Software that runs on Arduino is written by you, the users. The visible text that creates the software is usually called a SKETCH.
Usually Sketches are written using the Arduino "IDE" (Integrated Development Environment) which includes an editor, menus for files, tools and reference material. Many Examples are provided so you can see what Arduino Sketches look like, and which you can try out and change. The IDE also includes many "Libraries" which is code that people have written and made available to anyone to use. There is also a Library Manager which can help you find Libraries that were written by others and shared and which you can install in your copy of the IDE.
CLICK HERE to go to this page which shows you how to install the Arduino IDE, use the IDE, and start to understand Physical Computing which is what Arduino is all about. Physical Computing is quite different than writing software for personal computers where the only physical inputs are the Mouse and Keyboard. With Arduino you can connect and control literally hundreds of different devices, and write software that creates new Intelligent Devices.
Combining Arduino Sketches
CLICK HERE to go to this page which will help you with this very common problem: adding two or more working sketches into one sketch.
Putting a bunch of code together and then getting a Zillion error messages is No Fun!
FIRST! Start by understanding both sketches and the resources and libraries they use before attempting to stitch them together.
The Arduino Language and Library References
Arduino code is written in a version of the well-known and widely used language C++. Arduino provides many easy-to-use additions to C++ and you will see many sketches are pretty easy to understand.
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See the Arduino Language Reference HERE for details and short examples of all of the Arduino Language.
And see the Arduino Library Reference HERE for details of the many very useful libraries you can use in your Sketches.
Troubleshooting Arduino Sketches and hardware
CLICK HERE for the Arduino online guide to Troubleshooting.