Rust println! returning two copies of each line

help
1

I have a small toy compartmental model that iteratively solves a system of ode's using a rk4 algorithm from a ode_solvers crate. For some reason when I get printout from each iteration, only for monitoring/testing purposes I get two copies of each line from "println!"

Have I coded something incorrectly or could it possible be a bug in the ode solver? Thx. J.

The code to create the ode solver object is this:

mpl ode_solvers::System<f64, State> for TwoPool {
    fn system(&self, _: Time, y: &State, dy: &mut State) {

and the line further down that outputs the results is this:

	// print the outputs of the model at each integration iteration 
	println!("PoolSizes   A={:.3}, B={:.3}, Tot={:.3}", y[0], y[1], y[2]);

and this is a section of the output I get:

PoolSizes   A=22.231, B=14.926, Tot=37.157
PoolSizes   A=22.231, B=14.926, Tot=37.157
PoolSizes   A=22.288, B=14.956, Tot=37.243
PoolSizes   A=22.288, B=14.956, Tot=37.243
PoolSizes   A=22.344, B=14.986, Tot=37.330
PoolSizes   A=22.344, B=14.986, Tot=37.330
PoolSizes   A=22.401, B=15.015, Tot=37.416
PoolSizes   A=22.401, B=15.015, Tot=37.416
PoolSizes   A=22.457, B=15.045, Tot=37.502
PoolSizes   A=22.457, B=15.045, Tot=37.502
PoolSizes   A=22.513, B=15.075, Tot=37.588

Any thoughts why I'm getting doubles? Thx. J

2

I don't know ODE so I don't know if getting the same data twice is a bug or just how it works but I am quite sure println! would never print the same line twice if invoked once.

1 Like
3

Rk4 evaluates the system four times on each integration step, that’s probably what you’re seeing.

4

I wondered the same but would expect to get four copies? I may have to ask the writer of the ode crate. the Rk4 might be variable ... up to 4 evaluations per iteration if required? It could be finding a suitable solution after the second evaluation in each case? I will try to find out and report back.

5

No, see the source. If you print out all the parameters (x, y, dy), they should be different on each call as far as I can see.