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Introduction File Declaration and Initialization Creating and Opening File Closing File EOF Reading from and Writing into a File Extra : Random Access Files 2

Almost all programs developed before is interactive, where input is entered via keyboard and output is via screen or monitor. This type of processing is not suitable if it involves huge amount of input or output to be entered or displayed on the screen at one time. Therefore, file processing can solve the problem mentioned. 3

Storage of data in variables and arrays is temporary – all data are lost when a program terminates. Files are used for permanent retention of large amounts of data. A file is a group of related records. * record – is a group of related fields. 4

To implement file processing in C, it is advisable to include #include in your program. To use file for input and output, a file pointer variable has to be declared. FILE *in_file; => in_file is a pointer to a FILE structure FILE *out_file; => out_file is a pointer to a FILE structure in_file and out_file are also known as internal file names 5

File pointer initialization has the following format : internal_filename =fopen(external_filename, mode); For example, to declare and initialize the file pointer variables in_file and out_file : FILE *in_file; FILE *out_file; in_file = fopen (“c:data.txt”, “r”); out_file = fopen (“c:results.txt”, “w”); 6 modeexternal filenameinternal filename

Format: internal_filename =fopen(external_filename, mode); Each file must be opened before it can be accessed or processed. When opening a file, external file name needs to be related to the internal file name using fopen function. fopen is the stdio library function used to open or create a file. Internal file name is the name that the C system uses to identify a file among others that a program might process. External file name is the name given at “save file as” outside the program e.g. “student.dat”, “records.out”, “data.txt”, etc. Mode is to indicate the process to be made onto a file. 7

Basics mode are: “r” : open file to read “w” : open file to write “a” : append data to the end of an already existing file “r+” : open and create file for update, i.e. read and write, does not overwrite previous contents “w+” : open and create file for update, overwrite “a+” : append, open or create file for update 8

There is a possibility of a file fails to open. Could be the particular file does not exist. Therefore, need to check or verify whether the file is successfully opened. If file fails to open, need to stop the program, use exit(- 1). A file pointer whose value equals to NULL(empty or ‘0’) is called a null pointer. if (in_file == NULL) {printf(“\nFile fails to open\n”); exit(-1); } 9

You can also combine file initialization and file opening verification, using statement: if ((in_file = fopen(“student.dat”, “r”)) == NULL) { printf(“\nFile fails to open\n”); exit(-1); } * NULL = empty or ‘0’ 10

Each opened file needs to be closed. Format: fclose(internal_filename); Examples: fclose(in_file); fclose(out_file); 11

Usually you don’t know how many data you want to read from file. Therefore, need to check whether you have reached end of file. End-of-file (EOF) character marks the end of the entire file. Function feof is used to detect EOF. Format: feof(internal_filename) 12

FILE *in_file; in_file = fopen(“student.dat”, “r”); if(in_file == NULL) { printf(“Error opening file\n”); exit(-1); } while(!feof(in_file)) { //statements to process data } fclose(in_file); 13

Format: fscanf (internal file name, format control string, input list); fscanf(in_file, “%d”, &iMarks); fgetc (internal file name); cCh = fgetc(in_file); ▪ fgets (string variable, size, internal file name); fgets(acName, 10, in_file); 14

Format: fprintf (internal file name, format control string, output list); fprintf(out_file, “%d”, iMarks); fputc (character expression, internal file name); fputc(cCh, out_file); fputc(“4”, out_file); fputs (string expression, internal file name); fputs(acName, out_file); fputs(“Jane”, out_file); 15

#include int main(void) { FILE *in_file; FILE *out_file; int iMarks, iTotal = 0, iCount = 0; float fAvg; in_file = fopen("student.dat", "r"); out_file= fopen("student.out", "w"); if(in_file == NULL) { printf("Error opening file\n"); exit(-1); } 16 while(!feof(in_file)) { fscanf(in_file,"%d",&iMarks); ++iCount; iTotal = iTotal + iMarks; fprintf(out_file, " %d ",iMarks); } fAvg = iTotal /iCount; fprintf(out_file, "\n%.2f\n", fAvg); fclose(in_file); fclose(out_file); return 0; }

student.dat student.out Input file name Data in input file Output file name Display data in output file

In sequential access file, records in a file created with the formatted output function fprintf are not necessarily the same length. Individual records of a random access file are normally fixed in length. This record can be accessed directly without searching through other records. Thus, file searching process will be faster. Random access is suitable to be used in large database systems such as in airline reservation systems, banking systems and other kind of transaction processing systems. 18

Because every record in randomly access file normally fixed in length, data can be inserted in random access file without destroying other data. Data stored previously can also be updated or deleted without rewriting the entire file. 19

Function fwrite : to transfer a specified numbers of byte beginning at a specified location in memory into a file. The data is written beginning at the location in the file indicated by the file position pointer. Function fread : transfers a specified number of bytes from the file specified by the file position to an area in memory with a specified address. 20

When writing an integer, instead of using fprintf(fPtr, “%d”, iNumber); which could print as few as 1 digit or as many as 11 digits, we can use fwrite(&iNumber, sizeof(int), 1, fPtr); which always writes 4 bytes from variable iNumber to the file represented by fPtr. 21

fread is used to read 4 bytes integer into variable number. The fread and fwrite functions are capable of reading and writing arrays of data to and from a disk. The third argument in the fread and fwrite is the number of element in array that should be read from disk or written to disk. The preceding fwrite function call, writes a single integer to disk, so third argument is 1. File processing program rarely writes a single field to a file. Normally, we write one struct at a time. 22

#include struct clientData { int iAcctNum; char acLastName[15]; char acFirstName[15]; float fBalance; }; int main() { int iIndex; struct clientData sBlankClient = {0, “ “, “ “, 0.0}; FILE *cfPtr; if((cfPtr = fopen(“credit.txt”, “w”)) = = NULL) printf(“file cant be open”); else{ for (iIndex= 1; iIndex<=100; iIndex++) fwrite(&sBlankClient, sizeof(struct ClientData), 1, cfPtr); fclose(cfPtr); } return 0; } 23 This program shows how to open a randomly access file, define a record format using structure, write a data to disk, and close the file. This program initializes all 100 records of a file “credit.txt” with empty struct using function fwrite

#include struct clientData { int iAcctNum; char acLastName[15]; char acFirstName[15]; float bBalance; }; int main () { FILE *cfPtr; struct clientData sClient; if ((cfPtr = fopen(“credit.txt”, “r+”))==NULL) printf(“file cant be open”); else { print(“Enter account number(0 to end input): ”); scanf(“%d”, &sClient.iAcctNum); 24 while (sClient.iAcctNum != 0) { printf(“Enter lastname, firstname, balance”); scanf(“%s %s %f, &sClient.acLastName, &sClient.acFirstName, &sClient.fBalance); fseek(cfPtr, (sClient.iAcctNum – 1) * sizeof(struct clientData), SEEK_SET); fwrite(&sClient, sizeof(struct clientData), 1, cfPtr); printf(“Enter account number”); scanf(“%d”, &sClient.iAcctNum); } //end of while statements } //end of else statements fclose(cfPtr); return 0; } //end of main

Output: 25 Enter account number (0 to end) ? 29 Enter lastname, firstname, balance ?Brown Nancy Enter account number (0 to end) ? 30 Enter lastname, firstname, balance ?Dunn Stacy Enter account number (0 to end) ? 31 Enter lastname, firstname, balance ?Barker Doug 0.00 Enter account number (0 to end) ? 0

The statement fseek(cfPtr,(sClient.iAcctNum–1) *sizeof(struct clientData),SEEK_SET); positions the file position pointer for the file reference by cfPtr to the byte location calculated by (iAccountNum-1)*sizeof(struct clientData); Because of the account number is between 1 to 100 but the byte positioning starts from 0, the account number needs to be subtracted with 1 (minus 1). 26

#include struct clientData { int iAcctNum; char acLastName[15]; char acFirstName[15]; float fBalance; }; int main () { FILE *cfPtr; struct clientData sClient; if((cfPtr = fopen(“credit.txt”, “r”)) = = NULL) printf(“file cant be open”); else{ printf(“%-6s%-16s%-11s%10s\n”, “Acct”, “Last Name”, “ First Name”, “Balance”); while (!feof(cfPtr)) { fread(&sClient, sizeof(struct clientData), 1, cfPtr); if (sClient.iAcctNum != 0) printf(“(“%-6s%-16s%-11s%10.2f\n”,”sClient.iAcctNum, sClient.acLastName, sClient.acFirstName, sClient.fBalance); }} fclose (cfPtr); return 0; } 27

Output: 28 AcctLast NameFirst NameBalance 29 BrownNancy DunnStacey BarkerDoug0.00 fread(&sClient, sizeof(struct clientData), 1, cfPtr); Reads the number of bytes determined by sizeof(struct clientData) from the file reference by cfPtr and stores the data in the structure sClient.

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