C File Processing - I

Math 130

B Smith:

Eval: 3. Energy and excitement helped for avg graphics. Gen interest. A bit wordy, henced redundant in some parts since most is discussed. Good for handout, bad for lectures.

B Smith:

Eval: 3. Energy and excitement helped for avg graphics. Gen interest. A bit wordy, henced redundant in some parts since most is discussed. Good for handout, bad for lectures.

B Smith:

4/13/2005 9:54 AM: Rate: 3, low discussion, implement, “Your turn…” Time: 50 minutes

B Smith:

4/13/2005 9:54 AM: Rate: 3, low discussion, implement, “Your turn…” Time: 50 minutes

B Smith:

Rate:2/4 More examples needed, fewer words.

B Smith:

Rate:2/4 More examples needed, fewer words.

B Smith:

Sp06: rate 2.5 Worked mostly from examples 11.1 and 11.2. Need better examples.

B Smith:

Sp06: rate 2.5 Worked mostly from examples 11.1 and 11.2. Need better examples.

Overview

• C Functions to Open and Close Streams

• Declaring, Opening, and Closing Files

• Reading and Writing Files

• Example

Opening a File

• We have been working with programs that have read from the “standard input” (the keyboard) written to the “standard output” (the monitor)

• How can we access a file which is not already “connected” to the program?

• File processing is performed using a FILE structure/data-type. First you must declare an instance of the FILE structure. Examples:

FILE* inFile;FILE* prices;FILE* fp;

FILE* inFile;FILE* prices;FILE* fp;

B Smith:

In C a stream is referred to as a FILE. See attached notes.

B Smith:

In C a stream is referred to as a FILE. See attached notes.

Opening a File

• Use fopen() to open a file• fopen() takes an external name such as

“proj01.c” as an argument• fopen() returns an internal file name for use in

subsequent reads and writes to the file this “handle” is a file pointer returned from fopen()

//prototypeFILE *fopen(char* FileName, char *Mode);

FILE *fp;

fp = fopen(“proj01.c”, “r”);

fopen()

• The file pointer returned from fopen() points to a file structure

• A file structure contains file details such as buffer location current character position in the buffer whether the file is being read or written

FILE *fp;

fp = fopen(“proj01.c”, “r”);

fopen()

• The first argument of fopen is the name of the file, taken as a character string

• The second argument is the “mode,” used to indicate how the file is to be used

FILE *fp;

fp = fopen(“proj01.c”, “r”);

fopen()

• The files opened by fopen() will generally be read, written, or appended allowed modes reflect this

• mode r: read

• mode w: write

• mode a: append, or add to

file name and mode should be enclosed in double quotes

FILE *fp;

fp = fopen(“proj01.c”, “r”);

pgm 11.1 from Bronson textbook

#include <stdlib.h>#include <stdio.h>int main(){ FILE *inFile; char fileName[13];

printf("\nEnter a file name: "); gets(fileName);

inFile = fopen(fileName,"r"); /* open the file */

if (inFile == NULL) { printf("\nThe file cannot be opened."); printf("\nPlease check that the file currently exists."); exit(1); /* all open streams are closed */ }

printf("\nThe file has been successfully opened for reading"); return 0;}

Opening a File for Writing:

• This creates a new file makes the file available for output from the function

opening the file if a file with the same name exists, the old file is

erased

outFile = fopen(“prices.bnd”,”w”);

pgm 11.2 – Writing a File example

#include <stdlib.h>#include <stdio.h>int main(){ int i; FILE *outFile; /* FILE declaration */ float price[] = {39.95,3.22,1.03}; /* a list of prices */ char *descrip[] = { "Batteries", /* a list of */

"Bulbs", /* descriptions */ "Fuses"};

outFile = fopen("prices.txt","w"); /* open the file */ if (outFile == NULL) { printf("\nFailed to open the file.\n"); exit(1); } for(i = 0; i < 3; ++i) fprintf(outFile,"%-9s %5.2f\n",descrip[i],price[i]);

fclose(outFile); return 0;}

system(“start notepad.exe prices.txt”); //to view file

Note File Size

Appending To a File• This makes an existing file available for data to be added to end of

file

• If the file opened for appending does not exist, a new file is created

• This new file is available to receive output from the program

• In append mode, data is written to the end of the file

• NB: in write mode, data is written starting at the beginning

outFile = fopen(“prices.bnd”,”a”);

Reading from a file

• Open the file and save the returned pointer• inFile = fopen(“sales.dat”,”r”);

• Check for existence of file if (inFile == NULL) ...

• requests to open a nonexistent file will return a NULL address

• Read the data using functions similar to scanf(), gets(), and getchar()

Bailing Out

• Errors should be communicated back to the OS By convention, this is done with exit(1) or exit(0)

• Errors are also typically reported back to the user

• Conventionally, exit will return a value of 0 for success a nonzero value to indicate failue

• exit calls fclose for each open output file

Closing a File

• Use fclose() to close the files you’ve opened

• There is a limit to the maximum number of files you can have open

• Use the same internal pointer name to close the file

• Open files are normally closed automatically by the operating system upon exit

Writing to Files

• These functions are almost identical to those used for displaying to a monitor

fputc(c, filename) Write a single character to a file

fputs(string, filename) Write a string to the file

fprintf(filename, “format”, args) Write the values of the arguments to the file according to the format string

fputc(‘z’, outFile); Write a ‘z’ to the file

fputs(“Nemo was a fish”, outFile) Write the string to the file

fprintf(outFile, “%s %d”, descrip, price)

B Smith:

Not clear here what’s a proto and what’s a function example!

B Smith:

Not clear here what’s a proto and what’s a function example!

Reading Files• Very similar to reading data from the keyboard

all detect EOF. fgetc() and fscanf() return EOF when the marker is detected fgets() returns a NULL when it detects the end of a file

fgetc(filename) Read a single character from the file

fgets(stringname,n, filename) Read n-1 characters from the file and store in stringname

fscanf(filename, “format”, &args) Read values for the listed arguments

fgetc(infile); // Read the next character from the file

fgets(message, 10, inFile) //Read file until: 9 characters, OR \n, OR EOF

fscanf(inFile, “%f”, &price) //Read a floating point number

Reading data from a file

#include <stdlib.h>#include <stdio.h>int main(){ char descrip[10]; float price; FILE *inFile;

inFile = fopen("prices.txt","r"); if (inFile == NULL) { printf("\nFailed to open the file.\n"); exit(1); }

while (fscanf(inFile,"%s %f",descrip,&price) != EOF){ printf("%-9s %5.2f\n",descrip,price); } fclose(inFile);

return 0;}

B Smith:

Not a great example since it uses the same field names! The more logical thing to do would be to read a new file with fields chosen by the user!

B Smith:

Not a great example since it uses the same field names! The more logical thing to do would be to read a new file with fields chosen by the user!