neurobs presentation video

Chapter 1 - Variables

Note: In Presentation, the end of a statement is always marked by a semi-colon. This allows you to use more than one line for a statement. If you get an error in your scenario, it oftentimes is because of a missing semi-colon.

Presentation has several types of information that can be stored that are considered basic types. The distinction between basic types and other types should become clearer once other types and their parameters are introduced. This section will only use basic type variables in the examples. The basic types in Presentation are strings, integers (ints), doubles, booleans (bools), and rgb colors (rgb_colors).

Basic Types

Note: We will frequently give full scenario examples so you can test out things for yourself. If you are reading this course within the compiled documentation distributed with Presentation, you may open these examples in Presentation by clicking on the links in the example box. We will sometimes use the command term.print( ... ) , which prints to the area labeled "Terminal" in the Editor tab , as a simple way to create output. Also keep an eye on the area labeled "Status", which may have important messages, such as "Press Enter to Start".

Note: We have added lines here to print each of the red/green/blue/alpha values separately, with a tab ("\t") between each.

To review, a variable may be declared and initialized using the following syntax:

The initizializing, in (), is optional, but it is good practice to do so to avoid unexpected behavior. For example, if you declare an integer variable without giving it an initial value, and then start adding to that variable without checking the starting value, you will be adding to an unknown quantity. If some aspect of your experiment is dependent on the value of that variable, the program may not work as you expect.

Naming variables

You can give your variables whatever names you wish, adhering to the following rules:

• Variable names should include only letters, numbers, and underscores (_).
• A variable name must start with a letter.
• Your variable name cannot be the same as any other special word that is part of PCL, like "begin" or "loop".

For ease of programming, it is best to make variable names easy to remember and clear. For example, if you are working on a gambling task and you want a variable to store the total amount of money the user has earned, you might make a variable called total_money or TotalMoney. It also helps that you remain consistent in how you name your variables (e.g., capitalization, underscores, etc.), as it will help you to remember the name of the variable as you continue to work with it in your code.

Assigning values to variables

Changing types

Once you have declared a variable, you cannot change the type of that variable. If you are familiar with programming, you may know that this is not always the case in other programming languages. In Presentation, even though a variable cannot be changed to a different type, there are conversion methods so that you can convert the information held in one variable to another variable. The methods used to convert information from one type to another are called conversion methods . Conversion methods can be useful, for example, if you want to include the value of a number in a string. For instance, if participants play a game for points in your scenario, you might want to display the number of points they had onscreen as text. You can only display strings on screen, so you would need to convert the number of points, stored in an integer variable, into a string. In that case, you might use something like:

Scope of Variables

The examples above are pretty short, so they don't demonstrate the importance of where you declare your variables. Where a variable is declared will determine where it can be used, or its scope . You must declare a variable before using it. A more complicated point about scope is that if you declare a variable inside a sub-part of your scenario - for example a loop or a function - then you can't use it outside that sub-part. So, if you declare a variable inside an conditional, or if statement, that variable does not exist outside of that statement and results in an error:

In this example, because j was declared inside of an if statement, it cannot be used outside of it. However, because k was declared outside of the if statement, you may use it both inside and outside of the following if statement.

Global scope indicates that a variable exists in all parts of the program after the variable is declared. You can establish a variable with global scope by declaring it outside of any subsections of the scenario. Declaring a variable with narrow scope (only when necessary) avoids having to worry about losing track of different points in the scenario where the variable may be changed.

A variable is used to store information. When you declare your variable, you must specify its type (the sort of information it will store, such as int, string or bool) and you can optionally initialize it (give it a starting value). Once you declare a variable, the type of that variable cannot change, but the value can. There are methods for converting information from one type of variable to another type of variable, for example if you need to use an integer value as a string. Finally, we introduced the concept of scope. Scope refers to where in a program a particular variable exists.

In the following exercises, initialize an appropriate variable for the type of data that will be needed for the described portion of an experiment. Print the value of your variable to the terminal window. You may use the experiment/scenario file below.

Example: You are running a task and want to keep track of the number of blocks you have run.

• Your paradigm is a go/no-go paradigm. You want to declare a variable to store whether the next stimulus is supposed to be responded to or not. The default case is that the stimulus should not be responded to.
• You want to have a variable to denote the number of trials of a particular type in your paradigm. There are 32 of those trials in the paradigm.
• You want to print your participant's name to the screen and need a variable to store the name. His name is John Smith.
• In your paradigm, you want to set a stimulus to change color based on user response. You need a variable to keep track of the color of the object so you can set the stimulus to that color. The initial color is red. Note: Please see the rgb_color initialization chapters above for how to print the values to the terminal.
• In your paradigm, you rotate a dot about the origin based on the amount of time the scenario has taken, moving 1.5 degrees every 100 ms. You need a variable to keep track of the current angle of the dot with respect to the origin. The dot starts at a 45 degree angle.
• In your paradigm, participants earn a point for each trial they respond to correctly, and you want a variable to keep track of the number of points they have. They do not have any points when they start.

Additional relevant documentation readings

The following is a page in the Presentation documentation that may be of use to you in further understanding the material presented in chapter 1.

• Basic PCL Types