Trap in arithmetic

Using Arduino Programming Questions
1

Hi,
I found an error in a simple computation which might be a trap for many beginners in programming. I am not really a beginner, but I fell into this trap, which took me a lot of time to find out.

case 1:
#include <SoftwareSerial.h>

uint32_t thresh = 800;
int32_t tmic_d0, tmic_filt;
SoftwareSerial mySerial(6, 5);  // RX, TX (DO=TX)

setup() {
 mySerial.begin(38400);
  delay(500);

  tmic_d0=15868;
  tmic_filt=15533;
  mySerial.print(tmic_filt);  mySerial.print(" - ");
  mySerial.print(tmic_d0);  mySerial.print(" = "); 
  mySerial.print(tmic_filt-tmic_d0);

  if (tmic_filt -tmic_d0 < thresh) mySerial.print(" < ");
  else mySerial.print(" >= ");
 mySerial.println(thresh);

while(1);
}
case 2:
  same code as in case1 with 1 line changed:
  int32_t thresh = 800;

Serial Monitor shows in case1:

15533 - 15868 = -335 >= 800

which is wrong!

But in case 2 it is ok:

15533 - 15868 = -335 < 800

Comparisons between signed and unsigned variables are handled by the compiler as if both variables were unsigned!
(-335 interpreted as unsigned is more than 4 billion)

This happened to me with an ATtiny84. But I guess it is the same with any other controller.

2

The compiler can catch many of these common bugs: go to the preferences in the IDE and enable all compiler warnings. It'll warn about the comparison of signed and unsigned integers for case 1.

3

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