I’m having some trouble optimizing a function in R using l-bfgs-b. I keep getting an error message saying that “finite values of fn” are needed. I’m not exactly sure what that means or how to solve it.

I’m using the optimize function in R to find the minimum value of my function, and it works fine when I use other optimization methods like “Brent” or “Golden Section.” However, when I try to use l-bfgs-b, I get the error message. Here’s the code I’m using:

“`{r}

f <- function(x) {return(x^2 - 4*x + 4)}
optimize(f, interval=c(-Inf, Inf), method="L-BFGS-B")
```
When I run this code, I get the error message: "Error in optim(par = par, fn = fn, gr = gr, method = method, lower = lower, : L-BFGS-B needs finite values of 'fn'"
I'm not quite sure what to do here. I tried looking up some information on the l-bfgs-b algorithm, but I wasn't able to find anything that helped me solve this problem. Any ideas?

Optimization of a function in R: L-BFGS-B needs finite values of 'fn'

levinkunn
Teacher

Hello there and thank you for asking this question. It seems that you are having issues optimizing your function in R using the L-BFGS-B optimization algorithm due to an error regarding finite values of fn. One possible cause of this error could be a problem with your input values.

The L-BFGS-B algorithm requires an initial estimate of the function’s parameters. It is possible that these initial values are not appropriate for the problem you are trying to solve, leading to the error you are seeing. You might want to try adjusting these initial values to see if that resolves the issue.

Another possible reason for the error could be that your function is returning an infinite or NaN value for some inputs. This could cause the optimization algorithm to fail. To address this issue, you could try modifying your function to return an appropriate value for all inputs. You might want to check your code for any division by zero or other mathematical errors that could cause this issue.

In addition, you could try changing the optimization algorithm you are using. L-BFGS-B is just one of many optimization algorithms available in R. Depending on the specifics of your problem, a different algorithm might perform better. Some popular options include Nelder-Mead, conjugate gradient, and simulated annealing.

Another possibility is that the optimization algorithm is hitting a boundary constraint. This can happen when the optimizer tries to test a value outside the boundaries of the feasible region. You might want to check your optimization function to ensure that boundary constraints are being properly handled.

Finally, make sure that your input data is properly formatted, with no missing or invalid values. This could also cause the optimization algorithm to fail. If you have any missing data, you might consider imputing those values before running the optimization.

In summary, there are several possible reasons why you might be seeing the “need finite values of fn” error when using L-BFGS-B optimization in R. Possible solutions include adjusting initial values, modifying your function to return appropriate values for all inputs, changing the optimization algorithm, properly handling boundary constraints, and ensuring that input data is properly formatted. Good luck!

The problem with your code is that the function you are trying to optimize needs to have finite values throughout its domain, but it seems that the function might not be defined for certain input values. You need to make sure that there are no undefined values in the input domain of the function before optimizing it. Try checking whether there are any input values that might cause an undefined or infinite result and make sure to exclude them from your optimization process. If you cannot avoid undefined values, you should add a constraint to your optimization process so that it doesn’t attempt to optimize over those values.

One possible way to approach this issue is to use an optimization algorithm that can handle undefined values, such as a genetic algorithm or a simulated annealing algorithm. These algorithms can explore the input domain more broadly and are less likely to get stuck in areas with undefined values. Another option is to use a more robust optimization algorithm than l-bfgs-b, which might be better suited to handle undefined values.

In general, it’s important to always make sure that your function is well-defined across its entire input domain before attempting to optimize it. This will not only help you avoid issues with optimization algorithms, but will also ensure that your function produces meaningful results across its entire range of input values.

One possible approach to this issue is to check your function for any infinite or NaN values. Since the ‘optim’ function is used to optimize a function, it also requires that the function returns finite values. To check for these values in your function, you can use the ‘is.finite’ function or the ‘is.nan’ function to identify any potentially problematic values.

Another possible issue is that the function you are trying to optimize may have a wrong argument order. Sometimes, the function that you are trying to optimize has multiple arguments, and if the order of the arguments is not correct, the optimization algorithm may fail to converge. Make sure that the argument order of your function is correct, as specified by the documentation of the optimization function you are using.

It is also recommended that you start optimizing using a very simple function, and check whether the optimizer can handle it correctly. If it can, then gradually increase the complexity of the function until you reach the desired level of optimization. This will help you identify any potential issues with your implementation early on in the process.

Another approach is to use a different optimization algorithm. If you have tried everything else, and your function still fails to optimize, it might be worth trying a different optimization algorithm. There are many optimization algorithms available, and some of them might be more suited to your particular problem than others. Try out different algorithms until you find one that works well for your function.

In summary, try to check for finite and NaN values in your function, ensure that the argument order is correct, start with a simple function to identify any potential implementation issues, and consider using a different optimization algorithm if the issues persist. These steps should help you successfully optimize your function.

One possible solution for your optimization problem is to try different optimization methods, such as Nelder-Mead, conjugate gradient or simulated annealing. You can do this by specifying the `method` argument in the `optim()` function. This may help overcome the issue you are having with the `L-BFGS-B` optimizer requiring finite values of `fn`.

Another thing you can try is to check if your function is returning finite values, as required by `L-BFGS-B`. You can do this by adding the line `if(any(!is.finite(par))) return(Inf)` at the beginning of your function. This will return `Inf` if any of the parameters are infinite or not a number, which can be used to terminate the optimization.

It may also help to scale your variables to have mean zero and standard deviation one. This can improve the convergence of some optimization methods, such as `L-BFGS-B`. You can do this using the `scale()` function in R.

Ultimately, the best solution for your problem may depend on the specifics of your function and dataset. Experiment with different optimization methods and preprocessing steps to find the best combination for your particular problem.