Scientific computing

(for the rest of us)

Pseudo-code

To facilitate the transition between diagram and code, one important step is to write pseudo-code, i.e. text that looks reasonably like code, but is not. This pseudo-code will not help the computer think about our problem, but it might help us think about the problem in ways that will make the actual programming easier.

Pseudo-code is mostly useful when it comes to working on a single function. The overall structure of the project can be done as a flowchart, and then each function can be written as pseudo-code. This is, in fact, a really good exercise to try with your own projects.

Let us try with an example - we want to sort numbers. We will write pseudo-code for a very, very inefficient algorithm:

let X be a list of numbers
let Y be an empty list of numbers

as long as X still has elements in it
    let x be the minimum of X
    add x to Y
    remove x from X

return Y

This is, essentially, what pseudo-code is: a way to explain in your own words what the function should do. This can be translated line-by-line into code, but this is rarely a good idea. The best way to represent an idea with code is rarely the same as the best way to represent it as pseudo-code.

Yet there are important lessons in this piece of pseudo-code. First, we know that we will need two variables (X and Y), a way to find x which is going to be the smallest value in X, and ways to add and remove elements from lists. Although this is not a full program yet, this is helpful to guide our search for information in, e.g., Julia’s documentation! We will also need to perform a series of steps as long as some condition is satisfied, which means that we will have to think about iteration and conditionals and all that.

The purpose of this material is to give you the basic bits of information to write this function, and much more!

To give you a little taste, we can write the Julia code:

X = rand(1:5, 12)
Y = empty(X)

while !(isempty(X))
    x, i = findmin(X)
    push!(Y, x)
    deleteat!(X, i)
end

Y'

1×12 adjoint(::Vector{Int64}) with eltype Int64:
 1  2  2  3  3  4  4  4  4  4  5  5
At this point, it is absolutely normal to not understand the code. Especially if you have little programming experience! After completing the first two or three sections of this material, you should be able to revisit it and understand what is going on, and even suggest an improvement to it! This will take time, and we will go through all the steps.

Pseudo-code is also useful to reason about the structure of the program you are about to start writing. A good example is when you do not know which functions to use (or write!) yet, or whether your functions are appropriately sized; if the pseudo-code is getting too long, it is a good sign that you may start thinking about smaller functions, and break down your problem into further (smaller) pieces.