c++ read and write TXT file collation method

As shown below:

#include "stdafx.h"
#include <iostream> 
// Include both read and write <fstream> The header file 
#include <fstream> 
#include <iomanip> 
using namespace std;
 
int main()
{
	//ifstream Flow from file to memory ifstream Represents a file input stream, meaning a file read operation 
	ifstream myfile("c://a.txt");
	//ofstream Flow from memory to files ofstream Represents a file output stream, meaning a write file operation 
	ofstream outfile("c://b.txt");
 
	fstream fin1("file1.txt", ios::in);
 
 
 
	/*********************************************************************************************************/
	// Examples of reading files 
	// If the file does not exist (in visual stdio 2017 In the environment, the file exists .vcxproj In the file folder) 
	if (!fin1) {
 
		// tried 1 Next, use cout That's fine. Now I don't understand why cerr Why is the standard library defined cerr This object 
		cerr << " Read failure " << endl;
		system("pause");
		return -1;
	}
	else {
		cout << " Read the success " << endl;
		// Note that the document is not empty 
		if (outfile.is_open())
		{
			char linestring[100];
 
			//memset Function in the socket Is used to empty arrays . Such as : The prototype is memset(buffer, 0, sizeof(buffer))
			//memset(linestring, 0, 100);
 
			//file.good() An error occurs while a file is being read or written, or the file is returned only after it has been read false;
			//eof At the end of the read 1 After the data, it's still zero False ", when trying to read again 1 Of the data, 
			// Because I found no data to read, I knew I was at the end, and then I changed the flag. eof into True
			if (myfile.good() && !myfile.eof()) {
 
				//  read info.txt the 1 Ok, deposit linestring
				myfile.getline(linestring, 100);
 
				// The character 1 a 1 A processing until encountered '/0' So far,  
				for (int i = 0; linestring[i] != 0; i++)  
					// through ASCII Code to ensure that the input character is a character  
					if (linestring[i] >= 65 && linestring[i] <= 90 || linestring[i] >= 97 && linestring[i] <= 122) { 
						// Saves alphabetic characters to a disk file  
						outfile.put(linestring[i]);
						cout << linestring[i] << "";
					}
				cout << endl;
				outfile.close();
			}			
		}	
	}
 
 
	/*********************************************************************************************************/
	// Examples of writing files 
	char ch;
	// Open the file as input  
	ifstream infile("f1.dat", ios::in); 
 
	// If the file doesn't exist 
	if (!infile) {
		cerr << "open error!" << endl;
		exit(1);
	}
	// Define output stream f3.dat file  
	ofstream outfile("f3.dat"); 
	if (!outfile) {
		cerr << "open error!" << endl;
		exit(1);
	}
	// When the read character is successful   
	while (infile.get(ch)) {
		if (ch <= 122 && ch >= 97)
			ch = ch - 32;
		outfile.put(ch);
		cout << ch;
	}
	cout << endl;
	infile.close();
	outfile.close();
 
	system("pause");
 
	return 0;
}