Ventana software support forum

Potential bug in STEP and RAMP initialization, and probably similar functions.

I have noticed that both RAMP and STEP are executed when TIME + TIME STEP/2 > start time and TIME +TIME STEP/2 > step time, respectively.
This could produce a bug when the start time (or step time) is not a multiple of the time step. For example, the function
with INITIAL TIME = 0; FINAL TIME = 3; TIME STEP = SAVEPER = 1

produces the output which has a negative value! This is because it is computing when TIME < start time. This value could make some models fail as the ramp return a value outside the expected value rang [0, slope*(final time - start time)].The expected output should be similar happens to step: with the same control variables produces: when the expected output would be: I know that is not desirable to have the start time not being a multiple of time step but this could happen sometimes. A possible workaround would be to use the same constant used to ZIDZ and XIDZ to avoid numerical error in comparison, i.e., start the RAMP and STEP when TIME + 1E-6 > start time or TIME + 1E-6 > step time.

I am using Vensim DSS 9.2.4 x64 in Windows.

Find the model attached. Thank you

For STEP, I think this is not a bug. The step has to happen somewhere, so the logic places it at the nearest time step. In this example, the step time of 0.45 is closer to 0 than to 1, so it's appropriate for the step to occur at 0.

For RAMP, I think you're right - it seems inappropriate to return a value outside the range of [0,slope*(finish time-start time)]. I'll propose a fix.

ZIDZ and XIDZ don't use the common x/(y+1e-6) to compute a result. They do a logical check on the denominator instead. This avoids side effects of computing the numerator when it won't be used (mainly consumption of random numbers). It also avoids numerical dilution when x and y are small relative to the tolerance.

BTW, whatever testing you're doing, please keep it up!