Files

1. Files EKT120: Computer Programming

2. Outline • Introduction • File Declaration and Initialization • Creating and Opening File • Closing File • EOF • Reading from and Writing into a File • Extra : Random Access Files EKT120: Computer Programming

3. Introduction • Almost all programs developed before, are interactive. • input is entered via keyboard • output is via screen or monitor. • not suitable if • it involves huge amount of input • output to be entered or displayed on the screen at one time. • Storage of data in variables and arrays is temporary – all data are lost when a program terminates. EKT120: Computer Programming

4. Solution: file processing EKT120: Computer Programming

5. What is a FILE? • A file is a group of related records. * record – is a group of related fields. FILE RECORD RECORD Field Field Field RECORD Field EKT120: Computer Programming

6. Files are used for permanent retention of large amounts of data. EKT120: Computer Programming

7. Steps of Using a File EKT120: Computer Programming

8. File Declaration • Use#include <stdlib.h> in your program. • Declare a file pointer variable. 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_fileandout_file are also known as internal file names. EKT120: Computer Programming

9. Initializing & Opening a file • Before accessing a file, it MUST BE OPENED. Format: in_filename = fopen (ext_filename, mode); A function to open/create file - Is in stdio.h library The process to be used on the file Name used inside a C Program to identify a file Actual Name of the file used EKT120: Computer Programming

10. Examples of Opening Files declare and initialize the file pointer variables in_file, out_file and cPtr: FILE *in_file; FILE *out_file; FILE *cPtr; in_file = fopen (“c:data.txt”, “r”); out_file = fopen (“c:results.txt”, “w”); cPtr = fopen (“myFile.dat”, “a”); EKT120: Computer Programming

11. File Modes • 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 EKT120: Computer Programming

12. Verifying File Opening • Check whether a file has successfully open. • It may fail if the particular file does not exist  fopen() will return a NULL. • Use exit(-1) to stop the program if (in_file == NULL) { printf(“\nFile fails to open\n”); exit(-1); } EKT120: Computer Programming

13. 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’ EKT120: Computer Programming

14. Closing File and fclose function • Each opened file needs to be closed. • Format: fclose(internal_filename); Examples: fclose(in_file); fclose(out_file); EKT120: Computer Programming

15. End-of-File (EOF) and feof function • 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) EKT120: Computer Programming

16. Example for EOF and feof function 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); EKT120: Computer Programming

17. Reading Data from a Text File • 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); EKT120: Computer Programming

18. Writing Data to a Text File • 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); EKT120: Computer Programming

19. Example Assume: int iA = 100, iB= 400; char cName[30] = “Marion Nasir”; float fX = 34.59; Q: Write iA, iB & cName into the file cPtr. fprintf (cPtr, “\n%d %d %s”, iA, iB, cName); EKT120: Computer Programming

20. Example 2 Q: Read the book title and year from the user, and then write it in a file cPtr. scanf (“%s %d”, &title, &bYear); fprintf (cPtr, “\n %s %d”, title, bYear); EKT120: Computer Programming

21. Example 3 Q: Read the cost of an item from the user, and then write it in a file, outFile. scanf (“%f”, &cost); fprintf (outFile, “\n %.2f”, cost); EKT120: Computer Programming

22. Example 4 Q: Read the book title and year from the the file cPtr. fscanf(cPtr, “%s %d”, &title, &bYear); EKT120: Computer Programming

23. Sample Program 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; } #include <stdio.h> #include <stdlib.h> 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); } EKT120: Computer Programming

24. Sample Input File and Output File 50 60 70 80 90 44 55 66 77 88 24 56 79 50 77 Input file name student.dat Data in input file Output file name student.out Display data in output file 50 60 70 80 90 44 55 66 77 88 24 56 79 50 77 64.00 EKT120: Computer Programming

25. Random Access Files • 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. EKT120: Computer Programming

26. Random Access File • 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. EKT120: Computer Programming

27. Creating a Randomly Accessed File • Function fwrite is used 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. EKT120: Computer Programming

28. Creating a Randomly Accessed File • 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. EKT120: Computer Programming

29. Creating a Randomly Accessed File • fread is used to read 4 bytes integer into variable number. • The fread and fwritefunctions 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. EKT120: Computer Programming

30. Creating a Randomly Accessed File – Example 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 <stdio.h> 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; }

31. Writing Data Randomly to a Randomly Accessed File #include <stdio.h> #include <stdlib.h> 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); 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(structclientData), SEEK_SET); fwrite(&sClient, sizeof(structclientData), 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

32. Writing Data Randomly to a Randomly Accessed File • Output: Enter account number (0 to end) ? 29 Enter lastname, firstname, balance ?Brown Nancy -24.54 Enter account number (0 to end) ? 30 Enter lastname, firstname, balance ?Dunn Stacy 314.33 Enter account number (0 to end) ? 31 Enter lastname, firstname, balance ?Barker Doug 0.00 Enter account number (0 to end) ? 0 EKT120: Computer Programming

33. Writing Data Randomly to a Randomly Accessed File • 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). EKT120: Computer Programming

34. Reading Data Randomly from a Randomly Accessed File #include <stdio.h> 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; }

35. Reading Data Randomly from a Randomly Accessed File • Output: • Acct Last Name First Name Balance • Brown Nancy -24.54 • 30 Dunn Stacey 314.33 • 31 Barker Doug 0.00 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. EKT120: Computer Programming

36. THANK YOU EKT120: Computer Programming