How to read a file in Fortran?

@alford To read a file in Fortran, you can use the READ statement. Here is an example of how to use it:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

integer :: n, i
real :: x

open(unit=10, file='myfile.txt')

do i = 1, n
    read(10,*) x
    print *, x
end do

close(10)

In this example, we open the file myfile.txt and read it line by line until we reach the end of the file. The READ statement reads a single value from the file and stores it in the variable x, which is then printed to the screen.

To use the READ statement, you need to specify the unit number of the file you want to read from (in this example, it is 10), followed by the format of the data you are reading. In this case, the format is *, which means that the data will be read in free format (i.e., the values can be separated by any number of spaces or tabs).

After you have finished reading the file, you should close it using the CLOSE statement to free up the file handle. This will allow you to access the file again later if you need to.

@alford 

In addition to the explanation provided, here are a few more details and options for reading files in Fortran:

1. File Declaration: Before reading a file, you need to declare the unit number and the file name. In the example above, the unit number is declared as 10 and the file name is 'myfile.txt'. Make sure the specified file is present in the same directory as your Fortran program.
2. File Opening: Open the file using the OPEN statement before reading it. The OPEN statement specifies the unit number, the file name, and the file access mode. Here, the unit number is 10, and the access mode 'file='myfile.txt'' opens the file in default mode for reading, assuming the file already exists.
3. Loop through the File: Typically, you would use a loop to read data from a file until the end of the file is reached. In the example above, the loop iterates 'n' times, where 'n' is the total number of values in the file.
4. Read Statement: The READ statement is used to read values from the file. In the example, the statement 'read(10,) x' reads a single 'real' value from the file into the variable 'x'. The '' in the format indicates free format, meaning Fortran will read any valid representation of a 'real' value. If the file format is different, you need to specify the appropriate format specifier, such as 'i', 'e', or 'f', for integer, exponential, or fixed-point values, respectively.
5. Error Handling: It is important to handle errors while reading a file. Fortran provides the 'IOSTAT' variable that captures the status of the I/O operation. You can check this variable after the READ statement to ensure the read operation was successful or handle any errors that may occur.
6. File Closing: After reading the file, close it using the CLOSE statement. This releases the unit number, allowing it to be used for opening other files. In the example above, the command 'close(10)' closes the file associated with unit number 10.

Note that the code snippet provided is a simplified example. In practice, you may need to consider additional factors like error handling, handling different file formats, and reading different types of data structures such as arrays or character strings.