Lecture 24

File I/O

Reading

p500-506, 506-516

Alternate way to execute programs

1. Locate your executable file: It may be in your Visual Studio Projects\project_name\Debug folder. (in may be in other name, you can do a search to find out where it is).
2. using window explorer, locate the folder and click on the executable file.
Or
1. Locate your executable file: It may be in your Visual Studio Projects\project_name\Debug folder.
2. Run DOS prompt by clicking on DOS command. Change to the folder where the executable file locates.
3. You can see the executable file.
You can type programname.exe to execute the program.
Here is the code

 #include<iostream>
using namespace std;

int main() {
    cout << "Hello World.\n";

    // the following few lines are useless
    // it mainly used to prevent the closure of DOS window
    char ch;
    cout << "Press any key to stop the program. ";
    cin >> ch; 

    return 0;
}

File

Write to a file

Example

#include<iostream>
      #include<fstream> // include the library
using namespace std;

int main() {
    ofstream outStream; // define an output file stream
    outStream.open("hello.txt"); // open a file named hello.txt
    outStream << "Hello World"; // write to the file
    outStream.close(); // close the output file stream
    return 0;
}

Output

If you run the program in Visual .net studio, then the a file named hello.txt will appear under folder Visual Studio Projects\project_name.
If you run the program using the method described above, the file hello.txt will appear in the folder contains the executable file. i.e. under folder Visual Studio Projects\project_name\Debug.
The content of the file hello.txt is
```
Hello World
```

Explanation

#include<fstrea>: file stream library.
ofstream outStream; declares an ofstream (output file stream) named outStream.
outStream.open("hello.txt") opens a file named hello.txt for writing.
outStream << "Hello World". Write Hello World to the file. Compared to cout << "Hello World".
outStream.close(): close the stream.

Read from a file

Example

#include<iostream>
#include<fstream> // include the library
using namespace std;

int main() {
    ifstream inStream; // declare an ifstream    
    inStream.open("number.txt"); // open file number.txt for reading

    int a;
    inStream >> a; // read from the file
    cout << a << endl;
 
    inStream.close(); // close the stream
    return 0;
    
}

Name the following file number.txt

Where to put the file?

In Visual .net studio, right click on the Resource files (it should be on your right hand side). Choose add, then add new items, then choose Text file .txt icon. Gives it a name (number.txt). Then type in 1999. Compile and execute the program.

If you execute the program by using the method described at the beginning of this lecture , save the file in the folder contains the executable file, i.e. Visual Studio Projects\project_name.
You may need to add these few lines to the program if you use this method
```
char ch;
cout << "Press any key to stop the program "
cin >> ch;
```

Output

Explanation

ifstream inStream; declare an ifstream (input file stream) named inStream.
inStream.open("number.txt"): open the file number.txt.
inStream >> a, read an integer and stored into the variable a. Compared to cin >> a
inStream.close(): close the stream.

One more example

#include<iostream>
#include<fstream> // include the library
    using namespace std;
      
    int main() {
    int size = 6; // assume we know that there only 6 students
      
      
    ifstream inStream; // declare an ifstream
    inStream.open("student.txt"); // open file student.txt for reading 
    
    ofstream outStream; // declare an ofstream
    outStream.open("female.txt"); // open file female.txt 
    
    char name[20];
    char gender;
     
    for(int i = 0; i < size; i++) {
        inStream >> name; // read the name
        inStream >> gender; // read the gender

        if(gender == 'F') {
            outStream << name << endl; // write the names to the file
        }
    }

    inStream.close(); // close the stream
    outStream.close(); // close the stream
    
    return 0;
}

Here is the file named student.txt

Betty F
Eric M
Lori F
Charles M
Mary F
Ken M

Here is the output named female.txt

Betty
Lori
Mary