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https://github.com/beliavsky/fortran-with-chatgpt

Questions about Fortran answered by ChatGPT-4, reviewed by a human
https://github.com/beliavsky/fortran-with-chatgpt

chatgpt chatgpt-4 fortran fortran-tutorial modern-fortran

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Questions about Fortran answered by ChatGPT-4, reviewed by a human

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# Fortran with ChatGPT

This is a Fortran tutorial, with my questions, in bold, answered by ChatGPT-4. I add a **Note** when I think an answer needs to be amended, and I test the codes produced. When the initial code from ChatGPT does not compile, it is often able to fix the code when fed the error message from gfortran. ChatGPT is "chatty", so some of its answers have been abbreviated.



This page discusses compiling. For other topics see



[Variables and Order of Operations](./Variables.md)  

[Conditionals](./Conditionals.md)  

[Loops](./Loops.md)  

[Arrays](./Arrays.md)  

[Allocatable Arrays](./AllocatableArrays.md)  

[Modules and Functions](./Modules.md)  

[Subroutines](./Subroutines.md)



---

**Let's write a modern Fortran tutorial. Show a "Hello World" Fortran program with a comment line and explain how to compile and run the program on Linux and Windows.**



Here is a simple Fortran "Hello World" program:



```Fortran

! This is a simple Fortran program

program HelloWorld

    implicit none

    print*, 'Hello, World!'

end program HelloWorld

```



Let's break down this code:



* `! This is a simple Fortran program` is a comment line in Fortran. Any line starting with ! is considered a comment and is not processed by the compiler.

* `program HelloWorld` starts the definition of a program called HelloWorld.

* `implicit none` is a statement that tells the Fortran compiler to require explicit declaration of all variables. It's considered good practice to include this in all your Fortran programs to avoid possible problems with undeclared variables or misspelled variable names.

* `print*, 'Hello, World!'` is a statement that prints "Hello, World!" to the console.

* `end program HelloWorld` ends the program definition.



Now, to compile and run this program:



**Linux:**

1. Save the program in a file named `helloworld.f90`.

2. Open a terminal window.

3. Navigate to the directory containing `helloworld.f90` using the `cd` command.

4. Compile the program with the gfortran compiler like this:

```

gfortran -o helloworld helloworld.f90

```

Here, `gfortran` is the name of the compiler, `-o helloworld` specifies the output file name, and `helloworld.f90` is the source file.



5. If there are no errors in your program, the compiler will create an executable file named `helloworld`.

6. Run the program like this:

```

./helloworld

```

**Windows:**

1. Save the program in a file named `helloworld.f90`.

2. Open a command prompt window.

3. Navigate to the directory containing `helloworld.f90` using the `cd` command.

4. Compile the program with the gfortran compiler like this:

```

gfortran -o helloworld.exe helloworld.f90

```

Here, `gfortran` is the name of the compiler, `-o helloworld.exe` specifies the output file name, and `helloworld.f90` is the source file.



5. If there are no errors in your program, the compiler will create an executable file named `helloworld.exe`.

6. Run the program like this:

```

helloworld.exe

```



7. Please note that this assumes you have the gfortran compiler installed on your system. If it is not installed, you need to download and install it. There are many tutorials available online for how to do this.

8. Furthermore, the process to compile and run the program can differ based on the development environment or the IDE you are using. For example, in some IDEs, you can compile and run the program with a single click.



---

**On Windows, I installed gfortran, opened a command prompt, and issued the command 'gfortran helloworld.f90' but got**

```

'gfortran' is not recognized as an internal or external command,

operable program or batch file.

```



If you receive the error message "'gfortran' is not recognized as an internal or external command, operable program or batch file" when trying to compile a Fortran program with gfortran on Windows, it means that the `gfortran` executable is not in your system's PATH.



To resolve this issue, you can try one of the following:



1. Add the directory containing the `gfortran` executable to your system's PATH environment variable. This allows the system to locate the `gfortran` executable when you run it from any directory.



2. Use an integrated development environment (IDE) that provides a graphical interface for compiling and running Fortran programs, such as Code::Blocks or Microsoft Visual Studio. These IDEs typically handle the configuration of the compiler for you.



Make sure that you have installed gfortran correctly and that the installation directory is added to your system's PATH environment variable.



---

**How do I add gfortran to my system's PATH?**



To add gfortran to your system's PATH on Windows, you can follow these steps:



1. Open the Start menu and search for "Environment Variables" and select "Edit the system environment variables" (or "Environment Variables" if you are using an older version of Windows).



2. In the System Properties window, click the "Environment Variables" button.



3. In the Environment Variables window, locate the "Path" variable under the "System variables" section and select it.



4. Click the "Edit" button to modify the "Path" variable.



5. In the Edit Environment Variable window, click the "New" button and enter the directory path where gfortran is installed. Typically, it is something like `C:\MinGW\bin` or `C:\Program Files\mingw-w64\mingw64\bin`, depending on your installation.



6. Click "OK" to close all the windows.



After adding gfortran to the system's PATH, you should be able to run the `gfortran` command from any directory in the command prompt without specifying the full path to the executable.



Note that you may need to open a new command prompt window for the changes to take effect.



---

**Can I add it to my PATH from the command prompt?**



Yes, you can add gfortran to your PATH from the command prompt on Windows using the `setx` command. Follow these steps:



1. Open a command prompt window.



2. Determine the directory where gfortran is installed. For example, if it is installed in `C:\MinGW\bin`, use that as the installation directory.



3. Run the following command to add gfortran to your PATH:

```

setx PATH "%PATH%;C:\MinGW\bin"

```

Replace `C:\MinGW\bin` with the actual installation directory of gfortran on your system.



4. After running the command, you should see a message indicating that the operation was successful.



5. Close the command prompt window and open a new command prompt for the changes to take effect.



Once you have added gfortran to your PATH, you should be able to run the `gfortran` command from any directory in the command prompt without specifying the full path to the executable.



---

**What will be the name of the executable if you compile with `gfortran helloworld.f90` on Linux and on Windows?**



When you compile a Fortran source file `helloworld.f90` with `gfortran helloworld.f90` on Linux, the resulting executable will be named `a.out` by default, as mentioned earlier.



On Windows, the default name of the executable generated by gfortran is `a.exe`. This is the default name used by gfortran for the compiled executable when an explicit output name is not provided.



If you want to specify a custom name for the executable on either Linux or Windows, you can use the `-o` option followed by the desired name, as shown in the previous response.



---

**Why is the suffix .f90 used? Is it mandatory?**



The `.f90` suffix is commonly used for Fortran source files that use the free form source code layout introduced with Fortran 90. This is the layout that allows for flexibility with white space and line breaks compared to the older fixed form layout, which was quite restrictive.



The suffix is not mandatory, but it is recommended for the following reasons:



It helps to identify the file type quickly. If you see a file with a `.f90` extension, you can immediately identify it as a Fortran 90 or later source code file.



Some Fortran compilers use the file extension to determine the source form to expect. For example, with the gfortran compiler, a `.f` or `.for` extension indicates fixed-form source, whereas `.f90`, `.f95`, `.f03`, `.f08` indicate free-form source.



So, while not mandatory, it is generally a good idea to use the `.f90` extension (or the appropriate extension for your Fortran version) for your Fortran source files.



---

**How can you tell a compiler to treat a source file with the .f extension as free form source?**



In Fortran, the `.f` file extension typically indicates a fixed-form source file. To instruct the compiler to treat a file with the `.f` extension as free-form source code, you can use compiler-specific flags or extensions.



For example, with the GNU Fortran compiler (gfortran), you can use the `.f90` extension for free-form source code, and it will be automatically recognized as such. If you specifically want to treat a file with the `.f` extension as free-form, you can use the `-ffree-form` flag when compiling:

```

gfortran -ffree-form filename.f

```

Other Fortran compilers may have different flags or conventions for indicating free-form source code. It's recommended to consult the documentation or user guide of your specific compiler for the appropriate options.



Alternatively, you can consider renaming the file to have the `.f90` extension to ensure it is treated as free-form source code by default.



---

**How do I do arithmetic in Fortran with ordinary numbers, without defining any variables? What are the available operators?**



In Fortran, you can perform arithmetic operations without explicitly defining variables. You can directly use the values or literals in the arithmetic expressions. The available operators for arithmetic operations in Fortran with ordinary numbers are:



Addition: `+`



Subtraction: `-`



Multiplication: `*`



Division: `/`



Exponentiation: `**`



Here's an example that demonstrates the use of these operators without explicitly defining variables:

```Fortran

program arithmetic_example

    implicit none

    integer :: result

    

    result = 2 + 3    ! Addition

    print*, "2 + 3 =", result

    

    result = 5 - 2    ! Subtraction

    print*, "5 - 2 =", result

    

    result = 4 * 3    ! Multiplication

    print*, "4 * 3 =", result

    

    result = 10 / 3   ! Division

    print*, "10 / 3 =", result

    

    result = 2 ** 4   ! Exponentiation

    print*, "2 ** 4 =", result

end program arithmetic_example

```

The output of this program will be:

```

2 + 3 = 5

5 - 2 = 3

4 * 3 = 12

10 / 3 = 3

2 ** 4 = 16

```