Strange behaviour with using max

Question

I have this really simple code. Let's say I have a variable X. Then all I want to do is create a random value R, if R > X then I want X to become R. But for some reason, my n_of_buttons_pressed climbs up and then sometimes drops back to lower values. I can not wrap my head around how it could get a lower value then a previous value.

#define N_OF_BUTTONS 60

int n_of_buttons_pressed;


void setup() {
  Serial.begin(9600);
  Serial.println("start");
}

void loop() {

  n_of_buttons_pressed = (int) max(random(0, N_OF_BUTTONS), n_of_buttons_pressed);

  Serial.print("n_of_buttons_pressed:");
  Serial.print(n_of_buttons_pressed);
  Serial.println();

  delay(100);

}

Answer 1

The max() “function” is actually a macro defined in Arduino.h as

#define max(a,b) ((a)>(b)?(a):(b))

This expands in your code into something like

buttons_pressed = random(0, N_OF_BUTTONS) > n_of_buttons_pressed) ? \
                  random(0, N_OF_BUTTONS) : n_of_buttons_pressed

The problem with the macro is that it evaluates at least one of its arguments twice. In this case, if the test is true, the variable buttons_pressed gets assigned another pseudo-random number.

The way around this is to store the pseudo-random number in a variable, to avoid random() being evaluated twice:

int pseudo_random = random(0, N_OF_BUTTONS);
buttons_pressed = max(pseudo_random, n_of_buttons_pressed);