Why are functions created after they are used in the code examples in the book?

Question

Arduino

I'm learning how to write sketches for Arduino from the book "Programming the Arduino. Basics of working with sketches" by Simon Monk. In the book, in code examples, functions are first called and only then written.
For example like this:

void loop()
{
  flash(20, delayPeriod);
  delay(3000);
}

void flash(int numFlashes, int d)
{
  for (int i = 0; i < numFlashes; i ++)
  {
    digitalWrite(ledPin, HIGH);
    delay(d);
    digitalWrite(ledPin, LOW);
    delay(d);
  }
}

The way it is written in the book does not work, it works, of course, if you transfer the creation of the function to its use -

void flash(int numFlashes, int d)
{
  for (int i = 0; i < numFlashes; i ++)
  {
    digitalWrite(ledPin, HIGH);
    delay(d);
    digitalWrite(ledPin, LOW);
    delay(d);
  }
}

void loop()
{
  flash(20, delayPeriod);
  delay(3000);
}

Is this the wrong code in the book, or am I missing something?

Answer 1

Previous versions of the IDE allowed this (when compiling, they automatically placed the functions as they should).
More recent versions of the IDE don't take it easy anymore and you have to strictly follow C++ standards.

Answer 2

You may be missing header files.

Answer 3

or just the functions are declared before they are used