Working through Purdum's book on Arduino

Development Boards Arduino
1

I am working through Jack Purdum’s book on programming C for the Arduino. The following code appears in Chapter 9.

void setup() {

Serial.begin(9600); // Serial link to PC

}

void loop() {

int number = 50;

int (*funcPtr)(int n); // This defines a pointer to function

funcPtr = DisplayValue; // This copies the lvalue of DisplayValue

number = (*funcPtr)(number);

Serial.print("After return from function, number = ");

Serial.println(number);

Serial.flush();

exit(0);

}

int DisplayValue(int val) //I don’t understand this statement.<<<<<<<<<<<<<

{

Serial.print("In function, val = ");

Serial.println(val);

return val * val;

}

This program runs fine but I do not understand the placement or the syntax of the statement indicate by the <<<<<<<<< above. Why no semicolon terminator? How does program know what “DisplayValue” is in Loop()?

What am I missing here?

2

DisplayValue() is a separate function, just like loop() is. You can create multiple functions in a program.

Specifically DisplayValue() is a function with one argument, a signed integer called “val” and it returns a signed integer. There’s no semicolon after the close paren since that’s not the end of the function, the closing bracket is.

This example is using a function pointer to call DisplayValue(). Most of the time, you don’t use function pointers, and would call the function directly:

number = DisplayValue(number);

In this case, you have a pointer to the function (the linker will fill it in automatically), and you use that to call it.

Function pointers are a more specialized case; you find them in callback routines, menus, task switchers, and other places where you want the code to call one of many functions based on some variable or choice.

/mike

3

The answer above is correct and if you had kept the indentation of the code it would be more obvious. I formatted the code and added some comments below.

void setup() {
  Serial.begin(9600); // Serial link to PC
} // this brace indicates the setup() ends here

void loop() {
  int number = 50;
  int (*funcPtr)(int n); // This defines a pointer to function
  funcPtr = DisplayValue; // This copies the lvalue of DisplayValue
  number = (*funcPtr)(number);
  Serial.print("After return from function, number = ");
  Serial.println(number);
  Serial.flush();
  exit(0);
} // this brace indicates the loop() ends here

int DisplayValue(int val) //I don't understand this statement.<<<<<<<<<<<<<
{
  Serial.print("In function, val = ");
  Serial.println(val);
  return val * val;
}

Read this page:

http://arduino.cc/en/Reference/FunctionDeclaration

(click on to open)

4

Ah! Yes.Thanks all. Makes sense when you look at it the right way.