Why does Stateflow evaluate the condition [x >= (1/2)] differently from [x >= 0.5]?

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I have created a minimal example (available here: http://dl.dropbox.com/u/99981625/fraction_in_condition.mdl) consisting of a single chart with a single state (A) which increments an output variable x in steps of 0.1. A transition from A to A is conditioned on [x >= y], where y is a literal constant. A conditional action resets x to zero. The output variable x is declared as a real double scalar and is given an initial value of zero. If y is the literal "1/2", the condition mysteriously evaluates true even when the debugger shows that x == 0 (or x < 0.5, in general). The conditional action resets the value of x, causing it to remain zero for all time. On the other hand, if y is the literal "0.5", then x increments from 0 to 0.5 before being reset to zero. I expected both conditions to behave as in this second example. Why don't they?
Thanks for any ideas.

Accepted Answer

Siddharth Shankar
Siddharth Shankar on 4 Nov 2012
Edited: Siddharth Shankar on 4 Nov 2012
If I had to guess, I would think that 1/2 results in integer division which always yields 0, and this is then cast to double (0.0) for the comparison, by which time the damage is done. If this hypothesis is correct ;), 1.0/2.0 should show you the same behavior as 0.5.
  1 Comment
Brian B
Brian B on 4 Nov 2012
MATLAB has made me casual about my literals! I should have thought about that. Making the suggested change produces the expected behavior.
It is good to know that Stateflow does not treat literals as doubles by default, as MATLAB does. Thanks for the insight.

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