Variable Scope

Variables in the C programming language, which Arduino uses, have a property called "scope". This is in contrast to languages such as BASIC where every variable is a global variable.

A global variable is one that can be seen by every function in a program. Local variables are only visible to the function in which they are declared. In the Arduino environment, any variable declared outside of a function (e.g. setup(), loop() ), is a global variable.

When programs start to get larger and more complex, local variables are a useful way to insure that only one function has access to its variables. This prevents programming errors when one function inadvertently modifies variables used by another function.

Example:

int gPWMval;  // any function will see this variable

void setup()
{
  // ...
}

void loop()
{
  int i;    // "i" is only "visible" inside of "loop"
  float f;  // "f" is only "visible" inside of "loop"
  // ...
}