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  • Writer's picturePuriphico

introduction to Arduino code syntax

This article details the basic syntax for defining constants, including libraries, making comments, and using punctuation in your Arduino programs.

Defining constants:

  • #define

    • The #define syntax allows the programmer to give a name to a constant value before the program is compiled, allowing any references to the constant to be replaced with the defined value when the program is running.

    • However, although convenient, using #define presents a few things to watch out for:

      • Firstly, proper syntax:

        • When writing “#define constantName value”, you cannot punctuate the line with a semicolon (unlike other lines of code).

        • Additionally, you must be careful to not involuntarily include an “=” in between the constantName and the value that is being defined.

  • Const

    • Writing const before a variable in the code setup() or before will change the variable to a “read-only” format, meaning that its value cannot be changed later in the program. In other words, it is constant. It serves as a more popular alternative to #define.

    • When using arrays (a data type referencing a collection of variables), you cannot use const.

    • A benefit of using const over #define is that const allows you to specify a data type for the constant being defined, as you can see in this image:

In the following code, pi is declared as a float (a data type similar to an integer, except where decimals are included in the constant). For a challenge, try to identify the error in the code below:


const float pi = 3.14;

float x;

// ....

x = pi*2;

pi = 7;


The error: In the first line, pi is declared as a constant; for this reason, improper syntax is used in the last line, as it tries to modify the constant's value.

  • Variable Scope

    • Declaring variables and defining constants only becomes useful when they are declared in the right parts of a program; this is called variable scope, and it’s the idea that any variable is limited to the place in which it’s defined. For instance, when a variable/constant is declared outside both the setup and the loop functions, it is called a global variable, meaning it can be used throughout the program. On the other hand, if a variable is declared within a for-loop, it can only be used within that loop (if you try to reference a local variable in a segment of code where it was not originally defined, the program will return an undefined value).

Including Libraries:

  • #include<library.h>

    • Using #include allows programmer to use and reference premade libraries in their program.

    • Careful: both #include and #define functions do not use semicolons, and employing one results in an error.

    • When using an external component in your circuit, like a LCD display for instance, you would include that component’s premade library in your program so that the microcontroller would recognize it (this is a simplified explanation of how libraries work). Here is an example of what that looks like in practice:

In this image, take note of the syntax when referencing libraries:

In this video, take note of how I access the premade libraries by clicking "Tools" and then finding the "Manage Libraries" tab:

Writing Comments:

  • Comments

    • Comments are ignored by the processor and serve as a way to inform yourself and others about the way a program works.

  • Single-Line Comments //

    • These comments have no coding functionality, but merely allow you to add your own thoughts or reminders in the code. When using the // syntax, the comment cannot take up more than one line of code.

  • Block Comment /* */

    • Serve the same purpose as single-line comments, but using this syntax allows for a multi-line comment: the comment must start with /* and end with */. Everything in between these two symbols will be ignored by the compiler and will not occupy any space in your program.

    • Usually, I use a block comment when I need to modify an aspect of code or identify the aspect of a program that is causing a given error but not remove it entirely in the process; in these instances, I comment the original code and modify it outside of the comment.

In the below image, you can see that I have commented to remind myself which pin refers to which color of LED:


  • Semicolon ;

    • Semicolons are used to end a statement (line of code).

    • Forgetting to include a semicolon will prompt a compiler error; however, it is important to remember that #include and #define do not use semicolons.

    • Even if you are writing code within an if statement, you must always punctuate each line with a semicolon.

  • Curly Braces {}

    • The opening brace must be followed by a closing brace. Arduino IDE software packs a useful feature that allows you to check that the brackets align properly by clicking on (or near) one brace; once you do this, the paired brace will be highlighted.

    • These braces are used in:

      • Conditional statements

      • Loops

        • While

        • Do

        • For

      • Functions

      • All of these will be detailed in an upcoming article, which you will soon find linked in a comment on this post.

Here is an example of the braces in one of my programs:


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