Debugging odin models can be challenging because:
Here, we outline some strategies for debugging, and describe the new features that aim to make this easier.
print()
As of odin 1.4.5, you can print the value of some variables in the middle of running your model. We will expand and change this functionality in future versions, your feedback is very welcome.
Consider the simple model below, which illustrates the idea:
gen <- odin::odin({ deriv(x) <- x initial(x) <- 1 print("x: {x}") }, debug_enable = TRUE) mod <- gen$new() mod$run(c(0, 0.1))
Here we've told odin that we want to watch the variable x
and print its value at every evaluation (the third line of the model code) and to include generated code that actually prints the debugging information (debug_enable = TRUE
). When we run the model it prints out the time in square brackets then the debug information following. Notice that we only requested the solution at times 0 and 0.1 but the debug information shows every point in time that the ODE solver evaluated this system of equations.
While this function shares its name with R's print()
it has entirely different functionality.
Importantly, adding the print()
statement to a model has no effect unless it is compiled with debug_enable = TRUE
. It's safe to leave these statements in with no performance cost as if debug_enable = FALSE
(the default) odin will simply not generate any related code.
print
format stringsFor print formatting, we use glue to drive the formatting, and if you have used that package the format will feel familiar.
The most simple usage is as above; you can refer to variables within {curly braces}
; so long as your variable is a scalar this will work. Outside of curly braces the string is printed verbatim.
If your model takes many steps, or if you want to narrow down on a problem, you may want to enable conditional display of your debug information. Use the argument when =
to control display, such as
print("x: {x}", when = x > 1)
which will display the value of x
when it is greater than 1. You can chain together expressions with parentheses and &&
or ||
and reference any value in your system. For example:
printf("{x} {y} {z}", when = x > (x + y + z) / 2 && a < 1)
You can control the way that quantities are displayed through the use of formatting options. The formatting is the same as used by R, so you can experiment in the console easily. The default is to print as a generic floating point number, so this:
print("x: {x}, y: {y}")
is roughly equivalent to writing
sprintf("x: %f, y: %f", x, y)
See ?sprintf
for more information; but this defaults to 6 decimal places of precision. This may not be appropriate if you are dealing with numbers that are very large or very small; these both look a bit silly:
sprintf("x: %f, y: %f", 1e-7, 1e7)
The first loses all information - the only non-zero parts of the number fall after the precision cut-off, while in the second the 6 decimal places just add noise. So above we might prefer:
sprintf("x: %g, y: %g", 1e-7, 1e7)
which we could write in odin's approach as
print("x: {x; g}, y: {y; g}")
Anything after the ;
is interpreted as a format specifier. You could also do
print("x: {x; g}, y: {y; .2f}")
which would format y
to 2 decimal places. We follow here the example
of the sprintf transformer example in
glue
by not including the %
placeholder, but allow all formats that the
underlying library supports.
This is an experimental interface, and it has not been exposed to much real-world use. As such it is possible that you might write fairly innocent looking code and it produce a compiler error rather than a nicer R error - please let us know so we can fix this.
print
{x; d}
) for variables that are merely integer-like, or you will get unexpected junk output out.Add the following code to your website.
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