Guide To Learn
Including a pure attribute in your function and subroutine statements forces you to write side effect-free code. This has two principal benefits: Tip Write pure procedures whenever possible. As I’ll show you later in the book, using the pure attribute can get you a long way toward functional programming with Fortran.
A pure procedure, while advantageous from both program design and compiler optimization perspectives, does come with a number of restrictions: There are several more restrictions on pure procedures that are more situational and that you’re less likely to encounter. We’ll revisit this topic later in the book as we encounter these edge cases.
A Fortran procedure is pure when it doesn’t cause any observable side effects, such as I/O or modifying the value of a variable declared outside of the procedure. To define a procedure as pure, simply add the pure attribute to its function or subroutine statement, as shown in the following listing. Listing 3.17 Defining a pure, side effect-free function ❶ The pure attribute asserts that the function […]
Fortran lets you define a function or a subroutine in a way that prevents side effects. What exactly do I mean here by side effects? For a concrete example, consider the traffic grid in a major city. Roadwork on a busy road during rush hour will soon slow down the incoming traffic, causing a traffic jam […]
Recall the initialization part of our tsunami simulator from listing 2.13, which is repeated in the following listing for your convenience, where we set the initial conditions. Listing 3.15 Initializing the tsunami simulator ❶ The index at which the water perturbation will be centered ❷ The parameter that governs the width of the perturbation ❸ Sets the array […]
Whenever I write a new procedure, I use the rules of thumb shown in figure 3.8 to decide whether to make it a function or a subroutine. Figure 3.8 Deciding when to use a subroutine over a function This is a simple decision-making process that you can follow. If you know your procedure will cause […]
Let’s see the difference between a subroutine and a function in an example. The following listing defines a subroutine add that’s equivalent to our function sum from the previous subsection. Listing 3.13 A subroutine that calculates the sum of two integers ❶ Inputs arguments like before ❷ Outputs argument that’s returned to the caller Here, a and b are input arguments–notice the intent(in) attribute just like in the sum function–and res is the […]
I mentioned earlier that Fortran has two kinds of procedures: functions and subroutines. Many rules that we covered for functions apply to subroutines as well. They’re both designed to be reused many times, and both may have input and output arguments. Unlike functions, subroutines can’t be used in expressions and can only be invoked in a […]
You now understand how to define a function and how to call it from the main program. Finally, we get to the fun part–applying our new knowledge about functions to refactor our tsunami simulator. Let’s look back at the main time loop in our program, as reprised in the following listing. Listing 3.9 The time […]
Let’s jump straight into it and write our first custom function. We’ll take our cold front program from listing 3.2 and rewrite it to delegate the temperature calculation to an external function. This will allow us to easily compute the solution for a series of different input values. Specifically, we’ll iterate over several values of […]