Computer Programming Language
Week 11 Data Files

Objectives
Understand Text and Binary Files  Declaring, Opening, and Closing File Streams  Reading from and Writing to Text Files  Passing and Returning Filenames  Reading from and Writing to Binary Files (Optional)
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Files
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File: collection of data that is stored together under a common name, usually on a disk, magnetic tape, or CD-ROM Each file has a unique filename, referred to as the file’s external name
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For example, prices.dat and info.txt

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Text and Binary Files
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A file can be thought as an array of bytes.
Characters Displayed 768A

Code page: ASCII Code

Software Interpretation

Binary numbers, in decimal numbers: 55 54 56 65

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ASCII Table

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Other Code Page*
How this text file is stored in computer? Hello 您好

Encode simplified Chinese • GB2312 • GB18030 • UTF-8

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Declaring, Opening, and Closing File Streams
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To store and retrieve data outside a C program, you need two items:
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A file A file stream

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File stream: one-way transmission path used to connect a file stored on a physical device to a program
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Input file stream: receives data from a file into a program Output file stream: sends data to a file

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File Streams

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Declaring a File Stream
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File stream is a data structure For each file that your program uses, a file stream must be named (declared) and created (opened) Declare a file stream  FILE *inFile;  Asterisk is necessary, inFile is a pointer pointing to FILE type data.  The FILE data structure is declared in stdio.h

Opening a File Stream
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Opening a file stream (or opening the file):
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Establishes the physical communication link between the program and the data file Equates a specific external filename to the name declared in the FILE declaration statement outFile = fopen("prices.bnd","w"); fileOut = fopen("prices.dat", "wb"); inFile = fopen("prices.bnd","r");
 If a file opened for reading does not exist,

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Use fopen() (declared in stdio.h)
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fopen()

returns the NULL address value

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Open Mode

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Example: open an input file

if ((inFile = fopen("prices.dat","r")) == NULL )

passes its integer argument directly to the operating system 12 and then terminates program operation; declared in stdlib.h

Opening an output File Stream
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Approach in Program 10.1 does NOT work for output files  If a file exists having the same name as the file to be opened for writing, the existing file is erased and all its data is lost  The file can first be opened in input mode, simply to see if it exists
 If it does, the user is given

the choice of explicitly permitting it to be overwritten when it is subsequently opened in output mode
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Example: open an output file stream

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Sample run 1:
A file by the name prices.dat exists. Do you want to continue and overwrite it with the new data (y or n): n
The existing file will not be overwritten.

Sample run 2:
A file by the name prices.dat exists. Do you want to continue and overwrite it with the new data (y or n): y

The file has been successfully opened for output.
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Users choose file to open

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Closing a File Stream
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A file stream is closed using fclose()  fclose() breaks the link between the file’s external and internal names, releasing the internal file pointer name, which can then be used for another file  fclose(inFile); Because all computers have a limit on the maximum number of files that can be open at one time, closing files that are no longer needed makes good sense Open files existing at the end of normal program execution are closed by the operating system
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Writing to Text Files

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Prototypes in stdio.h Examples
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fputc('a',outFile); fputs("Hello world!",outFile); fprintf(outFile,"%s %n",descrip,price);

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Example: write to a text file

prices.dat:
Batteries 39.25 Bulbs 3.22 Fuses 1.03

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What to write and what is written

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Reading from a Text File

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Prototypes in stdio.h Examples
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fgetc(inFile); fgets(message,10,inFile); fscanf(inFile,"%lf",&price);

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fgetc() and fscanf() return EOF when the end-of-file is detected fgets() returns a NULL instead
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Example: read from a text file

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Example: read from a text file (another way)

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Move Access Point in a File
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rewind() resets the current position to the start of the file  rewind(inFile) fseek() allows the programmer to move to any position in the file  fseek(fileName, offset, origin)  Origin: SEEK_SET, SEEK_CUR, and SEEK_END ftell() returns the offset value of the next character that will be read or written  ftell(inFile);
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Examples of fseek() are
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fseek(inFile,4L,SEEK_SET); fseek(inFile,4L,SEEK_CUR); fseek(inFile,-4L,SEEK_CUR); fseek(inFile,0L,SEEK_SET); fseek(inFile,0L,SEEK_END); fseek(inFile,-10L,SEEK_END);

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Example: Read a file in reverse order

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Passing and Returning Filenames

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Passing and Returning Filenames (cont’d)

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Writing and Reading Binary Files* (Optional)
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Binary files store numerical values using the computer’s internal numerical code No number-to-character conversion when writing a number to a file, and no character-to-number conversion when a value is read from the file  Resulting file frequently requires less storage space than its character-based counterpart

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Example: Writing Binary Files*

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Example: Reading Binary Files*

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Standard Device Files: Virtual Files
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When a program is run, the keyboard used for entering data is automatically opened and assigned to the internal file pointer name stdin
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fscanf(stdin,"%d",&num);

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The output device used for display is assigned to the file pointer named stdout
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fprintf(stdout,"Hello World!");

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stderr is assigned to the output device used for system error messages  stderr and stdout often refer to the same device
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Correspondence between some I/O functions
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The character function pairs listed in Table 10.2 can be used as direct replacements for each other This is not true for the string-handling functions

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Other Devices
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Most IBM or IBM-compatible personal computers assign the name prn to the printer connected to the computer  fprintf("prn","Hello World!"); prn is not a pointer constant but the actual name of the device; as such, it must be enclosed in double quotes when used in a statement

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Output Redirection
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Redirect your program’s output to a file
>: Output to a new file; if the file exists, overwrite the file pgm10-5 > 3.txt  >>: Append the output to a existing file; if the file doesn’t exist, create the file pgm10-5 >> 3.txt
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Summary
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A data file is opened using the fopen() standard library function A file can be opened for reading, writing, or appending An internal filename must be declared as a pointer to a FILE Data files can be accessed randomly using rewind(), fseek(), and ftell() In addition to any files opened within a function, the standard files stdin, stdout, and stderr are automatically opened when a program is run
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