# mpe.numeric.md

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| ----------- | --------------: |
| mpe.numeric | R Documentation |

### Mean Percentage Error

#### Description

A generic S3 function to compute the _mean percentage error_ score for a\
regression model. This function dispatches to S3 methods in `mpe()` and\
performs no input validation. If you supply NA values or vectors of\
unequal length (e.g. `length(x) != length(y)`), the underlying `C++`\
code may trigger undefined behavior and crash your `R` session.

**Defensive measures**

Because `mpe()` operates on raw pointers, pointer-level faults (e.g.\
from NA or mismatched length) occur before any `R`-level error handling.\
Wrapping calls in `try()` or `tryCatch()` will _not_ prevent `R`-session\
crashes.

To guard against this, wrap `mpe()` in a "safe" validator that checks\
for NA values and matching length, for example:

{% code overflow="wrap" lineNumbers="true" %}
```r
safe_mpe <- function(x, y, ...) {
  stopifnot(
    !anyNA(x), !anyNA(y),
    length(x) == length(y)
  )
  mpe(x, y, ...)
}
```
{% endcode %}

Apply the same pattern to any custom metric functions to ensure input\
sanity before calling the underlying `C++` code.

#### Usage

```r
## S3 method for class 'numeric'
mpe(actual, predicted, ...)
```

#### Arguments

|                       |                                            |
| --------------------- | ------------------------------------------ |
| `actual`, `predicted` | A pair of \<double> vectors of length `n`. |
| `...`                 | Arguments passed into other methods        |

#### Value

A \<double> value

#### References

James, Gareth, et al. An introduction to statistical learning. Vol. 112.\
No. 1. New York: springer, 2013.

Hastie, Trevor. "The elements of statistical learning: data mining,\
inference, and prediction." (2009).

Virtanen, Pauli, et al. "SciPy 1.0: fundamental algorithms for\
scientific computing in Python." Nature methods 17.3 (2020): 261-272.

Pedregosa, Fabian, et al. "Scikit-learn: Machine learning in Python."\
the Journal of machine Learning research 12 (2011): 2825-2830.

#### Examples

```r
## Generate actual
## and predicted values
actual_values    <- c(1.3, 0.4, 1.2, 1.4, 1.9, 1.0, 1.2)
predicted_values <- c(0.7, 0.5, 1.1, 1.2, 1.8, 1.1, 0.2)

## Evaluate performance
SLmetrics::mpe(
   actual    = actual_values, 
   predicted = predicted_values
)
```

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