Read from file in cpp

The opening of the file is done using ofstream or fstream object of the file. Hence, we need to specify the mode of file opening along with the file name. But still it is considered to be a good practice to close the file after the desired operation is performed.

In order to read or write the file, we need to first open the file using the open function and define its mode. In order to release all the resources and free the allocated memory close function is used. We have used the if and else statement to check whether the file is [resent or not.

If the file is not found, a proper message is displayed on the console. In order to release all the resources and free the memory, the close function is used. Find centralized, trusted content and collaborate around the technologies you use most. Connect and share knowledge within a single location that is structured and easy to search. Using ifstream, I am able to open the text file and assign first number to a variable, but I don't know how to read the next number after the spaces.

It can depend, especially on whether your file will have the same number of items on each row or not. If it will, then you probably want a 2D matrix class of some sort, usually something like this:. Note that as it's written, this assumes you know the size you'll need up-front.

That can be avoided, but the code gets a little larger and more complex. In any case, to read the numbers and maintain the original structure, you'd typically read a line at a time into a string, then use a stringstream to read numbers from the line. This lets you store the data from each line into a separate row in your array.

If you don't know the size ahead of time or especially if different rows might not all contain the same number of numbers:. This does impose some overhead, but if different rows may have different sizes, it's an easy way to do the job. The input operator for number skips leading whitespace, so you can just read the number in a loop:.

But if you want to write into the same one, you could try with this:. You can use a 2D vector for storing the numbers that you read from the text file as shown below:. The output of the above program can be seen here. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?

Collectives on Stack Overflow. You must also include the iostream header file. Before performing any operation on a file, you must first open it. If you need to write to the file, open it using fstream or ofstream objects.

If you only need to read from the file, open it using the ifstream object. The three objects, that is, fstream, ofstream, and ifstream, have the open function defined in them. The function takes this syntax:. Code Explanation:. The fstream, ofstream, and ifstream objects have the close function for closing files.

Arithmetic Assignment Comparison Logical. Boolean Values Boolean Expressions. Create References Memory Address. Create Pointers Dereferencing Modify Pointers. Class Description ofstream Creates and writes to files ifstream Reads from files fstream A combination of ofstream and ifstream: creates, reads, and writes to files.