Declaring Structure Variables By defining a structure we have only created a format, the actual use of structures will be when we declare variables based on this format. We can declare structure variables in two ways- Using the structure tag With structure definition 1. Using the structure tag We can declare structure variables using a structure tag. This can be written as- struct student{ char name [20] ; int rollno; float marks; } struct student stul, stu2, stu3; Here stu1, stu2 and stu3 are structure variables that are declared using the structure tag student. Declaring a structure variable reserves space in memory. Each structure variable declared to be of type struct student has three members viz. name, rollno and marks. The compiler will reserve space: each variable sufficient to hold all the members. For example each variable of type struct student will occupy 26 (20+2+4) bytes.

21. 2. With structure definition: struct student{ char name [20] ; int rollno; float marks; }stul,stu2,stu3; Here stul, stu2, and stu3 are variables of type struct student . When we declare a variable while defining the structure template, the tagname is optional. So we can also declare them as- struct { char name [20] ; int rollno; float marks; }stul,stu2,stu3;

22. Initialization Of Structure Variables The syntax of initializing structure variables is similar to that of arrays. All the values are given in curly braces and the number, order' and type of these values should be same as in the structure template definition. The initializing values can only be constant expressions. struct student { char name [20] ; int rollno; float marks; }stul={"Mary H", 25, 98}; struct student stu2={"John H", 24, 67. 5}; We cannot initialize members while defining the structure. struct student { char name [20] ; int rollno; float marks=99; / * Invalid * / }stu; This is invalid because there is no variable called marks, and no memory is allocated for structure definition.

23. If the number of initializers is less than the number of members then the remaining members are initial with zero. For example if we have this initialization- struct student stu1 = {"Mary"}; Here the members rollno and marks of stul will be initialized to zero. This is equivalent to the initialization- struct student stu1 = {"Mary", 0, 0};

24. Accessing Members of a Structure For accessing any member of a structure variable, we use the dot ( . ) operator which is also known as the period or membership operator. The format for accessing a structure member is structvariable.member Here on the left side of the dot there should be a variable of structure type and on right hand side there should be the name of a member of that structure. For example consider the following structure- struct student { char name [20] ; int rollno; float marks; }; struct student stu1, stu2; name of stu1 is given by – stu1.name rollno of stu1 is given by – stu1.rollno marks of stu1 is given by – stu1.marks name of stu2 is given by - stu2.name rollno of stu2 is given by - stu2.rollno marks of stu2 is given by - stu2.marks

25. Assignment of Structure Variables We can assign values of a structure variable to another structure variable if both variables are defined as the same structure type. For example- A program to assign a structure variable to another structure variable.

26. struct student { char name [20] ; int rollno; float marks; } ; int main ( ) { struct student stul = {"Oliver",12,98}; struct student stu2; stu2=stul; printf("stul : %s %d %. 2f\n", stul.name, stul.rollno, stul.marks) ; printf("stu2 : %s %d %. 2f\n", stu2.name, stu2.rollno, stu2.marks) ; }

27. Size of Structure: We may need to find out the size of structure in some situations like reading or writing to files. To find out the size of a structure by sizeof operator, we can either use the structure variable name or the tag name with the struct keyword. For example- sizeof(struct student) sizeof(stu1) Above 2 expressions gave the same result.

28. Nested Structures (Structure Within Structure) The members of a structure can be of any data type including another structure type i.e. we can include a structure within another structure. A s t r u c t u r e v a r i ab l e can b e a m e m ber of ano t her s t r uc t u r e . T h i s i s c a l l ed n e s ting of structures. struct tagl { memberAl; m e m b e r A 2 ; struct tag2 { memberBl; memberB2; ……. member Bm; )var1; ……. member An; }var2; For accessing memberBl of inner structure we’ll write it as v a r2.v a rl.m e mbe r B l

29. Array of Structures We can declare array of structures where each element of array is of structure type. Array of structures can be declared as struct student stu[10]; Here stu is an array of l0 elements, each of which is a structure of type struct student, means each element of stu has 3 members, which are name, rollno and marks. These structures can be accessed through subscript notation. To access the individual members of these structures we'll use the dot operator as usual. All the structures of an array are stored in consecutive memory locations.

30. Program to understand array of structures #inc1ude<stdio.h> struct student { char name [20]; int rollno; float marks; }; int main ( ) { int i; struct student stuarr [10] ; for(i=0; i<10; i++) { printf ("Enter name, ro1lno and marks "); scanf("%s %d %f", stuarr[i].name, &stuarr[i].rollno, &stuarr[i].marks); } for(i=0; i<10; i++) printf("%s %d %f \n", stuarr[i].name, stuarr[i].rollno, stuarr[i].marks); }

31. S t ructu r es An d Functi o ns Structures may be passed as arguments to function in different ways. We can pass individual members, whole structure variable or structure pointers to the function. Similarly a function can return either a structure member or whole structure variable or a pointer to structure. Passing Structure Members As Arguments We can pass individual structure members as arguments to functions like any other ordinary variable.

32. #include<stdio.h> # i n cl u d e < s tri ng . h > struct student { char name [20]; int rollno; int marks; }; void display (char name[], int rollno, int marks); int main () { struct student stu1= {"John" , 12,87}; struct student stu2; strcpy(stu2.name, "Mary"); stu2.rollno=18; stu2.marks=90; display(stu1.name,stu1.rollno,stu1.marks); printf("\n"); display(stu2.name,stu2.rollno,stu2.marks); } void display(char name[], int rollno, int marks) { printf( "Name - %s \n" , name) ; printf("Rollno - %d\n", rollno) ; printf ("Marks - %d\n" ,marks); } Output:

33. Self Referential Structures A structure that contains pointers to structures of its own type is known as self- referential structure. For example- struct tag{ datatype member1 ; datatype member2; ……. struct tag *ptr1; struct tag *ptr2; }; Where ptr1 an ptr2 are structure pointers t at can point to structure variables of type struct tag, so struct tag is a self-referential structure. These types of structures are helpful in implementing data structures like linked lists and trees.

34. Unio n s Union is a derived data type like structure and it can also contain members of different data types. The syntax used for definition of a union, declaration of union variables and for accessing members is similar to that used in structures, but here keyword union is used instead of struct. The main difference between union and structure is in the way memory is allocated for the members. In a structure each member has its own memory location, whereas members of union share the same memory location. When a variable of type union is declared, compiler allocates sufficient memory to hold the largest member in the union. Since all members share the same memory location hence we can use only one member at a time. Thus union is used for saving memory. The concept of union is useful when it is not necessary to use all members of the union at a time.

35. The syntax of definition of a union is- union union_name{ datatype member1; datatype member2; ………… }; Comparison of memory allocated for a union and structure variable: struct stag{ char c; int i; float f; } union utag{ char c; int i; float f; }

36. Introduction to Files F i l e i s a c ol l e c ti o n o f d a t a t h at s t o r ed o n s e c o n d ary m e m o ry li k e harddisk of a computer. Types of Files in C A file can be classified into two types based on the way the file stores the data. They are as follows: Text Files Binary Files

37. 1. Text Files A t e x t f i l e co n t a i n s d a t a i n t h e f o rm o f A SCII c h ar ac t e r s and is generally used to store a stream of characters. Each line in a text file ends with a new line character (‘\n’). It can be read or written by any text editor. They are generally stored with .txt file extension. Text files can also be used to store the source code.

38. 2. Binary Files A binary file contains data in binary form (i.e. 0’s and 1’s) instead of ASCII characters. They contain data that is stored in a similar manner to how it is stored in the main memory. The binary files can be created only from within a program and their contents can only be read by a program. More secure as they are not easily readable. They are generally stored with .bin file extension.

39. The steps for file operations in C programming are as follows- Open a file Read the file or write data in the file Close the file The functions fopen( ) and fclose( ) are used for opening and closing the files respectively.

40. Opening a File: A file must be opened before any I/O operations can be performed on that file. The process of establishing a connection between the program and file is called opening the file. A structure named FILE is defined in the file stdio.h. A file pointer is a pointer to a structure of type FILE. A file pointer is a reference to a particular position in the opened file. It is used in file handling to perform all file operations such as read, write, close, etc. We use the FILE macro to declare the file pointer variable. The FILE macro is defined inside <stdio.h> header file. Syntax: FILE* pointer_name;

41. For opening a file in C, the fopen() function is used with the filename or file path along with the required access modes. Syntax of fopen(): FILE* fopen(const char *file_name, const char *access_mode); Parameters: Return Value is FILE* If the file is opened successfully, returns a file pointer to it. If the file is not opened, then returns NULL. file_name : name of the file when present in the same directory as the source file. Otherwise, full path. access_mode: Specifies for what operation the file is being opened.

42. Modes Description r This mode is used for opening an existing file for reading purpose only. The file to be opened must exist and the previous data of file is not erased. If the file cannot be opened fopen( ) returns NULL. rb Open for reading in binary mode. If the file does not exist, fopen( ) returns NULL. w If the file doesn't exist then this mode creates a new file for writing, and if the file already exists then the previous data is erased and the new data entered is written to the file. wb Open for writing in binary mode. If the file exists, its contents are overwritten. If the file does not exist, it will be created. a If the file doesn't exist, then this mode creates a new file, and if the file already exists, then the data entered is appended at the end of existing data. ab Open for append in binary mode. Data is added to the end of the file. If the file does not exist, it will be created.

43. Modes Description r+ This mode is same as "r" mode but in this mode, we can also write and modify existing data. The file to be opened must exist and the previous data of file is not erased. Since we can add new data to modify existing data so this mode is also called update mode. rb+ Open for both reading and writing in binary mode. If the file does not exist, fopen( ) returns NULL. w+ This mode is same as "w" mode but in this mode we can also read and modify the data. If the file doesn't exist then a new file is created and if the file exists then previous data is erased. wb+ Open for both reading and writing in binary mode. If the file exists, its contents are overwritten. If the file does not exist, it will be created. a+ This mode is same as the "a" mode but in this mode we can also read the data stored in the file. If the file doesn't exist, a new file is created and if the file already exists then new data is appended at the end of existing data. We cannot modify existing data in this mode. ab+ Open for both reading and appending in binary mode. If the file does not exist, it will be created.

44. Read the file fgetc( ): S y nt a x : int fgetc( FILE *fptr ); This function reads the character at the current file position in the specified file, and increments the file position. The return value of fgetc() has the type int . If the file position is at the end of the file, fgetc() returns EOF. For example: A program to understand the use of fgetc ( ) #include<stdio.h> int main ( ) { FILE *p; char ch; p = fopen("myfile.txt", "r") if((p == NULL) printf("This file doesn't exist\n"); else { while((ch=fgetc(p)) != EOF) printf ("%c", ch); } fclose (p) ; } If “myfile.txt” contains following information A BC DEF XYZ O u t p u t: ch=fgetc(p) ; while(ch!=EOF) { printf("%c",ch); ch=fgetc(p); } (OR )

45. fg e t s () : Syntax: char *fgets(char *str, int n, FILE *fptr); This function is used to read characters from a file, and these characters are stored in the string pointed to by str. It reads at most n-1 characters from the file, where n is the second argument. fptr is a file pointer which points to the file from which characters are read. This function reads characters from the file until either a newline or an end of file is encountered or until n-1 characters have been read. F o r ex a m pl e : A p r o g r am t o un d e r sta n d the use of fgetc ( ) #include<stdio.h> int main ( ) { FILE *p; char str[80]; p=fopen("myfile.txt", "r"); if(p == NULL) printf("This file doesn't exist\n"); else { while(fgets(str, 80, p) != NULL) printf("%s",str); } fclose (p) ; }

46. Write data in the file fputc ( ): Syntax: int fputc( int c, FILE *fptr); This function writes a character to the specified file at the current file position and then increments the file position pointer. On success, it returns an integer. representing the character written, and on error it returns EOF. For example: Input: Ou t p u t: #include<stdio.h> int main ( ) { FILE *p; char ch; p=fopen("myfile.txt", "w"); if(p == NULL) printf("File doesn't opened in write mode\n"); else { printf("Enter text :\n"); while((ch=getchar()) != EOF) fputc(ch, p); printf("Data written successfully"); } fclose (p) ; }

47. fputs( ) Synt a x : int fputs(const char *str, FILE *fptr); This function writes the null terminated string pointed to by str to a file. The null character that marks the end of the string is not written to the file. On success it returns the last character written and on error it returns EOF. For example: Input: Output: # i n c l ude< s t d i o . h > int main() { FILE *fptr; char str[80]; fptr=fopen("test.txt", "w"); if(fptr == NULL) printf("This file doesn't exist\n"); else { printf( "Enter the text:\n"); while(gets(str) != NULL){ fputs(str,fptr); fputs("\n",fptr); } printf("Data copied successfully"); } fclose(fptr); }

48. 1) ftell(): ftell() is used to find the position of the file pointer from the starting of the file. Syntax: long ftell(FILE *fp) For ex: # i n c l ude< s t d i o . h > int main() { FILE *p; char ch; p = fopen("test.txt", "r"); if(p == NULL) printf("This file doesn't exist\n"); else { printf("Initially the position of file pointer is %ld\n", ftell(p)); while((ch=fgetc(p)) != EOF) printf ("%c", ch); } Input: Ou t p u t: Note: If a document has 3 lines, at the end of each line we have \n. For each new line character \n, it took 2 bytes } printf("At the end the position of file pointer is %ld", ftell(p)); fclose (p) ;

49. 2) fseek(): This function is used for setting the file position pointer at the specified byte. Syntax: int fseek( FILE *fp,. long displacement, int origin ); Here fp is a file pointer displacement is a long integer which can be positive or negative and it denotes the number of bytes which are skipped backward ( if negative,) or forward ( if positive ) from the position specified in the third argument. This argument is declared as long integer so that it is possible to move in large files. origin is the position relative to which the displacement takes place. It can take one of these three values.

50. 3) rewind(): This function is used to move the file position pointer to the beginning of the file. Here fptr is a pointer of FILE type. The function rewind( ) is useful when we open a file for update. Syntax: rewind(FILE *fptr); Note: rewind(fptr) is equivalent to fseek(fptr, 0L, 0):

51. For example: #include<stdio.h> # i n c l ud e < st d l ib . h > int main() { FILE *fp; fp=fopen("test.txt", "r+"); if(fp==NULL) { printf("Error in opening file\n"); exit(1) ; } printf("position pointer ->%ld\n", ftell(fp)); fseek(fp,0,2) ; printf("position pointer ->%ld\n", ftell(fp)); rewind(fp) ; printf("Position pointer ->%ld\n", ftell(fp)); fclose(fp) ; } Input: Output: