Mind Your Parens

I was cleaning up my code of magic numbers and added a few #define statements for the frequency values. But after uploading the program to the board I didn't understand why the the colors on the light were not changing at the correct rate. So I added a couple serial output debug prints saw that my frequency was not set properly.

Code

// Time constants
#define SECONDS_PER_MIN   60.0
#define MINUTES_PER_HOUR  60.0
#define SECONDS_PER_HOUR  SECONDS_PER_MIN * MINUTES_PER_HOUR 

// Cycle time for ColorClock object 0
// Should equal 0.00333____
#define CC0_CYCLE_TIME_HOURS 12.0 / SECONDS_PER_HOUR

//...

Serial.print("SECONDS_PER_HOUR:     ");
Serial.println(SECONDS_PER_HOUR);
Serial.print("CC0_CYCLE_TIME_HOURS: ");
Serial.println(CC0_CYCLE_TIME_HOURS);

Serial Output

I was expecting CC0_CYCLE_TIME_HOURS to print as 0.00333 or even 0.00. Why was it printing 12.00??

CC0_CYCLE_TIME_HOURS: 12.00
SECONDS_PER_HOUR:     3600.00

I figured out that it had something to do with my #define statement. I stared at the code for way too long before deciding to do away with the SECONDS_PER_HOUR constant and just hard code the values directly:

#define CC0_CYCLE_TIME_HOURS 12.0 / 3600.00

And this worked, so whatever. I just figured that the Arduino compiler is weird and doesn't treat preprocessor directives the same as gcc. I started writing this post to complain about the Arduino compiler, so to illustrate my point, I coded up the following simple program to compile on my desktop.

Code

#include <stdio.h>

#define SECONDS_PER_MIN   60.0
#define MINUTES_PER_HOUR  60.0
#define SECONDS_PER_HOUR  SECONDS_PER_MIN * MINUTES_PER_HOUR 

#define CC0_CYCLE_TIME_HOURS 12.0 / SECONDS_PER_HOUR

int main()
{
  printf("\nSECONDS_PER_HOUR:     %f", SECONDS_PER_HOUR);
  printf("\nCC0_CYCLE_TIME_HOURS: %f", CC0_CYCLE_TIME_HOURS);

  return 0;
}

But to my surprise, the console output displayed the same value as the Arduino serial out!

Console Output

SECONDS_PER_HOUR:     3600.000000
CC0_CYCLE_TIME_HOURS: 12.000000

The Problem

I stared at the code again and realized that the lack of parenthesis in the definition for SECONDS_PER_HOUR was the source of my problem. The #define macro is a simple find and replace, so what the compiler saw was:

#define SECONDS_PER_MIN   60.0
#define MINUTES_PER_HOUR  60.0

// The compiler sees:
// #define SECONDS_PER_HOUR  60.0 * 60.0
#define SECONDS_PER_HOUR  SECONDS_PER_MIN * MINUTES_PER_HOUR 

// The compiler sees:
// #define CC0_CYCLE_TIME_HOURS 12.0 / 60.0 * 60.0
#define CC0_CYCLE_TIME_HOURS 12.0 / SECONDS_PER_HOUR

The compiler was basically seeing 12 / 60  60 instead of 12 / (60  60). And because of order of operations, CC0_CYCLE_TIME_HOURS was indeed 12, not the 0.003333 that I was hoping for 🤦‍♀️

Lesson learned 😳