C: How to Program
Week 17
2007/June/18
1
Chapter 11 – File Processing
•
Outline
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
11.10
Introduction
The Data Hierarchy
Files and Streams
Creating a Sequential Access File
Reading Data from a Sequential Access File
Random Access Files
Creating a Randomly Accessed File
Writing Data Randomly to a Randomly Accessed File
Reading Data Randomly from a Randomly Accessed File
Case Study: A Transaction-Processing Program
2
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
11.2 The Data Hierarchy
• 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
11.2 The Data Hierarchy
• Data Hierarchy (continued):
– File – group of related records
• Example: payroll file
– Database – group of related files
Judy
Judy
01001010
1
Green
Record
Field
Byte (ASCII character J)
Sally
Black
Tom
Blue
Judy
Green
Iris
Orange
File
Randy Red
Bit
Fig. 11.1 The data hierarchy
5
11.2 The Data Hierarchy
• Data files
– Record key
• Identifies a record to facilitate the retrieval of specific records
from a file
– Sequential file
• Records typically sorted by key
6
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
11.3 Files and Streams
• 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
8
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
/* 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
/* 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
Enter
? 100
? 200
? 300
? 400
? 500
? ^Z
the account, name, and balance.
EOF to end input.
Jones 24.98
Doe 345.67
White 0.00
Stone -42.16
Rich 224.62
11
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
11.4 Creating a Sequential Access File
Computer system
Key combination
UNIX systems
<return> <ctrl> d
IBM PC and compatibles
Macintosh
<ctrl> z
<ctrl> d
Fig. 11.4 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
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
11.4 Creating a Sequential Access File
User has access to this
Only the OS has access to this
Open File Table
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
0
1
:
:
7
:
FCB for “clients.dat”
4 The program calls an
7
3 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.
OS service that uses
data in the FCB to
control all input and
output to the actual
file on the disk.
15
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
Open a file for reading in binary mode.
wb
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. 11.6 File open modes.
16
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
/* 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
fclose( cfPtr ); /* fclose closes file */
} /* end else */
return 0; /* indicates successful termination */
} /* end main */
Account
100
200
300
400
500
Name
Jones
Doe
White
Stone
Rich
Balance
24.98
345.67
0.00
-42.16
224.62
– Opening a nonexistent file for reading is an error.
19
/* 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
/* 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
/* 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
/* 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
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
-42.16
? 3
Accounts with debit balances:
100
Jones
24.98
200
Doe
345.67
500
Rich
224.62
? 4
End of run.
24
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 0.00 400 Jones 32.87
(old data in file)
If we want to change White's name to Worthington,
300 Worthington 0.00
300 White 0.00 400 Jones 32.87
Data gets overwritten
300 Worthington 0.00ones 32.87
25
11.6 Random-Access Files
• 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
0
100
200
300
400
500
}byte offsets
100
bytes
}
100
bytes
}
100
bytes
}
100
bytes
}
}
}
100
bytes
100
bytes
26
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
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
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
/* Fig. 11.11: 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 1-100 */
/* 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
/* 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
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
/* Fig. 11.12: 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
/* 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
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 -24.54
Enter account number
? 96
Enter lastname, firstname, balance
? Stone Sam 34.98
Enter account number
? 88
Enter lastname, firstname, balance
? Smith Dave 258.34
Enter account number
? 33
Enter lastname, firstname, balance
? Dunn Stacey 314.33
Enter account number
? 0
35
11.8 Writing Data Randomly to a Randomly
Accessed File
36
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
/* Fig. 11.15: 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
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
Acct
29
33
37
88
96
Last Name
Brown
Dunn
Barker
Smith
Stone
First Name
Nancy
Stacey
Doug
Dave
Sam
Balance
-24.54
314.33
0.00
258.34
34.98
40
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
/* Fig. 11.16: 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
/* 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
/* 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
/* 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
/* 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 ( 1 - 100 ): " );
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
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
/* 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 ( 1 - 100 ): " );
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
/* 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 ( 1 - 100 ): " );
scanf( "%d", &accountNum );
/* move file pointer to correct record in file */
fseek( fPtr, ( accountNum - 1 ) * sizeof( struct clientData ), SEEK_SET );
49
/* 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
/* 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
After choosing option 1 accounts.txt contains:
Acct
29
33
37
88
96
Last Name
Brown
Dunn
Barker
Smith
Stone
First Name
Nancy
Stacey
Doug
Dave
Sam
Balance
-24.54
314.33
0.00
258.34
34.98
running option 2:
Enter account to update ( 1 - 100 ): 37
37
Barker
Doug
0.00
Enter charge ( + ) or payment ( - ): +87.99
37
Barker
Doug
87.99
running option 2:
Enter new account number ( 1 - 100 ): 22
Enter lastname, firstname, balance
? Johnston Sarah 247.45
52
Chapter 13 - The Preprocessor
Outline
13.1
13.2
13.3
13.4
13.5
13.6
13.7
13.8
13.9
13.10
Introduction
The #include Preprocessor Directive
The #define Preprocessor Directive: Symbolic Constants
The #define Preprocessor Directive: Macros
Conditional Compilation
The #error and #pragma Preprocessor Directives
The # and ## Operators
Line Numbers
Predefined Symbolic Constants
Assertions
53
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
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
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 3.14159
– Everything to right of identifier replaces text
#define PI = 3.14159
• Replaces “PI” with "= 3.14159"
– Cannot redefine symbolic constants once they have been
created
56
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 = ( 3.14159 * ( 4 ) * ( 4 ) );
57
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 = 3.14159 * 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
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
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
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
13.5 Conditional Compilation
• Debugging
#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
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
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
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
13.8 Line Numbers
• #line
– 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
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 19 2001").
The time the source file is compiled (a string literal of
the form "hh:mm:ss").
67
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