Review/Questions s3

IPC144 - Files Agenda:

1 Review/Questions 2 More scanf 3 Files 4 Try it! 5 Homework

Review/Questions

More scanf() (see example: fun36.c )

%[] - an additional format specifier available for scanf is %[] where the square brackets contain a list of valid characters. "-" character inside the square [] brackets indicates a range. "^" character inside the square [] brackets indicates that all characters should be accepted except those inside the square brackets. %30s number specifies maximum number of characters to be read.

File is a named area of the disk storage. C provides a set of predefined functions to open, read, write, and close files. The standard distinguishes between text files and binary files. Different functions are available for accessing text and binary files.

When a file is open – it is loaded into the memory. We need to declare a pointer that will be used to access the file.

Syntax:

FILE *fp; /* defines a file pointer fp */ fopen and fclose functions fopen() function is used to open a file. fopen() function expects two arguments: the name of the file, as a string, and the mode, also given as a character string. Returns a pointer to a file or NULL if error occurred and file could not be open.

Mode If the file exists If the file does not exist “r” Opens the file for reading Error “w” Opens a new file for writing Creates a new file “a” Opens a file for appending(writing at the end of Creates a new file the file) “r+” Opens the file for reading and writing Error “w+” Opens a new file for reading and writing Creates a new file “a+” Opens the file for reading and writing at the end Creates a new file of the file.

Syntax: FILE *fp; /* define a file pointer fp */ char file_name[] = “AnyName.txt”; /* character array to store a filename */ fp = fopen(file_name, “w”); /* open file for writing, if the file exists the old content is discarded */

A binary files may be opened by adding a ‘b’ to the mode string. E.g. fopen(“file.dat”, “wb”); /* will open binary file file.dat for writing. */

When a file is opened, a file position marker is set to some location in the file. It identifies the next place in the file to read or write data. When a file is open in mode “r”, “w”, “r+”, or “w+”, the file position marker is set to the beginning of the file. When a file is opened in mode “a” or “a+”, the file position marker is set to the end of the file. After each input/output operation, the file position marker is automatically repositioned just after the data that were read or written. fclose() function is used to close the file. fclose() expects one argument, a pointer to FILE. It will return 0 (zero) if the file is successfully closed, and EOF, if for some reason file can not be closed. fprintf and fscanf functions fprintf() is used to write to a file. It works the same as printf(), but with one extra argument, the pointer to a file, which was opened for "w" or "a" access.

Example:

/* fun43.c – simple example of writing to a file ( no error checking ) */ #include main(){ FILE *fp; /* declare a pointer to a FILE */ char lname[] = “Smith”; char fname[] = “John”; double salary = 2338.00; char position[] = “Manager”;

fp = fopen(“records.dat”, “w”); /* open file records.dat */

fprintf(fp, "%s,%s:%.2lf,%s\n", lname, fname, salary, position);

/* write a record to a file */

fclose(fp);

/* close the file */ } fscanf() is used to read from a file. Since fscanf() reads from a file, it requires a pointer to a file as one of it’s arguments.

Example: /* fun44.c – example of reading from a file ( no error checking) */ #include main(){ FILE *fp; /* declare a pointer to a FILE */ char lname[31]; char fname[31]; double salary; char position[31];

fp = fopen(“records.dat”, “r”); /* open file records.dat */

fscanf(fp, "%[^,],%[^:]:%lf,%[^\n]\n", lname, fname, &salary, position);

/* read a record from a file */

printf(“Last Name: %s\nFirst Name: %s\nSalary: %.2lf\nPosition: %s\n”, lname, fname, salary, position);

/* display record */

fclose(fp);

/* close the file */ } Example: /* fun45.c – example of writing to a file providing error checking */

#include main(){

FILE *fp; /* declare a pointer to a FILE */ char lname[31]; char fname[31]; double salary; char position[31]; int i;

fp = fopen(“records.dat”, “w”); /* open file records.dat */ if(fp){ /* same as if( fp != NULL ) */ for(i = 0; i < 3; i++){ printf(“Last Name: “); gets(lname); printf(“First Name:”); gets(fname); printf(“Salary:”); scanf(“%lf”, &salary); printf(“Position:”); gets(position); fprintf(fp, "%s,%s:%.2lf,%s\n", lname, fname, salary, position); } printf(“%d records written to the file\n”, i); fclose(fp); } else printf(“Cannot open records.dat file\n”); } /* fun46.c - example of reading from a file */ #include main(){ FILE *fp; /* declare a pointer to a FILE */ char lname[31]; char fname[31]; double salary; char position[31]; int i;

fp = fopen("records.dat", "r"); /* open file records.dat */ if(fp){ /* if fp is not NULL */ printf("%-13s%-13s%10s%10s\n", "Last Name", "First Name", "Salary", "Position"); for(i = 0; i < 3; i++){ fscanf(fp, "%[^,],%[^:]:%lf,%[^\n]\n", lname, fname, &salary, position); /* read a record from a file */

printf("%-13s%-13s%10.2lf%10s\n", lname, fname, salary, position);

/* display record */ } fclose(fp); /* close the file */ }else printf("Can not open file!\n"); } Built in Function rewind()

Function rewind() is used to move the file position indicator to the beginning of the file

Syntax:

#include void rewind( FILE *stream );

Example (see walk19.c)

Read the content of the file

When working with files remember that you’ll need to read the content of the file into some variables as you can’t directly manipulate data in the file.

Example 1 (fun50.c) – read one character at a time from the file, until the last character is reached (EOF)

while ( fscanf(pin, "%c", &ch) != EOF )

Example 2 (fun49.c) – read one word at a time from the file, until the last character is reached (EOF)

while ( fscanf(pin, "%s", word) != EOF )

Example 3 (fun46.c) – read using format specifiers from the files formatted with delimiters.

fscanf(fp, "%[^,],%[^:]:%lf,%[^\n]\n", lname, fname, &salary, position);

Example 4 (fun53.c) – write to a file using fputs()

fputs(name,fp);

Example 5 (fun55.c) – read from file using getc()

while((ch = getc(pin)) != EOF)

Example 6 (fun67.c) – read from a file a set number of characters using fgets(). Read file up to 79 characters at a time will stop if 79 characters read or at \n or at EOF

while(fgets(line,80,fp)!= NULL) Read Data from File into Arrays The following example ( fun66.c ) demonstrates how to read a few records from a text file into arrays. #include main(){ FILE *pin; char description[100][50]; char sale[100]; int quantity[100]; double price[100]; int i = 0, num_records = 0;

pin = fopen("tempfile.txt","r"); if(pin == NULL){ printf("Error: file can't be open..\n"); } else { /* read from the file until EOF reached */ while(fscanf(pin, "%[^,],%c,%d,%lf\n", description[i], &sale[i], &quantity[i], &price[i]) != EOF){ i++; } fclose(pin);

printf("%-15s %12s %10s %15s\n","DESCRIPTION", "SALE (Y/N)", "QUANTITY", "UNIT PRICE");

/* display the content of the arrays */ num_records = i; for(i = 0; i < num_records; i++){ printf("%-15s %12c %10d %15.2lf\n",description[i], sale[i], quantity[i], price[i]); } } } Format of the file is DESCRIPTION,SALE(Y/N),QUANTITY,UNIT PRICE The content of tempfile.txt file:

PENCIL,Y,7,7.50 PAPER,N,200,3.99 INK,N,50,2.99 STAPLES,N,15,0.75

Try it! Modify fun66.c as follows: 1. Display a listing of all items on sale Homework #13 (email your solution: [email protected])

Write a program that will allow the user to add a record to a text file emp.dat. Each record consists of the employee first and last name, employee number (1000 – 6000), and department number (1 – 10).