ENG120. Misc. Topics

Transcription

1 ENG120 Misc. Topics

2 Topics Files in C Using Command-Line Arguments Typecasting Working with Multiple source files Conditional Operator 2

3 Files and Streams C views each file as a sequence of bytes File ends with the end-of-file marker Or, file ends at a specified byte Stream created when a file is opened Provide communication channel between files and programs Opening a file returns a pointer to a FILE structure Example file pointers: stdin - standard input (keyboard) stdout - standard output (screen) stderr - standard error (screen) FILE structure File descriptor Index into operating system array called the open file table File Control Block (FCB) Found in every array element, system uses it to administer the file 3

4 Files and Streams Read/Write functions in standard library fgetc Reads one character from a file Takes a FILE pointer as an argument fgetc( stdin) equivalent to getchar() fputc Writes one character to a file Takes a FILE pointer and a character to write as an argument fputc('a', stdout) equivalent to putchar('a') fgets Reads a line from a file fputs Writes a line to a file fscanf / fprintf File processing equivalents of scanf and printf 4

5 Dealing with Files C imposes no file structure No notion of records in a file Programmer must provide file structure Creating a File FILE *myptr; Creates a FILE pointer called myptr myptr = fopen("myfile.dat", openmode); Function fopen returns a FILE pointer to file specified Takes two arguments file to open and file open mode If open fails, NULL returned fprintf Used to print to a file Like printf, except first argument is a FILE pointer (pointer to the file you want to print in) 5

6 Dealing with Files feof(file pointer) Returns true if end-of-file indicator (no more data to process) is set for the specified file fclose( FILE pointer) Closes specified file Performed automatically when program ends Good practice to close files explicitly Details Programs may process no files, one file, or many files Each file must have a unique name and should have its own pointer 6

7 Open Modes Mode Description r Open a file for reading. w Create a file for writing. If the file already exists, discard the current contents. a Append; open or create a file for writing at end of file. r+ Open a file for update (reading and writing). w+ Create a file for update. If the file already exists, discard the current contents. a+ Append; open or create a file for update; writing is done at the end of the file. 7

8 3 #include <stdio.h> 4 5 int main() 6 { 7 int account; 8 char name[ 30 ]; 9 double balance; 10 FILE *cfptr; /* cfptr = clients.dat file pointer */ if ( ( cfptr = fopen( "clients.dat", "w" ) ) == NULL ) 13 printf( "File could not be opened\n" ); 14 else { 15 printf( "Enter the account, name, and balance.\n" ); 16 printf( "Enter EOF to end input.\n" ); 17 printf( "? " ); 18 scanf( "%d%s%lf", &account, name, &balance ); while (!feof( stdin ) ) { 21 fprintf( cfptr, "%d %s %.2f\n", 22 account, name, balance ); 23 printf( "? " ); 24 scanf( "%d%s%lf", &account, name, &balance ); 25 } fclose( cfptr ); 28 } return 0; 31 } 8

9 Enter the account, name, and balance. Enter EOF to end input.? 100 Jones 24.98? 200 Doe ? 300 White 0.00? 400 Stone ? 500 Rich ? 9

10 Reading Data from a Sequential Access File Reading a sequential access file Create a FILE pointer, link it to the file to read myptr = fopen( "myfile.dat", "r" ); Use fscanf to read from the file Like scanf, except first argument is a FILE pointer fscanf( myptr, "%d%s%f", &myint, &mystring, &myfloat ); Data read from beginning to end File position pointer Indicates number of next byte to be read / written Not really a pointer, but an integer value (specifies byte location) Also called byte offset rewind( myptr ) Repositions file position pointer to beginning of file (byte 0) 10

11 1 /* Fig. 11.7: fig11_07.c 2 Reading and printing a sequential file */ 3 #include <stdio.h> 4 5 int main() 6 { 7 int account; 8 char name[ 30 ]; 9 double balance; 10 FILE *cfptr; /* cfptr = clients.dat file pointer */ if ( ( cfptr = fopen( "clients.dat", "r" ) ) == NULL ) 13 printf( "File could not be opened\n" ); 14 else { 15 printf( "%-10s%-13s%s\n", "Account", "Name", "Balance" 16 fscanf( ); cfptr, "%d%s%lf", &account, name, &balance ); while (!feof( cfptr ) ) { 19 printf( "%-10d%-13s%7.2f\n", account, name, balance 20 ); fscanf( cfptr, "%d%s%lf", &account, name, &balance ); 21 } fclose( cfptr ); 24 } return 0; 27 } Account Name Balance 100 Jones Doe White Stone Rich

12 Unformatted File I/O Unformatted I/O functions fwrite Transfer bytes from a location in memory to a file fread Transfer bytes from a file to a location in memory Example: fwrite( &number, sizeof( int ), 1, myptr ); &number Location to transfer bytes from sizeof( int ) Number of bytes to transfer 1 For arrays, number of elements to transfer In this case, "one element" of an array is being transferred myptr File to transfer to or from 12

13 Unformatted File I/O Writing structs fwrite( &myobject, sizeof (struct mystruct), 1, myptr ); sizeof returns size in bytes of object in parentheses To write several array elements Pointer to array as first argument Number of elements to write as third argument 13

14 1 /* Fig : fig11_11.c 2 Creating a randomly accessed file sequentially */ 3 #include <stdio.h> 4 5 struct clientdata { 6 int acctnum; 7 char lastname[ 15 ]; 8 char firstname[ 10 ]; 9 double balance; 10 }; int main() 13 { 14 int i; 15 struct clientdata blankclient = { 0, "", "", 0.0 }; 16 FILE *cfptr; if ( ( cfptr = fopen( "credit.dat", "w" ) ) == NULL ) 19 printf( "File could not be opened.\n" ); 20 else { for ( i = 1; i <= 100; i++ ) 23 fwrite( &blankclient, 24 sizeof( struct clientdata ), 1, cfptr ); fclose( cfptr ); 27 } return 0; 30 } 14

15 Random Access fseek 15 Sets file position pointer to a specific position fseek( pointer, offset, symbolic_constant ); pointer pointer to file offset file position pointer (0 is first location) symbolic_constant specifies where in file we are reading from SEEK_SET seek starts at beginning of file SEEK_CUR seek starts at current location in file SEEK_END seek starts at end of file

16 Unformatted File Read fread Reads a specified number of bytes from a file into memory fread( &client, sizeof (struct clientdata), 1, myptr ); Can read several fixed-size array elements Provide pointer to array Indicate number of elements to read To read multiple elements, specify in third argument 16

17 Using Command-Line Arguments Pass arguments to main on DOS or UNIX Define main as int main( int argc, char *argv[] ) intargc Number of arguments passed char*argv[] Array of strings Has names of arguments in order argv[ 0 ] is first argument Example: $ copy input output argc: 3 argv[0]: "copy" argv[1]: "input" argv[2 ]: "output" 17

18 1 /* Fig. 14.3: fig14_03.c 2 Using command-line arguments Notice */ argc and 3 #include <stdio.h> argv[] in main 4 5 int main( int argc, char *argv[] ) 6 { 7 FILE *infileptr, *outfileptr; 8 int c; argv[1] is the second 9 10 if ( argc!= 3 ) argument, and is being read. 11 printf( "Usage: copy infile outfile\n" ); argv[2] is the third 12 else argument, and is being 13 if ( ( infileptr = fopen( argv[ 1 ], "r" ) )!= written to. NULL 14 ) 15 if ( ( outfileptr = fopen( argv[ 2 ], "w" ) )!= 16 NULL ) 17 while ( ( c = fgetc( infileptr ) )!= EOF )18 fputc( c, outfileptr ); else 21 printf( "File \"%s\" could not be opened\n", 22 argv[ 2 ] ); outfileptr. 23 else 24 printf( "File \"%s\" could not be opened\n", argv[ 1 ] ); return 0; 27} Loop until End Of File. fgetc a character from infileptr and fputc it into

19 Typecasting in C You can use type casts to explicitly convert types. Syntax: ( type-name ) cast-expression Example: int *p=(int *) malloc(20); int x = (int) 3.5; 19

20 Legal Typecasts 20

21 Implicit Casting (automatic transformation) works in a way that a variable (operand) of data type that is smaller in length (than data type of second variable) (operand), transforming internally to variable of data type with longer number length. It may sound mixed up, but here is an example: short int -> int -> unsigned int ->long int - > unsigned long int -> float -> double-> long double 21

22 int a; unsigned long b; Implicit Casting Example float f, g; double d; g = a + f; // a transforms to float d = a + b; // a and b transform to unsigned long, // adding result is produced in unsigned // long domain and then the result type // unsigned long is transformed // to double 22

23 Notes on Compiling Multiple-Source- File Programs Programs with multiple source files Function definition must be in one file (cannot be split up) Global variables accessible to functions in same file Global variables must be defined in every file in which they are used Example: If integer myglobal is defined in one file To use it in another file you must include the statement extern int myglobal; extern States that the variable is defined in another file Function prototypes can be used in other files without an extern statement Have a prototype in each file that uses the function 23

24 Notes on Compiling Multiple-Source- Keyword static File Programs Specifies that variables can only be used in the file in which they are defined Programs with multiple source files Tedious to compile everything if small changes have been made to only one file Can recompile only the changed files Procedure varies on system UNIX: make utility 24

25 Conditional (? :) Operator average = (n > 0)? sum / n : 0; Equivalent to: if(n > 0) average = sum / n; else average = 0; 25