C++ open files using open()- programmingknow

C++ open files using open()

As started earlier, the function open() can be used to open multiple files that use the same stream object. for example we may want to process a set of files sequentially. in such cases, we may create a single stream object and use it to open each in turn.

This is done as follows

file-stream-class stream-object;
stream-object.open(“filename”);

Example
ofstream outfile; // create stream(for output)
outfile.open(“DATA1”); // connect stream to DATA1
……..
……..
outfile.close(); // disconnect stream from DATA1
outfile.open(“DATA2”); // connect stream to DATA2
…….
…….
outfile.close(); // disconnect stream from DATA2
….
…..

The above example of program segment opens two files in sequence for writing the data. Note that the first file should be closed before opening the second one. This is necessary because a stream can be connected to only one file at a time see in the below program

WORKING WITH MULTIPLES FILES
// creating files with open() function
#include <iostream> #include <fstream> int main() { std::ofstream fout; //create the output stream fout.open("country"); // connect "country" to it fout<<"United states \n"; fout<<"united kingdom \n"; fout<<"south korea \n"; fout.close(); // disconnect the country fout.open("capital"); // and connect the capital fout<<"washington \n"; fout<<"London \n"; fout<<"seoul \n"; fout.close(); // disconnect the "capital" // reading the files const int n=80; // size of line char line[n]; std::ifstream fin; // create input stream fin.open("country"); // connect country to it std::cout << "the contents of country file \n"; while (fin) // check end of file fin.getline(line,n); // ready a line std::cout << line; // display it fin.close(); // disconnect country fin.open("capital"); // and connect to capital std::cout << "\n contents of capital file \n"; while(fin) { fin.getline(line,n); std::cout << line; } fin.close(); }

output:
Contents of country file
United states
united kingdom
south korea
contents of capital file
washington
london
seoul

two file stream working on one file picture

At times we may require to use two or more files simultaneously. for example we may require to merge to sorted files into a third sorted file. This means both the sorted files have to be kept open for reading and the third on kept open for writing. In such cases, we need to create two separate input streams for handling the two input files and one output stream for handling the output file.

READING FROM TWO FILES SIMULTANEOUSLY
// Reads the files created in program
#include < iostream> #include <fstream> #include <stdlib.h> int main() { const int SIZE=80; char line[SIZE]; std::ifstream fin1,fin2; // create two input streams fin1.open("country"); fin2.open("capital"); for (int i=1;i<=10;i++) { if (fin1.eof()!=0) { std::cout << "Exit from country \n"; exit (1); } fin1.getline(line,SIZE); std::cout << "capital of " << line; if(fin2.eof()!=0) { std::cout << "Exit from capital \n"; exit (1); } fin2.getline(line,SIZE); std::cout << line << "\n"; } }