Editing Custom Expressions (section)

== Index expressions ==

The custom expressions are calculated at each time step of the simulation, however there are some cases where it is useful to have access to earlier values of a given variable. This is possible using ''index expressions''. These use a square bracket syntax to specify the time (in seconds) for the variable you want. For example, the expression:
 m / m[0]
would give the ratio of the current mass to the launch mass at time 0. Similarly, 
 m - m[t-1]
would give you the change in mass over the last second.
 
You can specify any valid sub-expression inside the square brackets, the only restriction is that you can't nest another index/range expression inside the square brackets.

When indexed expressions are calculated, interpolation is used to get the value exactly at the specified time, independent of the time steps of the simulation.

If you specify a time smaller than 0 or greater than t then it will be clipped to 0 or t respectively. You can't access data that has not been calculated yet.