关于growthrates包中getClass("nonlinear_fit")功能及自定义类plot报错的问询

Hey there! Let's break down your questions about the growthrates package and your custom S4 class issue:

1. What does getClass("nonlinear_fit") do? #

In R's S4 object system, getClass("nonlinear_fit") retrieves the full definition of the nonlinear_fit class from the growthrates package. This includes:

• Its inheritance hierarchy (your console output shows it extends growthrates_fit, meaning it inherits all slots and methods from that parent class)
• The slots (data components) that objects of this class must contain (like fit, par, FUN, etc., which you're already using when creating instances)
• Any class-specific metadata or constraints

It's a handy way to inspect how the package's core objects are structured, which helps when working with or extending the package's functionality.

2. Why does plot(obj2) throw an error? #

The error Error in as.double(y) : cannot coerce type 'S4' to vector of type 'double' happens because:

• The nonlinear_fit class (used for obj) is part of the growthrates package, which includes a custom plot method tailored to handle objects of this type (and its parent growthrates_fit class). That's why plot(obj) works smoothly.
• Your custom nonlinear class, however, has no associated plot method. When you call plot(obj2), R falls back to its default generic plot function, which doesn't know how to interpret your S4 object's structure—hence the coercion error when it tries to treat the S4 object as a numeric vector.

Fixes you can try: #

Option 1: Inherit from the package's base class

Let your nonlinear class inherit from growthrates_fit (just like nonlinear_fit does). This way, it will automatically inherit all existing methods (including plot) from the parent class:

setClass("nonlinear", 
         contains = "growthrates_fit",
         representation( fit = "modFit", par = "numeric", FUN = "function_growthmodel", obs = "data.frame", rsquared = "numeric" ))

Option 2: Write a custom plot method for your class

If you want full control over the plotting behavior, define a specific plot method for your nonlinear class. Here's a basic example that plots your observed data and the fitted curve:

setMethod("plot", signature(x = "nonlinear"), function(x, main = "Custom Nonlinear Growth Fit", ...) {
  # Plot observed data
  plot(x@obs$time, x@obs$y, pch = 16, col = "black", main = main, xlab = "Time", ylab = "Growth", ...)
  # Add the fitted curve
  curve(x@FUN(x = x, par = x@par), add = TRUE, col = "red", lwd = 2)
})

After implementing either option, plot(obj2) should work as expected.

内容的提问来源于stack exchange，提问作者Chang il