Strange Variable Problem

Forum 2005-2010 (read only) Software Syntax & Programs
1

Hello,

I'm developing some software for an unmannned autonomous vehicle, and I've run into a very strange problem -- I have a float variable called "heading", and for testing purposes, I ended up with code like this:

Serial.println("---------TESTING");
if (heading == heading) {
Serial.println("Success");
}
Serial.println("TESTING---------");

The strange thing is, even though the first and last lines do print, the "Success" line doesn't! I initially wrote this code because the "heading" variable wasn't storing the proper value, and sort of out of desperation I just figured I would make sure that the variable itself was ok. Not sure where my logic was on that one, but it was late : P

Anyway, what would be a reason that a variable would not register as equal to itself?

2

Can it be because it's undefined ?

3

Good thought, but it's defined as a global variable:

float heading;

..earlier in the file (and given a value, albeit in a slightly complicated way, just before the test).

4

Erg, sorry about the trouble...it was one of those really annoying hard-to-spot programming mistakes.

Thanks though!

5

In general you can run into problems when you try to test for equality using floats, although it is strange that a float would not register the same as itself. Try changing the code to see if it's close to itself:

if (heading > heading - 0.0001 && heading < heading + 0.0001)
serial.println("success");

I've used 0.0001 in my example, but you should choose something that's three or four orders of magnitude smaller than whatever is currently in "heading".

  • Ben
6

it was one of those really annoying hard-to-spot programming mistakes.

Which was? :slight_smile:

--Phil.