Data Files. Computer Basics

Transcription

1 Unit 10 Data Files Computer Basics

2 Contents What is a data file? Basic operations with data files: Opening a data file Closing a data file Types of data files Text Files Reading text files Writing text files Binary Files Reading binary files Writing binary files Computer Basics - 2

3 What is a data file? Related information stored on auxiliary memory devices (hard disks, diskettes, CDs, flash pens,...). They allow the user to store information permanently, and to access and alter that information whenever necessary In C, an extensive set of library functions is available for creating and processing data files Reading from a data file is a similar operation to reading data entered by the keyboard Writing to a data file is a similar operation to writing data on the screen Will be using sequential files Operations with data files: Opening a data file: fopen() Closing a data file: fclose() Reading data files: fscanf(), fread() Writing data files: fprintf(), fwrite() Computer Basics - 3

4 Basic operations with data files But when working with a data file: The first step is to establish a buffer area In the buffer area information is stored while being transferred between the computer s memory and the data file The buffer area is established by writing: FILE *ptvar; Where: FILE (uppercase letters required) is a special structure type that establishes the buffer area. The structure contains all the information needed by the functions that handle data files Included in stdio.h ptvar is a pointer variable that indicates the beginning of the buffer area. The pointer ptvar is often referred to as a stream pointer, or simply a stream Computer Basics - 4

5 Basic operations with data files Opening a data file: fopen() A data file must be opened before it can be created or processed Once the data file is opened and until it is closed it is possible to carry out as many operations as the user desires without any further call to the function The library function fopen: Associates the file name with the buffer area Also specifies how the data file will be utilized, i.e., as a read-only file, a write-only file, or a read/write file, in which both operations are permitted Computer Basics - 5

6 Basic operations with data files Opening a data file: fopen() It has this prototype: FILE *fopen (char * file_name, char *mode); file_name is a string of characters that make up a valid filename and may include a path especification mode is a string that determines how the file will be opened (see next slide) The fopen() function returns a pointer: Error: null pointer Success: pointer to the beginning of the buffer area associated with the file Computer Basics - 6

7 Basic operations with data files Opening a file: Mode r Open an existing file for reading only w Open a new file for writing only. If a file with the specified file_name currently exists, it will be destroyed and a new file created in its place a Open an existing file for appending (i.e., for adding information at the end of the file). A new file will be created if the file with the specified file_name does not exist r+ Open an existing file for both reading and writing w+ Open a new file for both reading and writing. If a file with the specified file_name currently exists, it will be destroyed and a new file created in its place a+ Open a new file for both reading and appending. A new file will be created if the file with the specified file_name does not exist b Add to any of the previous modes when working with a binary file Computer Basics - 7

8 Basic operations with data files Closing a data file: fclose() Closes a file that was opened by a call to fopen() Writes any data still remaining in the buffer to the file an does a formal operating-system-level close on the file Disconnects the file from the program and frees the pointer to the file Frees the buffer area, making it available for reuse Failure to close a file invites all kind of trouble, including lost data, destroyed files, and possible intermittent errors in your program Before changing the present access mode to a data file, it has to be closed and opened again with the new access mode parameter Do not open a file more than once if it has not been previously closed Computer Basics - 8

9 Basic operations with data files Closing a data file: fclose() It has this prototype: int fclose(file *ptvar); ptvar: file pointer returned by the call to the function fopen() that opened the file that the user wants to close now The fclose() function returns: 0: if the close operation has been successful EOF: if an error has ocurred In either case, the operation is completed Generally, fclose() will fail only when a disk has been prematurely removed from the drive or there is no more space on the disk Computer Basics - 9

10 Basic operations with data files #include <stdio.h> int main(void) FILE *pfile; pfile = fopen( data.dat, w ); Declaring a pointer to a file Opening the data.dat file for writing (w) if (pfile == NULL) Checking the outcome of the opening operation printf( \n Error cannot open the designated file );... /* The file is used for writing */ /* File processing is terminated */ if (fclose(pfile)!= 0) printf( \n Error on closing the file ); return 0; Combining the call to the closing operation and its outcome Computer Basics - 10

11 Types of data files Types of data files: Text Named collection of characters saved in secondary storage (e.g., on a disk). The characters can be interpreted as individual data items, or as components of strings or numbers. A text file has no fixed size Can be viewed using a text editor Binary A file created by executing a program that stores directly in the file the computer s internal representation of each file component Only understandable by a program with information on the type, format, and structure of the file data elements Cannot be created or modified in a word processor Negative side to binary file usage: A binary file created on one computer system might not be readable on another type of computer Text files are readable and portable, but they do not use disk space as efficiently as binary files. In a text file, numbers are stored on the disk as strings instead of as numerical values Example: integer type variable with a value of (text: 8 bytes; binary: 4 bytes). Computer Basics - 11

12 Types of data files Creating a file: A data file must be created before it can be processed Ways to create a file: Directly: Using a text editor or a word processor (only valid for text files) Indirectly: Writing a program that enters information into the computer and then writes it out to the data file (text and binary files) Different library functions are used for reading and writing text and binary files: Text files: fprintf, fscanf, getc, putc, fgets, fputs Binary files: fwrite, fread, fseek Computer Basics - 12

13 Text Files Consisting of consecutive ASCII characters Data file opening and closing See previous slides Since the files consist of strings: Library functions for reading and writing strings already discussed can be utilized The pointer to the buffer area has to be included as an additional parameter putc(character, pointer) getc(pointer) fputs(string, pointer) fgets(string, number of characters, pointer) fprintf(pointer, control string, variables) fscanf(pointer, control string, variables) Computer Basics - 13

14 Text Files Functions fprintf() and fscanf( ): The function itself converts the stream of ASCII characters into the binary integers, type double mantissas and exponents, and characters strings that are the representation in main memory of the same data, and vice versa fprintf(): Behaves exactly like printf() except that it operates with a file that has to be specified Returned value: Success: returns the number of bytes written on the data file Failure: returns EOF fscanf(): Behaves exactly like scanf() except that it operates with a file that has to be specified Returned value: Success: returns the number of data items successfully assigned a value Failure: returns 0 (no values assigned to data items) EOF: if the end of file has been reached Computer Basics - 14

15 Text Files /* Program: Reads the file "pepe.txt" that contains floating-point numbers and adds them up*/ #include <stdio.h> int main(void) int n; /* Value returned by fscanf */ FILE *pfile; /* Pointer to file */ double num1,num2; /* numbers read from the data file */ double sum; /* Sum of the numbers */ pfile = fopen("pepe.txt", "r+"); if (pfile == NULL) printf("error: Cannot open \"pepe.txt\"\n"); else sum = 0.; /* Initializing the sum to 0*/ n = fscanf(pfile, %lf %lf", &num1,&num2); while (n!= EOF ) /* iterate until an end of file is detected */ sum+=num1+num2; n = fscanf(pfile, %lf%lf ", &num1,&num2); printf( The sum is: %lf\n", sum); if( fclose(pfile)!= 0) printf("error on closing the file\n"); return(0); If pepe.txt contains: The result would be: The sum is: Computer Basics - 15

16 Text Files Functions fgets(), fputs(), getc() or fgetc(), putc() or fputc() They are functions that write or read a character (fgetc(), getc(), fputc(), putc()) or a string (fgets(), fputs()) No format conversion takes place (they work with characters) Reading a character: int fgetc(file *pointer_to_file) or int getc(file *pointer_to_file) Returns the next character from the file pointed by pointer_to_file Returned value: Success: character just read Failure: returns EOF EOF: if the end of the file is reached Writing a character: int fputc(int ch, FILE * pointer_to_file) or int putc(int ch, FILE * pointer_to_file) Write the character ch to the file pointed by pointer_to_file Returned value: Success: character just written Error: EOF Computer Basics - 16

17 Text Files #include <stdio.h> #include <stdlib.h> #include <ctype.h> int main(void) FILE *point; char c; point=fopen("data.txt","w"); if (point==null) else printf("\n ERROR cannot open the designated file"); do c = toupper(getchar()); putc(c, point); while (c!= '\n'); if (fclose(point)!= 0) return (0); printf("error on closing the file"); Computer Basics - 17

18 Text Files /* Program: PrintFile * Description: Reads the contents of a data file on a character-by-character basis, * and displays them on the screen */ #include <stdio.h> #define SIZE_STRING 256 int main(void) char file_name[size_string]; /* Name of the file to be opened */ char letter; /* character read from the data file, and displayed on the screen */ FILE *pfile; /* Pointer to the file to be printed */ printf( Enter the name of the file: "); gets(file_name); pfile = fopen(file_name, "r"); if (pfile == NULL) printf("error on opening the file \"%s\"\n", file_name); else letter = fgetc(pfile); while(letter!= EOF) printf("%c", letter); letter = fgetc(pfile); fclose(pfile); return (0); Computer Basics - 18

19 Text Files Reading a string: char * fgets(char *str, int num, FILE *pointer_to_file). Reads up to num-1 characters from the file pointed by pointer_to_file and stores them in the character array pointed by str Characters are read until either a newline ( \n ) or an EOF is received or until the specified limit is reached. After the characters have been read, a null ( \0 ) is stored in the array immediately after the last character read. If fgets has room to store the entire string, it will include \n before \0. If the string is truncated, no \n is stored Returned value: Success: str (pointer to the string) Failure or EOF reached: a NULL pointer Writing a string: int fputs(const char *str, FILE *pointer_to_file) Writes the contents of the string pointed to by str to the file pointed by pointer_to_file. The null terminator ( \0 ) is not written Returned value: Success: nonnegative number Error: EOF Computer Basics - 19

20 Text Files #include <stdio.h> #include <ctype.h> #include <stdlib.h> int main(void) Example 1: OPTION A, Program that enters a string from the keyboard, then writes it out to a data file. Finally, the string is read back from the file and displayed on the screen FILE *point; char str[80]; char str2[80]; point=fopen( sample.dat","w"); if (point==null) printf("\n Error CANNOT OPEN THE FILE FOR WRITING"); else puts("\nenter a string (CR to quit):\n"); gets(str); fputs(str,point); if (fclose(point)!= 0) printf( Error on closing the file ); else point=fopen( sample.dat","r"); if (point==null) printf("\nerror - CANNOT OPEN THE FILE FOR READING"); else fgets(str2,80,point); puts("\nthe file has been read and contains:\n"); puts(str2); if (fclose(point)!= 0) printf( Error on closing the file ); return (0); Computer Basics - 20

21 #include<stdlib.h> #include <stdio.h> #include <ctype.h> int main(void) Text Files FILE *point; char str[80]; char str2[80]; Example 1: OPTION B, Program that enters a string from the keyboard, then writes it out to a data file. Finally, the string is read back from the file and displayed on the screen point=fopen( sample.txt","w+"); // For reading and writing if (point==null) else printf("\n Error CANNOT OPEN THE FILE FOR READING AND WRITING"); puts("\nenter a string (CR to quit) :\n"); gets(str); fputs(str,point); /* reset the file position indicator to start of the file */ rewind(point); WATCH OUT! fgets(str2,80,point); puts("\nthe file has been read and contains:\n"); puts(str2); if (fclose(point)!= 0) printf( Error on closing the file ); return(0); Computer Basics - 21

22 Text Files #include <stdio.h> #include <ctype.h> #define SIZE 80 int main(void) FILE *point; char str[size]; char str2[size]; char file_name[size]; int n; printf( Enter a name for the file ); gets(file_name); point=fopen(file_name,"w"); if (point == NULL) printf("\n Error Cannot open file for writing ); else puts("\nenter a string (CR to quit):\n"); gets(str); fprintf(point,"%s\n",str); if (fclose(point)!= 0) printf( Error on closing the file ); else point=fopen(file_name,"r"); if (point == NULL) printf("\n Error Cannot open file for reading"); else puts("\nthe file has been read and contains:\n"); n = fscanf(point, %[^\n]\n",str2); if (n == EOF n == 0) printf( Error on reading string 2 ); else puts(str2); if (fclose(point)!= 0) printf( Cannot close the file ); return(0); Computer Basics - 22

23 Text Files Example: Storing structures in text files #include <stdio.h> #include <stdlib.h> #include <string.h> #define N 2 /* Size of the array of structures (Number of products) */ /* Declaring a structure for the products */ typedef struct char name[12]; double price; T_PROD; int main(void) T_PROD product [N]; /* Array of products in the warehouse */ int i; /* counter for looping*/ FILE *f; /* File descriptor for the text file */ /*** Initializing the array of structures ***/ strcpy(product[0].name,"cd Rosana"); product [0].price=25.63; strcpy(product[1].name,"cd Titanic"); product[1].price=26.28; The text file would contain: CD Rosana CD Titanic /*** Creating the file ***/ f = fopen("data.txt","w"); if (f ==NULL) printf( Cannot open file data.txt\n"); else for (i = 0; i<n; i++) fprintf(f,"%s\n",product[i].name); fprintf(f,"%.2f\n",product[i].price); fclose(f); return(0); Computer Basics - 23

24 Binary Files Data elements in a binary file are stored as they are in memory A binary file cannot be created, modified or viewed (only by sheer luck a combination of 8 consecutive bits might represent an existing character code) Permit to organize data into blocks containing contiguous bytes of information These blocks can represent complex data structures, such as arrays and structures Are declared in exactly the same way as text files. The open and close functions are used just as they are used for text files stdio However, different stdio library functions are used for writing fwrite() and reading fread() of blocks of any type of data size_t fwrite(void *buffer, size_t num_bytes, size_t count, FILE *fp); size_t fread (void *buffer, size_t num_bytes, size_t count, FILE *fp); For fwrite(), buffer is a pointer to the information that will be written to the file. For read(), buffer is a pointer to a region of memory that will receive the data from the file The value of count determines how many items are read or written, with each item being num_bytes bytes in length fp is a pointer to a previously opened file Computer Basics - 24

25 fread() function Example 1 // Program that reads all the records in a file items.dat // and displays the contents on the screen // The number of records is not known in advance #include <stdio.h> #include <stdlib.h> typedef struct char name[20]; int stock; float unit_price; T_ITEM; int main(void) FILE *p_file; T_ITEM item; if((p_file = fopen ("items.dat", "rb"))==null) printf("error. Cannot open designated read file\n"); else while(fread(&item, sizeof(t_item), 1, p_file) == 1) printf("\nname of the item => %s\n", item.name); printf("number of items => %d\n", item.stock); printf("unit price => %.2f\n", item.unit_price); fclose(p_file); return 0; In the example, we take into consideration that fread() returns the number of items actually read. If fewer elements are read than are requested in the call, either an error has occurred or the end of the file has been reached. In this case, only 1 element is read, consequently the while loop keeps reading new records until the value returned by fread() is different from 1. Computer Basics - 25

26 fread() function Example 2 #include <stdio.h> #include <stdlib.h> int main(void) FILE *fp; int i; double bal[5] = 1.1, 2.2, 3.3, 4.4, 5.5; fp=fopen("test.dat", "wb+"); if (fp==null) printf("\ncannot open designated write file\n"); else fwrite(bal, sizeof(double), 5, fp); rewind(fp); if(fread(bal, sizeof(double), 5, fp)!=5) printf("\nfile read error\n"); else for(i=0; i<5; i++) printf("%.2f ", bal[i]); The program writes five double numbers from the bal array to a disk file called test and then reads them back. The value returned by fread() 5 in this case is used to test the outcome of the read operation. The example illustrates the point that fread() may return values different from 1. fclose(fp); return 0; Computer Basics - 26

27 Binary Files Example: Storing structures in binary files #include <stdio.h> #include <stdlib.h> #include <string.h> #define N 2 /* Size of the array of structures (number of products) */ /* Declaring a structure for the products */ typedef struct char name[12]; double price; T_PROD; int main(void) T_PROD product [N]; /* Array of products in the warehouse */ int i; /* counter for looping*/ FILE *f; /* File descriptor for the binary file */ /*** Initializing the array of structures ***/ strcpy(product [0].name,"CD Rosana"); product[0].price=25.63; strcpy(product[1].name,"cd Titanic"); product[1].price=26.28; Outcome: you need to write a program to show the information recorded on disk /*** Creating the file ***/ f=fopen("data.dat","wb"); if (f==null) else Opening in binary mode printf( Cannot open file data.dat\n"); for(i=0; i<n; i++) fwrite(&product[i], sizeof(product[i]),1,f); fclose(f); return(0); Address of the structure to be written to the file Size of the structure Computer Basics - 27

28 Binary Files #include <stdio.h> #include <stdlib.h> typedef struct char name[30]; int age; T_RECORD; int main(void) FILE *point; T_RECORD record; T_RECORD record2; point=fopen( info.dat","wb"); if (point==null) printf("\n Cannot open the file "); else printf( Enter name:\n"); gets(record.name); printf( Enter age:\n"); scanf("%d\n",&record.age); fwrite(&record, sizeof(t_record), 1, point); if (fclose(point)!= 0) printf( Error on closing the file ); else point=fopen( info.dat","rb"); if (point==null) printf("\n Cannot open the file "); else puts("\nthe contents of the file are:\n"); fread(&record2, sizeof(t_record), 1, point); printf("name: "); puts(record2.name); printf( AGE: "); printf("%d",record2.age); if (fclose(point)!= 0) printf( Error on closing the file ); return (0); Computer Basics - 28

29 Binary Files #include <stdio.h> #include <stdlib.h> #define N 5 typedef struct char name[30]; int age; T_RECORD; int main(void) FILE *point; T_RECORD record [N]; int count; Reading one structure at a time point=fopen( test.dat","wb"); if (point==null) printf("\n Cannot open file "); else for (count=0;count<n;count++) fflush(stdin); printf( Enter name %d:\n, count+1); gets(record[count].name); printf( Enter age %d:\n, count+1); scanf("%d",&record[count].age); fwrite(&record[count], sizeof(t_record), 1, point); Example Option 1 fclose(point) ; point=fopen( test.dat ","rb"); if (point==null) printf("\ncannot open file for reading "); else puts("\nthe contents of the file are:\n"); for (count=0;count<n;count++) fread(&record[count], sizeof(t_record),1,point); printf("name %d:, count+1); puts(record [count].name); printf( AGE %d:, count+1); printf( %d\n, record[count].age); fclose(point); return(0); Computer Basics - 29

30 Binary Files #include <stdio.h> #include <stdlib.h> #define N 5 typedef struct char name[30]; int age; T_RECORD; int main(void) FILE *point; Reading all the N structures with a single fread() T_RECORD record [N]; int count; point=fopen( test.dat","wb"); if (point==null) printf("\n Cannot open file "); else for (count=0;count<n;count++) fflush(stdin); printf( Enter name %d:\n, count+1); gets(record[count].name); printf( Enter age %d:\n, count+1); scanf("%d",&record[count].age); fwrite(&record[count], sizeof(t_record), 1, point); Example Option 2 fclose(point) ; point=fopen( test.dat ","rb"); if (point==null) printf("\ncannot open file for reading "); else puts("\nthe contents of the file are:\n"); fread(record, sizeof(t_record), N, point); for (count=0;count<n;count++) printf("name %d:, count+1); puts(record [count].name); printf( AGE %d:, count+1); printf( %d\n, record[count].age); fclose(point); return(0); Computer Basics - 30

31 fseek() and Random-Access I/O Random read and write operations can be performed using the C I/O system with the help of fseek(), which sets the file position indicator to point to any record in the file Prototype: size_t fseek(file *fp, long int displacement, int origin); fp : It is a file pointer returned by a call to fopen() displacement : Number of bytes from origin, which will become the new current position origin : Defined by one of the following arguments: SEEK_SET: Count from the beginning of the file SEEK_CUR: Count from the current pointer position SEEK_END: Count from the end of the file Returned value: Success: 0 (the file pointer has moved properly) Failure: a nonzero value Example: fseek(fp,sizeof(t_product,seek_set); /* Points to second record in file */ fseek(fp,9*sizeof(t_product),seek_set); /* Points to the 10th product */ fseek(fp,2*sizeof(t_product),seek_cur); /* Skips 2 products */ Computer Basics - 31

32 fseek(): a short example typedef struct char name[40]; char street [30]; char city[30]; char province[20]; int postal_code; T_INFO; The following function seeks to the specified structure of type T_INFO void find(int client_num) FILE *fp; T_INFO info; if((fp=fopen("mail.dat", "rb"))==null) printf("cannot open file.\n"); else // find the proper structure fseek(fp, client_num*sizeof(t_info), SEEK_SET); // read data into memory fread(&info, sizeof(t_info), 1, fp); fclose(fp); Computer Basics - 32

33 Binary Files: Example of fseek() #include <stdio.h> #include <stdlib.h> #include <string.h> typedef struct char name[30]; int age; T_RECORD; void Enter_data (T_RECORD record[ ]); void Display_data (T_RECORD record[ ]); void Create_file (T_RECORD record[ ]); void Load_file (T_RECORD record[ ]); void Update_file (void); int main(void) T_RECORD record[10]; Enter_data(record); Create_file(record); Display_data(record); Update_file(); Load_file(record); Display_data(record); return(0); void Enter_data (T_RECORD record[ ]) int count; printf( Enter name and age for 5 people: \n"); for (count=0;count<5;count++) fflush(stdin); printf( Enter name of person %d:\n, count+1); gets(record[count].name); printf( Enter age of person %d:\n, count+1); scanf("%d",&record [count].age); return; void Display_data (T_RECORD record[ ]) int count; printf("\n:::::present contents of the file:\n"); for (count=0;count<5;count++) printf("name %d:, count+1); puts(record[count].name); printf( AGE %d:, count+1); printf("%d\n",record[count].age); return; 1/4 Computer Basics - 33

34 Binary Files: Example of fseek() void Create_file(T_RECORD record[ ]) FILE *point; int count; printf("\nwriting INFO TO THE DATA FILE...\n"); point=fopen( info.dat","wb"); if (point==null) printf("\nerror on opening the file"); else for(count=0;count<5;count++) fwrite(&record[count],sizeof(t_record),1,point); /* An alternative to previous loop fwrite(record,sizeof(t_record),5,point); */ printf("\n:::::file CREATED SUCCESSFULLY:::::\n"); fclose(point); return; 2/4 void Load_file(T_RECORD record[ ]) FILE *point; int count=0; printf("\nreading THE FILE...\n"); point=fopen( info.dat","rb"); if (point==null) printf("\n Error on opening the file"); else while ( fread(&record[count],sizeof(t_record), 1,point) == 1) count++; printf("\n:::::file READ SUCCESSFULLY:::::\n"); fclose(point) ; return; Computer Basics - 34

35 Binary Files: Example of fseek() void Update_file(void) FILE *point; T_RECORD record; int count=0; char str[30]; 3/4 fflush(stdin); printf("\nenter name to search for:\n"); gets(str); printf("\nreading THE FILE AND SEARCHING FOR NAME OF THE PERSON...\n"); point=fopen( info.dat","rb+"); if (point==null) printf("\nerror"); else do fread(&record,sizeof(t_record),1,point); count++; while ((strcmp(record.name,str)!=0)&&(count<5)); (CONTINUES IN NEXT SLIDE) Computer Basics - 35

36 Binary Files: Example of fseek() if (strcmp(record.name,str)==0) printf("\n...name FOUND...\n"); printf("\n...current DATA:\n"); printf("name: %s\n",record.name); printf( AGE: %d\n",record.age); printf("\n:::::enter NEW DATA:::::::::\n"); fflush(stdin); printf( Enter name:\n"); gets(record.name); printf( Enter age:\n"); scanf("%d",&record.age); printf("\n...updating DATA FILE...\n"); fseek(point,(count-1)*sizeof(t_record),seek_set); fwrite(&record,sizeof(t_record),1,point); printf("\n...file UPDATING COMPLETED...\n"); else printf("\n::::name NOT FOUND IN THE FILE::::\n"); fclose(point); return; 4/4 Computer Basics - 36

37 feof() function Determines when the end of the file pointed by fp has been encountered Prototype: int feof (FILE *fp); fp : It is a file pointer returned by a call to fopen() Returned value: any value different from 0 (true): if the end of the file has been reached (we try to read a record and an EOF is encountered) 0: otherwise (a record is read normally) Permits to read a file when the number of records is not known in advance, by reading records until the end of the file is encountered feof() can be applied to binary files as well as text files Computer Basics - 37

38 feof(): example of binary file // Program that illustrates the use of the function feof() with a binary file #include <stdio.h> typedef struct char name[15]; int stock; double unit_price; T_ITEM; int main(void) FILE *fp; T_ITEM item; fp = fopen("items.dat", "rb"); if(fp==null) printf("\nerror. Cannot open designated read file\n"); else fread(&item, sizeof(t_item), 1, fp); while(!feof(fp)) printf("\nname=%-15s Stock=%d Price=%7.2f", item.name, item.stock, item.unit_price); fread(&item, sizeof(t_item), 1, fp); fclose(fp); return 0; Computer Basics - 38

39 feof(): example of text file // Program that illustrates the use of the function feof() with a text file #include <stdio.h> int main(void) FILE *fp; char name[15]; int stock; double unit_price; fp = fopen("items.txt", "r"); if(fp==null) printf("\nerror. Cannot open designated read file\n"); else fscanf(fp,"%s %d %lf", name, &stock, &unit_price); while(!feof(fp)) printf("\nitem=%-15s Stock=%3d Price=%8.2f", name, stock, unit_price); fscanf(fp,"%s %d %lf", name, &stock, &unit_price); fclose(fp); return 0; Computer Basics - 39

40 APPENDIX Computer Basics - 40

41 Function fgets() (1/4) Format: fgets(s, n, stream) One of the uses of this function is to assure that the number of characters assigned to a string variable is not exceeded This function copies characters from stream into the string s until it has read n-1 characters or a newline character, whichever comes first. The terminator '\0' is automatically added at the end of the string s Example (for reading data from the keyboard): fgets (book.title, 20, stdin); fgets behaves a bit differently from gets. Consider this code fragment: char line [80]; printf ( Type in a line of data.\n > ); gets(line); Computer Basics - 41

42 Function fgets() (2/4) If the user responds to the prompt as follows, Type in a line of data. > Here is a short sentence. The value stored in line would be Here is a short sentence. \0... The \n character representing the <return> or <enter> key pressed at the end of the sentence is not stored. Like scanf, gets() can overflow its string argument if the user enters a longer data line that will fit. The stdio library s file version of gets(), fgets(), behaves somewhat differently. Function fgets takes three arguments: the output parameter string, a maximum number of characters to store (n) and the file pointer to the data file. Function fgets() will never store more than n 1 characters from the data file, and the final character stored will always be '\0'. However, the next to last character may or may not be '\n'. If fgets() has room to store the entire line of data, it will include '\n' before '\0'. If the line is truncated, no '\n' is stored. Computer Basics - 42

43 Function fgets() (3/4) As mentioned earlier, when using gets(), it is possible to overrrun the array that is being used to receive the characters entered by the user because gets() provides no bounds checking. When used with stdin (to read from the keyboard), the fgets() function offers an useful alternative because it can limit the number of characters read and then prevents overrruns. The only trouble is that fgets() does not remove the newline character and gets() does, so you will have to manually remove it to avoid unwanted line jumps when the information is shown, for example, on the screen. Function Clean_String(char *string) can be used to remove the unwelcome newline character '\n'. Or the following code: lg=strlen(string); if(string[lg-1]== \n string[lg-1]= \0 ; Computer Basics - 43

44 Function fgets() (4/4) Code of Clean_String(): void Clean_String (char *string) int i=0; while(string[i]!= \n && string[i]!= \0 ) i++; if(string[i]== \n string[i]= \0 ; Example of the use of the above function is illustrated with the following group of instructions: printf("intro DNI of salesman => "); fgets(salesman.dni,10,stdin); Clean_String(salesman.dni); Computer Basics - 44