C: How to Program. Week /June/18

Transcription

1 C: How to Program Week /June/18 1

2 Chapter 11 File Processing Outline 11.1 Introduction 11.2 The Data Hierarchy 11.3 Files and Streams 11.4 Creating a Sequential Access File 11.5 Reading Data from a Sequential Access File 11.6 Random Access Files 11.7 Creating a Randomly Accessed File 11.8 Writing Data Randomly to a Randomly Accessed File 11.9 Reading Data Randomly from a Randomly Accessed File Case Study: A Transaction-Processing Program 2

3 11.1 Introduction Data files Can be created, updated, and processed by C programs Are used for permanent storage of large amounts of data Storage of data in variables and arrays is only temporary 3

4 Data Hierarchy: 11.2 The Data Hierarchy Bit smallest data item Value of 0 or 1 Byte 8 bits Used to store a character Decimal digits, letters, and special symbols Field group of characters conveying meaning Example: your name Record group of related fields Represented by a struct or a class Example: In a payroll system, a record for a particular employee that contained his/her identification number, name, address, etc. 4

5 11.2 The Data Hierarchy Data Hierarchy (continued): File group of related records Example: payroll file Database group of related files Judy Green Record Judy Field Byte (ASCII character J) Sally Black Tom Blue Judy Green File Iris Orange Randy Red 1 Bit Fig The data hierarchy 5

6 Data files 11.2 The Data Hierarchy Record key Identifies a record to facilitate the retrieval of specific records from a file Sequential file Records typically sorted by key 6

7 11.3 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) 7

8 FILEstructure File descriptor 11.3 Files and Streams 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 8

9 11.3 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 9

10 /* Fig. 11.3: fig11_03.c Create a sequential file */ #include <stdio.h> int main() { int account; /* account number */ char name[ 30 ]; /* account name */ double balance; /* account balance */ FILE *cfptr; /* cfptr = clients.dat file pointer */ /* fopen opens file. Exit program if unable to create file */ if ( ( cfptr = fopen( "clients.dat", "w" ) ) == NULL ) { printf( "File could not be opened\n" ); } /* end if */ else { printf( "Enter the account, name, and balance.\n" ); printf( "Enter EOF to end input.\n" ); printf( "? " ); scanf( "%d%s%lf", &account, name, &balance ); 10

11 /* write account, name and balance into file with fprintf */ while (!feof( stdin ) ) { fprintf( cfptr, "%d %s %.2f\n", account, name, balance ); printf( "? " ); scanf( "%d%s%lf", &account, name, &balance ); } /* end while */ fclose( cfptr ); /* fclose closes file */ } /* end else */ return 0; /* indicates successful termination */ } /* end main */ 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 ? ^Z 11

12 11.4 Creating a Sequential Access File C imposes no file structure No notion of records in a file Programmer must provide file structure Creating a File FILE *cfptr; Creates a FILE pointer called cfptr cfptr = fopen("clients.dat", "w"); Function fopen returns a FILE pointer to file specified Takes two arguments file to open and file open mode If open fails, NULL returned 12

13 11.4 Creating a Sequential Access File Computer system UNIX systems IBM PC and compatibles Macintosh Key combination <return> <ctrl> d <ctrl> z <ctrl> d Fig End-of-file key combinations for various popular computer systems. Opening an existing file for writing ( w ) when, in fact, the user wants to preserve the file discards the contents of the file without warning. Forgetting to open a file before attempting to reference it in a program is a logic error. 13

14 11.4 Creating a Sequential Access File 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) 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 14

15 11.4 Creating a Sequential Access File 3 User has access to this 1 cfptr = fopen( clients.dat, w ); fopen returns a pointer to a FILE structure (defined in <stdio.h>) newptr 2 FILE structure for clients.dat contents a descriptor, i.e., a small integer that is an index into the Open File Table 7 When the program issues an I/O call the program locates the descriptor (7) in the FILE structure, and uses the descriptor to find the FCB in the Open File Table. Only the OS has access to this Open File Table 0 1 : : 7 FCB for clients.dat : 4 The program calls an OS service that uses data in the FCB to control all input and output to the actual file on the disk. 15

16 11.4 Creating a Sequential Access File 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. rb wb Open a file for reading in binary mode. Create a file for writing in binary mode. If the file already exists, discard the current contents. ab Append; open or create a file for writing at end of file in binary mode. rb+ Open a file for update (reading and writing) in binary mode. wb+ Create a file for update in binary mode. If the file already exists, discard the current contents. ab+ Append; open or create a file for update in binary mode; writing is done at the end of the file. Fig File open modes. 16

17 11.5 Reading Data from a Sequential Access File Reading a sequential access file Create a FILE pointer, link it to the file to read cfptr = fopen( "clients.dat", "r" ); Use fscanf to read from the file Like scanf, except first argument is a FILE pointer fscanf( cfptr, "%d%s%f", &accounnt, name, &balance ); 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( cfptr ) Repositions file position pointer to beginning of file (byte 0) 17

18 /* Fig. 11.7: fig11_07.c Reading and printing a sequential file */ #include <stdio.h> int main() { int account; /* account number */ char name[ 30 ]; /* account name */ double balance; /* account balance */ FILE *cfptr; /* cfptr = clients.dat file pointer */ /* fopen opens file; exits program if file cannot be opened */ if ( ( cfptr = fopen( "clients.dat", "r" ) ) == NULL ) { printf( "File could not be opened\n" ); } /* end if */ else { /* read account, name and balance from file */ printf( "%-10s%-13s%s\n", "Account", "Name", "Balance" ); fscanf( cfptr, "%d%s%lf", &account, name, &balance ); /* while not end of file */ while (!feof( cfptr ) ) { printf( "%-10d%-13s%7.2f\n", account, name, balance ); fscanf( cfptr, "%d%s%lf", &account, name, &balance ); } /* end while */ 18

19 fclose( cfptr ); /* fclose closes file */ } /* end else */ return 0; /* indicates successful termination */ } /* end main */ Account Name Balance 100 Jones Doe White Stone Rich Opening a nonexistent file for reading is an error. 19

20 /* Fig. 11.8: fig11_08.c Credit inquiry program */ #include <stdio.h> int main() { int request; /* request number */ int account; /* account number */ double balance; /* account balance */ char name[ 30 ]; /* account name */ FILE *cfptr; /* clients.dat file pointer */ /* fopen opens file; exits program if file cannot be opened */ if ( ( cfptr = fopen( "clients.dat", "r" ) ) == NULL ) { printf( "File could not be opened\n" ); } /* end if */ else { /* display request options */ printf( "Enter request\n" " 1 - List accounts with zero balances\n" " 2 - List accounts with credit balances\n" " 3 - List accounts with debit balances\n" " 4 - End of run\n? " ); scanf( "%d", &request ); 20

21 /* process user's request */ while ( request!= 4 ) { /* read account, name and balance from file */ fscanf( cfptr, "%d%s%lf", &account, name, &balance ); switch ( request ) { case 1: printf( "\naccounts with zero balances:\n" ); /* read file contents (until eof) */ while (!feof( cfptr ) ) { if ( balance == 0 ) { printf( "%-10d%-13s%7.2f\n", account, name, balance ); } /* end if */ /* read account, name and balance from file */ fscanf( cfptr, "%d%s%lf", &account, name, &balance ); } /* end while */ break; case 2: printf( "\naccounts with credit balances:\n" ); 21

22 /* read file contents (until eof) */ while (!feof( cfptr ) ) { if ( balance < 0 ) { printf( "%-10d%-13s%7.2f\n", account, name, balance ); } /* end if */ /* read account, name and balance from file */ fscanf( cfptr, "%d%s%lf", &account, name, &balance ); } /* end while */ break; case 3: printf( "\naccounts with debit balances:\n" ); /* read file contents (until eof) */ while (!feof( cfptr ) ) { if ( balance > 0 ) { printf( "%-10d%-13s%7.2f\n", account, name, balance ); } /* end if */ 22

23 /* read account, name and balance from file */ fscanf( cfptr, "%d%s%lf", &account, name, &balance ); } /* end while */ break; } /* end switch */ rewind( cfptr ); /* return cfptr to beginning of file */ printf( "\n? " ); scanf( "%d", &request ); } /* end while */ printf( "End of run.\n" ); fclose( cfptr ); /* fclose closes the file */ } /* end else */ return 0; /* indicates successful termination */ } /* end main */ 23

24 Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - End of run? 1 Accounts with zero balances: 300 White 0.00? 2 Accounts with credit balances: 400 Stone ? 3 Accounts with debit balances: 100 Jones Doe Rich ? 4 End of run. 24

25 11.5 Reading Data from a Sequential Access File Sequential access file Cannot be modified without the risk of destroying other data Fields can vary in size Different representation in files and screen than internal representation 1, 34, -890 are all ints, but have different sizes on disk 300 White Jones (old data in file) If we want to change White's name to Worthington, 300 Worthington White Jones Data gets overwritten 300 Worthington 0.00ones

26 Random access files 11.6 Random-Access Files Access individual records without searching through other records Instant access to records in a file Data can be inserted without destroying other data Data previously stored can be updated or deleted without overwriting Implemented using fixed length records Sequential files do not have fixed length records byte offsets 100 bytes 100 bytes 100 bytes 100 bytes 100 bytes 100 bytes 26

27 11.6 Random-Access Files Data in random access files Unformatted (stored as "raw bytes") All data of the same type (ints, for example) uses the same amount of memory All records of the same type have a fixed length Data not human readable 27

28 11.7 Creating a Randomly Accessed File 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 28

29 11.7 Creating a Randomly Accessed File 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 29

30 /* Fig : fig11_11.c Creating a random-access file sequentially */ #include <stdio.h> /* clientdata structure definition */ struct clientdata { int acctnum; /* account number */ char lastname[ 15 ]; /* account last name */ char firstname[ 10 ]; /* account first name */ double balance; /* account balance */ }; /* end structure clientdata */ int main() { int i; /* counter used to count from */ /* create clientdata with default information */ struct clientdata blankclient = { 0, "", "", 0.0 }; FILE *cfptr; /* credit.dat file pointer */ /* fopen opens the file; exits if file cannot be opened */ if ( ( cfptr = fopen( "credit.dat", "wb" ) ) == NULL ) { printf( "File could not be opened.\n" ); } /* end if */ else { 30

31 /* output 100 blank records to file */ for ( i = 1; i <= 100; i++ ) { fwrite( &blankclient, sizeof( struct clientdata ), 1, cfptr ); } /* end for */ fclose ( cfptr ); /* fclose closes the file */ } /* end else */ return 0; /* indicates successful termination */ } /* end main */ 31

32 11.8 Writing Data Randomly to a Randomly Accessed File fseek 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 32

33 /* Fig : fig11_12.c Writing to a random access file */ #include <stdio.h> /* clientdata structure definition */ struct clientdata { int acctnum; /* account number */ char lastname[ 15 ]; /* account last name */ char firstname[ 10 ]; /* account first name */ double balance; /* account balance */ }; /* end structure clientdata */ int main() { /* create clientdata with default information */ struct clientdata Client ; FILE *cfptr; /* credit.dat file pointer */ /* fopen opens the file; exits if file cannot be opened */ if ( ( cfptr = fopen( "credit.dat", "rb+" ) ) == NULL ) { printf( "File could not be opened.\n" ); } /* end if */ else { 33

34 /* require user to specify account number */ printf( "Enter account number" " ( 1 to 100, 0 to end input )\n? " ); scanf( "%d", &client.acctnum ); /* user enters information, which is copied into file */ while ( client.acctnum!= 0 ) { /* user enters last name, first name and balance */ printf( "Enter lastname, firstname, balance\n? " ); /* set record lastname, firstname and balance value */ fscanf( stdin, "%s%s%lf", client.lastname, client.firstname, &client.balance ); /* seek position in file to user-specified record */ fseek( cfptr, ( client.acctnum - 1 ) * sizeof( struct clientdata ), SEEK_SET ); /* write user-specified information in file */ fwrite( &client, sizeof( struct clientdata ), 1, cfptr ); /* enable user to input another account number */ printf( "Enter account number\n? " ); scanf( "%d", &client.acctnum ); } /* end while */ 34

35 fclose( cfptr ); /* fclose closes the file */ } /* end else */ return 0; /* indicates successful termination */ } /* end main */ Enter account number ( 1 to 100, 0 to end input )? 37 Enter lastname, firstname, balance? Barker Doug 0.00 Enter account number? 29 Enter lastname, firstname, balance? Brown Nancy Enter account number? 96 Enter lastname, firstname, balance? Stone Sam Enter account number? 88 Enter lastname, firstname, balance? Smith Dave Enter account number? 33 Enter lastname, firstname, balance? Dunn Stacey Enter account number? 0 35

36 11.8 Writing Data Randomly to a Randomly Accessed File 36

37 11.9 Reading Data Randomly from a Randomly Accessed File 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 37

38 /* Fig : fig11_15.c Reading a random access file sequentially */ #include <stdio.h> /* clientdata structure definition */ struct clientdata { int acctnum; /* account number */ char lastname[ 15 ]; /* account last name */ char firstname[ 10 ]; /* account first name */ double balance; /* account balance */ }; /* end structure clientdata */ int main() { /* create clientdata with default information */ struct clientdata blankclient = { 0, "", "", 0.0 }; FILE *cfptr; /* credit.dat file pointer */ /* fopen opens the file; exits if file cannot be opened */ if ( ( cfptr = fopen( "credit.dat", "rb" ) ) == NULL ) { printf( "File could not be opened.\n" ); } /* end if */ else { 38

39 printf( "%-6s%-16s%-11s%10s\n", "Acct", "Last Name", "First Name", "Balance" ); /* read all records from file (until eof) */ while (!feof( cfptr ) ) { fread( &client, sizeof( struct clientdata ), 1, cfptr ); /* display record */ if ( client.acctnum!= 0 ) { printf( "%-6d%-16s%-11s%10.2f\n", client.acctnum, client.lastname, client.firstname, client.balance ); } /* end if */ } /* end while */ fclose( cfptr ); /* fclose closes the file */ } /* end else */ return 0; /* indicates successful termination */ } /* end main */ 39

40 Acct Last Name First Name Balance 29 Brown Nancy Dunn Stacey Barker Doug Smith Dave Stone Sam

41 11.10 Case Study: A Transaction Processing Program This program Demonstrates using random access files to achieve instant access processing of a bank s account information We will Update existing accounts Add new accounts Delete accounts Store a formatted listing of all accounts in a text file 41

42 /* Fig : fig11_16.c This program reads a random access file sequentially, updates data already written to the file, creates new data to be placed in the file, and deletes data previously in the file. */ #include <stdio.h> /* clientdata structure definition */ struct clientdata { int acctnum; /* account number */ char lastname[ 15 ]; /* account last name */ char firstname[ 10 ]; /* account first name */ double balance; /* account balance */ }; /* end structure clientdata */ /* prototypes */ int enterchoice( void ); void textfile( FILE *readptr ); void updaterecord( FILE *fptr ); void newrecord( FILE *fptr ); void deleterecord( FILE *fptr ); int main() { FILE *cfptr; /* credit.dat file pointer */ int choice; /* user's choice */ 42

43 /* fopen opens the file; exits if file cannot be opened */ if ( ( cfptr = fopen( "credit.dat", "rb+" ) ) == NULL ) { printf( "File could not be opened.\n" ); } /* end if */ else { /* enable user to specify action */ while ( ( choice = enterchoice() )!= 5 ) { switch ( choice ) { /* create text file from record file */ case 1: textfile( cfptr ); break; /* update record */ case 2: updaterecord( cfptr ); break; /* create record */ case 3: newrecord( cfptr ); break; 43

44 /* delete existing record */ case 4: deleterecord( cfptr ); break; /* display message if user does not select valid choice */ default: printf( "Incorrect choice\n" ); break; } /* end switch */ } /* end while */ fclose( cfptr ); /* fclose closes the file */ } /* end else */ return 0; /* indicates successful termination */ } /* end main */ /* create formatted text file for printing */ void textfile( FILE *readptr ) { FILE *writeptr; /* accounts.txt file pointer */ 44

45 /* create clientdata with default information */ struct clientdata client = { 0, "", "", 0.0 }; /* fopen opens the file; exits if file cannot be opened */ if ( ( writeptr = fopen( "accounts.txt", "w" ) ) == NULL ) { printf( "File could not be opened.\n" ); } /* end if */ else { rewind( readptr ); /* sets pointer to beginning of file */ fprintf( writeptr, "%-6s%-16s%-11s%10s\n", "Acct", "Last Name", "First Name", "Balance" ); /* copy all records from random-access file into text file */ while (!feof( readptr ) ) { fread( &client, sizeof( struct clientdata ), 1, readptr ); /* write single record to text file */ if ( client.acctnum!= 0 ) { fprintf( writeptr, "%-6d%-16s%-11s%10.2f\n", client.acctnum, client.lastname, client.firstname, client.balance ); } /* end if */ } /* end while */ fclose( writeptr ); /* fclose closes the file */ } /* end else */ } /* end function textfile */ 45

46 /* update balance in record */ void updaterecord( FILE *fptr ) { int account; /* account number */ double transaction; /* transaction amount */ /* create clientdata with no information */ struct clientdata client = { 0, "", "", 0.0 }; /* obtain number of account to update */ printf( "Enter account to update ( ): " ); scanf( "%d", &account ); /* move file pointer to correct record in file */ fseek( fptr, ( account - 1 ) * sizeof( struct clientdata ), SEEK_SET ); /* read record from file */ fread( &client, sizeof( struct clientdata ), 1, fptr ); /* display error if account does not exist */ if ( client.acctnum == 0 ) { printf( "Acount #%d has no information.\n", account ); } /* end if */ else { /* update record */ 46

47 printf( "%-6d%-16s%-11s%10.2f\n\n", client.acctnum, client.lastname, client.firstname, client.balance ); /* request transaction amount from user */ printf( "Enter charge ( + ) or payment ( - ): " ); scanf( "%lf", &transaction ); client.balance += transaction; /* update record balance */ printf( "%-6d%-16s%-11s%10.2f\n", client.acctnum, client.lastname, client.firstname, client.balance ); /* move file pointer to correct record in file */ fseek( fptr, ( account - 1 ) * sizeof( struct clientdata ), SEEK_SET ); /* write updated record over old record in file */ fwrite( &client, sizeof( struct clientdata ), 1, fptr ); } /* end else */ } /* end function updaterecord */ 47

48 /* delete an existing record */ void deleterecord( FILE *fptr ) { struct clientdata client; /* stores record read from file */ struct clientdata blankclient = { 0, "", "", 0 }; /* blank client */ int accountnum; /* account number */ /* obtain number of account to delete */ printf( "Enter account number to delete ( ): " ); scanf( "%d", &accountnum ); /* move file pointer to correct record in file */ fseek( fptr, ( accountnum - 1 ) * sizeof( struct clientdata ), SEEK_SET ); /* read record from file */ fread( &client, sizeof( struct clientdata ), 1, fptr ); /* display error if record does not exist */ if ( client.acctnum == 0 ) { printf( "Account %d does not exist.\n", accountnum ); } /* end if */ else { /* delete record */ 48

49 /* move file pointer to correct record in file */ fseek( fptr, ( accountnum-1 ) * sizeof( struct clientdata ), SEEK_SET ); /* replace existing record with blank record */ fwrite( &blankclient, sizeof( struct clientdata ), 1, fptr ); } /* end else */ } /* end function deleterecord */ /* create and insert record */ void newrecord( FILE *fptr ) { /* create clientdata with default information */ struct clientdata client = { 0, "", "", 0.0 }; int accountnum; /* account number */ /* obtain number of account to create */ printf( "Enter new account number ( ): " ); scanf( "%d", &accountnum ); /* move file pointer to correct record in file */ fseek( fptr, ( accountnum - 1 ) * sizeof( struct clientdata ), SEEK_SET ); 49

50 /* read record from file */ fread( &client, sizeof( struct clientdata ), 1, fptr ); /* display error if account already exists */ if ( client.acctnum!= 0 ) { printf( "Account #%d already contains information.\n", client.acctnum ); } /* end if */ else { /* create record */ /* user enters last name, first name and balance */ printf( "Enter lastname, firstname, balance\n? " ); scanf( "%s%s%lf", &client.lastname, &client.firstname, &client.balance ); client.acctnum = accountnum; /* move file pointer to correct record in file */ fseek(fptr, (client.acctnum-1) * sizeof( struct clientdata ), SEEK_SET); /* insert record in file */ fwrite( &client, sizeof( struct clientdata ), 1, fptr ); } /* end else */ } /* end function newrecord */ 50

51 /* enable user to input menu choice */ int enterchoice( void ) { int menuchoice; /* variable to store user's choice */ /* display available options */ printf( "\nenter your choice\n" "1 - store a formatted text file of acounts called\n" " \"accounts.txt\" for printing\n" "2 - update an account\n" "3 - add a new account\n" "4 - delete an account\n" "5 - end program\n? " ); scanf( "%d", &menuchoice ); /* receive choice from user */ return menuchoice; } /* end function enterchoice */ 51

52 After choosing option 1 accounts.txt contains: Acct Last Name First Name Balance 29 Brown Nancy Dunn Stacey Barker Doug Smith Dave Stone Sam running option 2: Enter account to update ( ): Barker Doug 0.00 Enter charge ( + ) or payment ( - ): Barker Doug running option 2: Enter new account number ( ): 22 Enter lastname, firstname, balance? Johnston Sarah

53 Chapter 13 - The Preprocessor Outline 13.1 Introduction 13.2 The #include Preprocessor Directive 13.3 The #define Preprocessor Directive: Symbolic Constants 13.4 The #define Preprocessor Directive: Macros 13.5 Conditional Compilation 13.6 The #error and #pragma Preprocessor Directives 13.7 The # and ## Operators 13.8 Line Numbers 13.9 Predefined Symbolic Constants Assertions 53

54 13.1 Introduction Preprocessing Occurs before a program is compiled Inclusion of other files Definition of symbolic constants and macros Conditional compilation of program code Conditional execution of preprocessor directives Format of preprocessor directives Lines begin with # Only whitespace characters before directives on a line 54

55 13.2 The #include Preprocessor Directive #include Copy of a specified file included in place of the directive #include <filename> Searches standard library for file Use for standard library files #include "filename" Searches current directory, then standard library Use for user-defined files Used for: Programs with multiple source files to be compiled together Header file has common declarations and definitions (classes, structures, function prototypes) #include statement in each file 55

56 13.3 The #define Preprocessor Directive: Symbolic Constants #define Preprocessor directive used to create symbolic constants and macros Symbolic constants When program compiled, all occurrences of symbolic constant replaced with replacement text Format #define identifier replacement-text Example: #define PI Everything to right of identifier replaces text #define PI = Replaces PI with "= " Cannot redefine symbolic constants once they have been created 56

57 13.4 The #define Preprocessor Directive: Macros Macro Operation defined in #define A macro without arguments is treated like a symbolic constant A macro with arguments has its arguments substituted for replacement text, when the macro is expanded Performs a text substitution no data type checking The macro #define CIRCLE_AREA( x ) ( PI * ( x ) * ( x ) ) would cause area = CIRCLE_AREA( 4 ); to become area = ( * ( 4 ) * ( 4 ) ); 57

58 13.4 The #define Preprocessor Directive: Macros Use parenthesis Without them the macro #define CIRCLE_AREA( x ) PI * x * x would cause area = CIRCLE_AREA( c + 2 ); to become area = * c + 2 * c + 2; Multiple arguments #define RECTANGLE_AREA( x, y ) ( ( x ) * ( y ) ) would cause rectarea = RECTANGLE_AREA( a + 4, b + 7 ); to become rectarea = ( ( a + 4 ) * ( b + 7 ) ); 58

59 13.4 The #define Preprocessor Directive: Macros If the replacement text for a macro or symbolic constant is longer than the remainder of the line, a backslash ( \ ) must be placed at the end of the line. #undef Undefines a symbolic constant or macro If a symbolic constant or macro has been undefined it can later be redefined 59

60 13.5 Conditional Compilation Conditional compilation Control preprocessor directives and compilation Cast expressions, sizeof, enumeration constants cannot be evaluated in preprocessor directives Structure similar to if #if!defined( NULL ) #define NULL 0 #endif Determines if symbolic constant NULL has been defined If NULL is defined, defined( NULL ) evaluates to 1 If NULL is not defined, this function defines NULL to be 0 Every #if must end with #endif #ifdef short for #if defined( name ) #ifndef short for #if!defined( name ) 60

61 13.5 Conditional Compilation Other statements #elif equivalent of else if in an if statement #else equivalent of else in an if statement "Comment out" code Cannot use /*... */ Use #if 0 code commented out #endif To enable code, change 0 to 1 61

62 Debugging 13.5 Conditional Compilation #define DEBUG 1 #ifdef DEBUG cerr << "Variable x = " << x << endl; #endif Defining DEBUG to 1 enables code After code corrected, remove #define statement Debugging statements are now ignored 62

63 13.6 The #error and #pragma Preprocessor Directives #error tokens Tokens are sequences of characters separated by spaces "I like C++" has 3 tokens Displays a message including the specified tokens as an error message Stops preprocessing and prevents program compilation #pragma tokens Implementation defined action (consult compiler documentation) Pragmas not recognized by compiler are ignored 63

64 # 13.7 The # and ## Operators Causes a replacement text token to be converted to a string surrounded by quotes The statement #define HELLO( x ) printf( Hello, #x \n ); would cause HELLO( John ) to become printf( Hello, John \n ); Strings separated by whitespace are concatenated when using printf 64

65 13.7 The # and ## Operators ## Concatenates two tokens The statement #define TOKENCONCAT( x, y ) x ## y would cause TOKENCONCAT( O, K ) to become OK 65

66 #line 13.8 Line Numbers Renumbers subsequent code lines, starting with integer value File name can be included #line 100 "myfile.c" Lines are numbered from 100 beginning with next source code file Compiler messages will think that the error occurred in "myfile.c" Makes errors more meaningful Line numbers do not appear in source file 66

67 13.9 Predefined Symbolic Constants Four predefined symbolic constants Cannot be used in #define or #undef Sym b o lic c o n sta n t De sc rip tio n LINE FILE DATE TIME The line number of the current source code line (an integer constant). The presumed name of the source file (a string). The date the source file is compiled (a string of the form "Mmm dd yyyy" such as "Jan "). The time the source file is compiled (a string literal of the form "hh:mm:ss"). 67

68 13.10 Assertions assert macro Header <assert.h> Tests value of an expression If 0 (false) prints error message and calls abort Example: assert( x <= 10 ); If NDEBUG is defined All subsequent assert statements ignored #define NDEBUG 68