Use this forum to discuss any issues relating to Systems Dynamics and Systems Thinking.
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Monte
- Senior Member
- Posts: 133
- Joined: Sat Aug 13, 2005 4:18 am
- Vensim version: PLE+
Post
by Monte »
Hi,
My problem is a rate variable computed as level/constant time, where constant time equals zero.
For example, let's consider the following equations:
travel speed (rate)=population (level)/travel time (constant time)
where, travel time=total distance/walking speed
At time=0, people do not walk yet, and thus walking speed equals 0 m/s at t=0, which is mathematically wrong.
I am looking for a solution to this problem. Thank you.
Monte
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Administrator
- Super Administrator
- Posts: 4590
- Joined: Wed Mar 05, 2003 3:10 am
Post
by Administrator »
In Vensim, I would use the ZIDZ (zero if divide by zero) function.
This is appropriate for the travel time=total distance/walking speed calculation.
No idea if the other SD packages have a function like this, but a simple 'if then else' will work in these instances.
Hope this helps.
Tony.
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Monte
- Senior Member
- Posts: 133
- Joined: Sat Aug 13, 2005 4:18 am
- Vensim version: PLE+
Post
by Monte »
Dear Tony,
Many thanks for your help.
Monte