Structured Programming II Text Files CHAPTER 2 Structured Programming II

Why we need Files Files are kept in Secondary Storage Units (Disks, Diskettes). Arrays use Primary Storage Units (RAM) of the computer. When you have to make a decision to use either a File or an Array you have to consider the following criterias: Speed of access to the data stored in. Unit Price of the Storage. Durability of the content. Amount of data have to be stored.

Why we need Files

Using Files in C Large amount of data are usually stored in files. Obtaining data (numbers or characters) from a file is called READING THE FILE. Placing data into a file is called WRITING TO THE FILE. PROCESSING A FILE, means obtaining data from a file and/or placing data into another. In order to process a file in C, you must first declare the FILE POINTER.

Declaration of a File Format : FILE * file_pointer_name; The second step is the OPENING of the declared file. File_pointer_name = fopen( file_name, Opening_mode); Example : my_file = fopen(“MYFILE.TXT”, “r+”);

File Opening Modes Op. Mode: Meaning: “r” : Read only obtain data from an existing file. “w” : Write only, place data onto a new file. “a” : Append only, appends data to the end of a new or existing file. “r+” : Read and Write “w+” : Write and Read “a+” : Append and read Once program completes its processing with a file, it has to close the used file with fclose() statement. Example : flose(input_file); /* “input_file” is a File Pointer */

“fopen()” Function fopen() takes two arguments: file name. opening mode. fopen returns a NULL pointer when it fails to open a file. Use the following code in order to test the file is opened successfully or not: Example : input_file = fopen(“Myfile.txt”,”r”); if (input_file==NULL) { printf(“\n An Error occurred, Myfile.txt can not opened !”); exit(1); } print(“\n Myfile.txt is opened successfully “);

“fscanf()” Function fscanf() is used to read data from a file. fscanf() function is similar with scanf() function, but needs a file pointer as the first argument. While scanf() is reading data from keyboard, fscanf() is used for reading data from a file. Example : fscanf(input_file,”%d”,&num); sum+= num;

Examples Example 1: Write a program in C that will generate random 100 integer numbers between 0-500, and will write them into the file “SALES.DAT”. Example 2: Write a program in C that will read the sales data from the file “SALES.DAT” (containing integer values) and calculate tax amounts ( tax rate is given 25%) and write them into the file “TAX.DAT”. Example 3: Write a program in C, that will read both “SALES.DAT” and “TAX.DAT” to calculate total sales and total tax to print on the monitor.

Examples Example 1: # include <stdio.h> #include <stdlib.h> void main() { FILE *out_file; int cnt; out_file = fopen(“SALES.DAT”,”w”); randomize(); for (cnt=0; cnt<50; cnt++) fprintf(out_file,“\n%d”,rand()%501); } fclose(out_file); } /* end of main */

Examples Example 2: # include <stdio.h> void main() { int sale_amo; FILE *in_file,*out_file; in_file = fopen(“SALES.DAT”,”r”); out_file = fopen(“TAX.DAT”,”w”); fscanf(in_file,”%d”,&sale_amo); while (sale_amo<>EOF) { fprintf(out_file,“\n%f”,sale_amo * 0.25); } fclose(in_file); fclose(out_file); } /* end of main */

Examples Example 3: # include <stdio.h> void main() { int tot_sales = 0, sale; float tot_tax = 0, tax; FILE *sale_file,*tax_file; sale_file = fopen(“SALES.DAT”,”r”); tax_file = fopen(“TAX.DAT”,”r”); fscanf(sale_file,”%d”,&sale); fscanf(tax_file,”%f”,&tax); while (sale<>EOF) { tot_sale += sale; } /* program is continuing on the next slide */

Examples Example 3: /* The beginning of the program is on the previous slide */ while (tax<>EOF) { fscanf(tax_file,”%f”,&tax); tot_tax += tax; } printf(“\n The total Sales are %d “,tot_sale); printf(“\n The total Taxes are %f “,tot_tax); fclose(sale_file); fclose(tax_file); } /* end of main */