FILE HANDLING in C Language-2 Lecture 27

Lecture 27: FILE HANDLING in C Language

FILE HANDLING in C Language-2

Lecture 27


FILE pointer in C

• The statement:

FILE *fptr1, *fptr2 ;

declares that fptr1fptr1 and fptr2fptr2 are pointer type variables of type FILEFILE. They can contain the address of a file descriptors.


Opening FILE• The statement: FILE *fptr1;fptr1 = fopen( “d:\\mydata.txt", "r");

would open the file d:\\mydata.txtd:\\mydata.txt for input (reading).

• The statement: FILE *fptr2; fptr2 = fopen(“d:\\results.txt", "w"); would open the file d:\\d:\\results.txtresults.txt for output (writing).


Opening Text Files• The statement: FILE *fptr2; fptr2 = fopen (“d:\\results.txt",“r+");would open the file d:\\d:\\results.txtresults.txt for both reading and writing.

• Once the FILE is open, it stay open until you close it or end of the program reaches (which will close all files.)


Testing for Successful Open• If the FILE was not able to be opened, then the

value returned by the fopenfopen routine is NULL.

• For example, let's assume that the file d:\\d:\\mmydata.txtydata.txt does not exist. Then:

FILE *fptr1;fptr1 = fopen ( “d:\\mydata.txt", “W") ;if (fptr1 == NULL) cout<< "File 'mydata' can’t be open”;

else cout << “File opens successfully”;


Writing in FILE• We can write any contents (data) in

the FILE using the following FILING function:

fprintf (paramter1, paramter2, paramter3);

FILE pointer

Signature of the variable

Actual name of the variable


Writing in FILE (Example 1)

The fprintffprintf function will write value from the number1 (memory location) to hard disk location (“d:\\mydata.txt”).

void main (void){

FILE *fptr;fptr = fopen( “d:\\mydata.txt” , ”w” ); // write mod

int number1 = 93;fprintf (fptr,”%d”, number1);

}



The fprintffprintf function will write value from the number1 & floatData (memory location) to hard disk location (“d:\\myfile.dat”).

void main (void){

FILE *fptr;fptr = fopen( “d:\\myfile.dat” , ”w” ); // write mod

long number1 = 93;float floatData = 34.63;fprintf (fptr,”%d%f”, number1,floatData);

}



The fprintffprintf function will write value from the number1, floatData and myCharacter (memory location) to hard disk location (“d:\\mydata.txt”).

void main (void){

FILE *fptr;fptr = fopen(“d:\\myfile.dat”,”w”); // write mod

long number1 = 93;float floatData = 34.63;

char myCharacter = ‘D’;fprintf (fptr,”%d%f%c”, number1,floatData,myCharacter);

}



The fprintffprintf function will write value from the string 1 (memory location) to hard disk location (“d:\\mydata.txt”).

void main (void){


char string1 [40];strcpy (string1, “My text information is Pakistan”);fprintf (fptr,”%s”, string1);

}


Writing in FILE (problem with “w” MOD)

• What would be the final data in the FILE name myfile.data when the above code executes 3 times.• The final data would be only (353) not (353353353).• The reason is that, with “w” MOD, on every run the previous data is REMOVED, and the new data is written from beginning of FILE.

void main (void){


int data = 353;fprintf (fptr,”%d”, data);

}


Solution Open in Append MOD “a+” or “w+”

• In append MOD, the previous data in the FILE is not removed on every new execution.

• The data in the FILE name “d:\\myfile.data” on the 3 runs would be now (353353353).

void main (void){

FILE *fptr;fptr = fopen(“d:\\myfile.dat”,”a+”); // write mod

int data = 353;fprintf (fptr,”%d”, data);

}


OUTLINE• How we can read data/information

from the FILES.• How we can close the FILES.


Reading from FILE• We can write any contents (data) in

the FILE using the following FILING function:

returnParameter fscanf (paramter1, paramter2, paramter3);

FILE pointer

Signature of the variable

Actual name of the variableEnd of file

indicator


Reading from FILE (Example 1)

The fscanffscanf function will read value from the hard disk location (“d:\\mydata.txt”) to number1 (memory location).

void main (void){

FILE *fptr;fptr = fopen( “d:\\mydata.txt” , ”r+” ); // random mod

int number1 = 93, number2 = 0; fprintf (fptr,”%d”, number1); fscanf (fptr,”%d”, &number2);}


Reading from FILE (Example 2)void main (void){

FILE *fptr;fptr = fopen( “d:\\myfile.dat” , ”r+” ); // random mod

long number1 = 93, read1 = 0;float floatData = 34.63, read2 = 0;fprintf (fptr,”%d%f”, number1,floatData);fscanf (fptr,”%d%f”,&read1,&read2);

}



FILE *fptr;fptr = fopen(“d:\\myfile.dat”,”r+”); // random mod

long number1 = 93, read1 = 0;float floatData = 34.63, read2 = 0;

char myCharacter = ‘D’, read3 = ‘ ‘;fprintf (fptr,”%d%f%c”, number1,floatData,myCharacter);

fscanf (fptr,”%d%f%c”,&read1, &read2, &read3);

cout << “Value are” << read1 << read2 << read3;}



FILE *fptr;fptr = fopen( “d:\\myfile.dat” , ”r+” ); // random mod

char string1 [40], string2 [40];strcpy (string1, “My text information is Pakistan”);fprintf (fptr,”%s”, string1);

fscanf (fptr,”%s”,string2);cout << string2;

}

Lecture 27: FILE HANDLING in C LanguageReading byte by byte Information from FILE (Example 5)void main (void){ FILE *fptr; fptr = fopen( “d:\\myfile.dat” , ”w” ); // write mod char string1 [40]; strcpy (string1, “My text information is Pakistan”);

fprintf (fptr,”%s”, string1); } void main (void){ FILE *fptr;

fptr = fopen( “d:\\myfile.dat” , ”r” ); // read mod char character; while (1) { fscanf (fptr,”%c”,&character);

cout << character; }}

write.cpp

read.cpp

The problem with this code is that it can read out of the file data from your disk.Control it using ‘End of file’ marker.


End of file• The end-of-file indicator informs the

program when there are no more data (no more bytes) to be processed.

• There are a number of ways to test for the end-of-file condition. One way is to use the value returned by the fscanffscanf function:int istatus;istatus = fscanf (fptr1, "%d", &var) ;if ( istatus == EOF ){ cout << “End-of-file encountered.”;}

Lecture 27: FILE HANDLING in C LanguageReading byte by byte Information from FILE (Example 6)void main (void){ FILE *fptr; fptr = fopen( “d:\\myfile.dat” , ”r” ); // read mod char character; while (1) {

int status = 0;status = fscanf (fptr,”%c”,&character);

if ( status == EOF )break;

cout << character; }}

read.cpp


Closing FILES• The statements:

fclose ( fptr1 ) ;fclose ( fptr2 ) ;

will close the files and release the file descriptor space from memory.

FILE HANDLING in C Language-2 Lecture 27

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