unexpected result: zero minus zero is greater than 10

Using Arduino Programming Questions
#1

Found some unexpected (for me) behavior in the IDE 1.0.5.
Tracked down my problem to one line of code:

#defined MAXROW = 10
uint16_t fileCount = 0
uint8_t   listStart = 0
				MAXROW < (fileCount - listStart)  evaluates to TRUE

Somehow, I don’t see how 10 is less than zero minus zero.
I recast listStart to uint16_t and now 10 is greater than zero.
What gives?

#2

try to print to the console, maybe it will help you see what is happening!
:wink:

#3

vanduino:

#defined MAXROW = 10

That's not how to write a #define

#define MAXROW 10
#4

Please post a complete sketch that actually compiles and that illustrates the problem. Shorter is better.

#5

I'm guessing this is a typo from when you posted message?

#defined MAXROW = 10
#6

vanduino:
Found some unexpected (for me) behavior in the IDE 1.0.5.
Tracked down my problem to one line of code:

#defined MAXROW = 10

uint16_t fileCount = 0
uint8_t   listStart = 0
MAXROW < (fileCount - listStart)  evaluates to TRUE




Somehow, I don't see how 10 is less than zero minus zero.
I recast listStart to uint16_t and now 10 is greater than zero.
What gives?

You are subtracting an 8 bit variable from a 16 bit variable to compare to what?

#7

vanduino:
Found some unexpected (for me) behavior in the IDE 1.0.5.
Tracked down my problem to one line of code:

#defined MAXROW = 10

uint16_t fileCount = 0
uint8_t   listStart = 0
MAXROW < (fileCount - listStart)  evaluates to TRUE

Four lines of pseudo-code.

Please post a sketch that actually compiles, and displays this problem.

#8

Plus what Jack Christensen said. :slight_smile:

#9

Yes, just a typo,
The code was embedded in a lot of verbosity. I isolated the line and bracketed it with print statements and yes, with all the constants and variables revealed, zero minus zero was greater than 10. Until I recast the uint8_t zero to uint16_t zero. Then 10 was greater than zero, like in the olden days.

I'll make a new sketch and try to recreate the behavior.

#10

Glad I'm not the only one that does that :wink:

#11

vanduino:
Yes, just a typo,
The code was embedded in a lot of verbosity. I isolated the line and bracketed it with print statements and yes, with all the constants and variables revealed, zero minus zero was greater than 10. Until I recast the uint8_t zero to uint16_t zero. Then 10 was greater than zero, like in the olden days.

I'll make a new sketch and try to recreate the behavior.

Mixed type math gave bad results back in the day too. That's why I spotted that.

#12

vanduino:
The code was embedded in a lot of verbosity. I isolated the line and bracketed it with print statements and yes, with all the constants and variables revealed, zero minus zero was greater than 10. Until I recast the uint8_t zero to uint16_t zero. Then 10 was greater than zero, like in the olden days.

I look forwards to the proof. :slight_smile:

#13

no need to cast, compiler is clever enough …

#define MAXROW 10

void setup()
{
  Serial.begin(115200);

  uint16_t fileCount = 0;
  uint8_t   listStart = 0;

  if (MAXROW < (fileCount - listStart)) Serial.print("TRUE");
  else Serial.print("FALSE"); 
}

void loop()
{
}
#14