---
title: "Getting Started with NNS: Clustering and Regression"
author: "Fred Viole"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{07. Getting Started with NNS: Clustering and Regression}
  %\VignetteEngine{knitr::rmarkdown}
  \usepackage[utf8]{inputenc}
---

```{r setup, include=FALSE, message=FALSE}
knitr::opts_chunk$set(echo = TRUE)
library(NNS)
library(data.table)
data.table::setDTthreads(1L)
options(mc.cores = 1)
RcppParallel::setThreadOptions(numThreads = 1)
Sys.setenv("OMP_THREAD_LIMIT" = 1)
```

```{r setup2, message=FALSE, warning=FALSE}
library(NNS)
library(data.table)
require(knitr)
require(rgl)
```


# Clustering and Regression
Below are some examples demonstrating unsupervised learning with NNS clustering and nonlinear regression using the resulting clusters.  As always, for a more thorough description and definition, please view the References.

## NNS Partitioning <span style="color:red">`NNS.part()`</span>
**`NNS.part`** is both a partitional and hierarchical clustering method.  `NNS` iteratively partitions the joint distribution into partial moment quadrants, and then assigns a quadrant identification (1:4) at each partition.

**`NNS.part`** returns a `data.table` of observations along with their final quadrant identification.  It also returns the regression points, which are the quadrant means used in **`NNS.reg`**.
```{r linear}
x = seq(-5, 5, .05); y = x ^ 3

for(i in 1 : 4){NNS.part(x, y, order = i, Voronoi = TRUE, obs.req = 0)}
```


### X-only Partitioning
**`NNS.part`** offers a partitioning based on $x$ values only **`NNS.part(x, y, type = "XONLY", ...)`**, using the entire bandwidth in its regression point derivation, and shares the same limit condition as partitioning via both $x$ and $y$ values.
```{r x part,results='hide'}
for(i in 1 : 4){NNS.part(x, y, order = i, type = "XONLY", Voronoi = TRUE)}
```

Note the partition identifications are limited to 1's and 2's (left and right of the partition respectively), not the 4 values per the $x$ and $y$ partitioning.
```{r res2, echo=FALSE}
NNS.part(x,y,order = 4, type = "XONLY")
```

## Clusters Used in Regression
The right column of plots shows the corresponding regression (plus endpoints and central point) for the order of `NNS` partitioning.
```{r depreg},results='hide'}
for(i in 1 : 3){NNS.part(x, y, order = i, obs.req = 0, Voronoi = TRUE, type = "XONLY") ; NNS.reg(x, y, order = i, ncores = 1)}
```


# NNS Regression `NNS.reg()`
**`NNS.reg`** can fit any $f(x)$, for both uni- and multivariate cases.  **`NNS.reg`** returns a self-evident list of values provided below.

## Univariate:
```{r nonlinear,fig.width=5,fig.height=3,fig.align = "center"}
NNS.reg(x, y, ncores = 1)
```

## Multivariate:
Multivariate regressions return a plot of $y$ and $\hat{y}$, as well as the regression points (`$RPM`) and partitions (`$rhs.partitions`) for each regressor.
```{r nonlinear multi,fig.width=5,fig.height=3,fig.align = "center"}
f = function(x, y) x ^ 3 + 3 * y - y ^ 3 - 3 * x
y = x ; z <- expand.grid(x, y)
g = f(z[ , 1], z[ , 2])
NNS.reg(z, g, order = "max", plot = FALSE, ncores = 1)
```

## Inter/Extrapolation
`NNS.reg` can inter- or extrapolate any point of interest.  The **`NNS.reg(x, y, point.est = ...)`** parameter permits any sized data of similar dimensions to $x$ and called specifically with **`NNS.reg(...)$Point.est`**.


## NNS Dimension Reduction Regression
**`NNS.reg`** also provides a dimension reduction regression by including a parameter **`NNS.reg(x, y, dim.red.method = "cor", ...)`**.  Reducing all regressors to a single dimension using the returned equation **`NNS.reg(..., dim.red.method = "cor", ...)$equation`**.
```{r nonlinear_class,fig.width=5,fig.height=3,fig.align = "center", message = FALSE}
NNS.reg(iris[ , 1 : 4], iris[ , 5], dim.red.method = "cor", location = "topleft", ncores = 1)$equation
```

```{r nonlinear_class2,fig.width=5,fig.height=3,fig.align = "center", message = FALSE, echo=FALSE}
a = NNS.reg(iris[ , 1 : 4], iris[ , 5], dim.red.method = "cor", location = "topleft", ncores = 1, plot = FALSE)$equation
```
Thus, our model for this regression would be:
$$Species = \frac{`r round(a$Coefficient[1],3)`*Sepal.Length `r round(a$Coefficient[2],3)`*Sepal.Width +`r round(a$Coefficient[3],3)`*Petal.Length +`r round(a$Coefficient[4],3)`*Petal.Width}{4} $$


### Threshold
**`NNS.reg(x, y, dim.red.method = "cor", threshold = ...)`** offers a method of reducing regressors further by controlling the absolute value of required correlation.
```{r nonlinear class threshold,fig.width=5,fig.height=3,fig.align = "center"}
NNS.reg(iris[ , 1 : 4], iris[ , 5], dim.red.method = "cor", threshold = .75, location = "topleft", ncores = 1)$equation
```

```{r nonlinear class threshold 2,fig.width=5,fig.height=3,fig.align = "center", echo=FALSE}
a = NNS.reg(iris[ , 1 : 4], iris[ , 5], dim.red.method = "cor", threshold = .75, location = "topleft", ncores = 1, plot = FALSE)$equation
```

Thus, our model for this further reduced dimension regression would be:
$$Species = \frac{\: `r round(a$Coefficient[1],3)`*Sepal.Length + `r round(a$Coefficient[2],3)`*Sepal.Width +`r round(a$Coefficient[3],3)`*Petal.Length +`r round(a$Coefficient[4],3)`*Petal.Width}{3} $$

and the `point.est = (...)` operates in the same manner as the full regression above, again called with **`NNS.reg(...)$Point.est`**.
```{r final,fig.width=5,fig.height=3,fig.align = "center"}
NNS.reg(iris[ , 1 : 4], iris[ , 5], dim.red.method = "cor", threshold = .75, point.est = iris[1 : 10, 1 : 4], location = "topleft", ncores = 1)$Point.est
```


# Classification
For a classification problem, we simply set **`NNS.reg(x, y, type = "CLASS", ...)`**.

**NOTE: Base category of response variable should be 1, not 0 for classification problems.**

```{r class,fig.width=5,fig.height=3,fig.align = "center", message=FALSE}
NNS.reg(iris[ , 1 : 4], iris[ , 5], type = "CLASS", point.est = iris[1 : 10, 1 : 4], location = "topleft", ncores = 1)$Point.est
```


# Cross-Validation `NNS.stack()`
The **`NNS.stack`** routine cross-validates for a given objective function the `n.best` parameter in the multivariate **`NNS.reg`** function as well as the `threshold` parameter in the dimension reduction **`NNS.reg`** version.  **`NNS.stack`** can be used for classification:

**`NNS.stack(..., type = "CLASS", ...)`** 

or continuous dependent variables:

**`NNS.stack(..., type = NULL, ...)`**.

Any objective function `obj.fn` can be called using `expression()` with the terms `predicted` and `actual`, even from external packages such as `Metrics`.

**`NNS.stack(..., obj.fn = expression(Metrics::mape(actual, predicted)), objective = "min")`**.


```{r stack,fig.width=5,fig.height=3,fig.align = "center", message=FALSE, eval=FALSE}
NNS.stack(IVs.train = iris[ , 1 : 4], 
          DV.train = iris[ , 5], 
          IVs.test = iris[1 : 10, 1 : 4],
          dim.red.method = "cor",
          obj.fn = expression( mean(round(predicted) == actual) ),
          objective = "max", type = "CLASS", 
          folds = 1, ncores = 1)
```

```{r stackevalres, eval = FALSE}
Folds Remaining = 0 
Current NNS.reg(... , threshold = 0.9350 ) | eval(obj.fn) = 1.000000 | MAX Iterations Remaining = 2
Current NNS.reg(... , threshold = 0.7950 ) | eval(obj.fn) = 0.973684 | MAX Iterations Remaining = 1
Current NNS.reg(... , threshold = 0.4400 ) | eval(obj.fn) = 0.894737 | MAX Iterations Remaining = 0
Current NNS.reg(. , n.best = 1 ) | eval(obj.fn) = 0.868421 | MAX Iterations Remaining = 12
Current NNS.reg(. , n.best = 2 ) | eval(obj.fn) = 0.736842 | MAX Iterations Remaining = 11
Current NNS.reg(. , n.best = 3 ) | eval(obj.fn) = 0.763158 | MAX Iterations Remaining = 10
Current NNS.reg(. , n.best = 4 ) | eval(obj.fn) = 0.736842 | MAX Iterations Remaining = 9
$OBJfn.reg
[1] 0.9733333

$NNS.reg.n.best
[1] 1

$probability.threshold
[1] 0.495

$OBJfn.dim.red
[1] 0.9666667

$NNS.dim.red.threshold
[1] 0.935

$reg
 [1] 1 1 1 1 1 1 1 1 1 1

$reg.pred.int
NULL

$dim.red
 [1] 1 1 1 1 1 1 1 1 1 1

$dim.red.pred.int
NULL

$stack
 [1] 1 1 1 1 1 1 1 1 1 1

$pred.int
NULL
```

# Increasing Dimensions
Given multicollinearity is not an issue for nonparametric regressions as it is for OLS, in the case of an ill-fit univariate model a better option may be to increase the dimensionality of regressors with a copy of itself and cross-validate the number of clusters `n.best` via:

**`NNS.stack(IVs.train = cbind(x, x), DV.train = y, method = 1, ...)`**.

```{r stack2, message = FALSE,fig.width=5,fig.height=3,fig.align = "center",results='hide', eval = FALSE}
set.seed(123)
x = rnorm(100); y = rnorm(100)

nns.params = NNS.stack(IVs.train = cbind(x, x),
                        DV.train = y,
                        method = 1, ncores = 1)
```

```{r stack2optim, echo = FALSE}
set.seed(123)
x = rnorm(100); y = rnorm(100)

nns.params = list()
nns.params$NNS.reg.n.best = 100
```

```{r stack2res, fig.width=5,fig.height=3,fig.align = "center",results='hide'}
NNS.reg(cbind(x, x), y, 
        n.best = nns.params$NNS.reg.n.best,
        point.est = cbind(x, x), 
        residual.plot = TRUE,  
        ncores = 1, confidence.interval = .95)
```



# Smoothing Option
Smoothness is not required for curve fitting, but the `NNS.reg` function offers an optional smoothed fit. This feature applies a smoothing spline to regression points generated internally using the partitioning method described earlier.

```{r smooth, fig.width=5,fig.height=3,fig.align = "center",results='hide'}
NNS.reg(x, y, smooth = TRUE)
```


# Imputation
Imputation in `NNS` is a direct application of nearest neighbor regression. When values of $y$ are missing, we use the observed $(X,y)$ pairs as the training set and the predictors of the missing rows as `point.est`.

A key insight is that even in univariate regressions, `NNS.reg` benefits from the increasing dimensions trick: by duplicating the predictor into a multivariate form, e.g. `cbind(x, x)`, the distance function underlying `NNS.reg` operates in a 2-D space. This sharpened distance metric allows a more robust donor selection, effectively turning univariate imputation into a special case of multivariate nearest neighbor regression.

For multivariate predictors, the same form applies directly — supply the full set of observed predictors in $x$, the observed responses in $y$, and the incomplete rows in `point.est`. With `order = "max", n.best = 1`, the imputation is always 1-NN donor-based: each missing $y$ is filled in by the response of its closest donor under the `NNS` hybrid distance. This ensures imputations remain strictly within the support of the observed data.

**Categorical data** is handled analogously, only requiring `NNS.reg(..., type = "CLASS")` in the procedure.

## Univariate Imputation

```{r uniimpute, eval=FALSE}
set.seed(123)

# Univariate predictor with nonlinear signal
n <- 400
x <- sort(runif(n, -3, 3))
y <- sin(x) + 0.2 * x^2 + rnorm(n, 0, 0.25)

# Induce ~25% MCAR missingness in y
miss <- rbinom(n, 1, 0.25) == 1
y_mis <- y
y_mis[miss] <- NA

# ---- Increasing dimensions trick ----
# Duplicate x so the distance operates in a 2D space: cbind(x, x).
# This sharpens nearest-neighbor selection even in a nominally univariate setting.
x2_train <- cbind(x[!miss], x[!miss])
x2_miss  <- cbind(x[miss],  x[miss])

# 1-NN donor imputation with NNS.reg
y_hat_uni <- NNS::NNS.reg(
  x         = x2_train,             # predictors (duplicated x)
  y         = y[!miss],             # observed responses
  point.est = x2_miss,              # rows to impute
  order     = "max",                # dependence-maximizing order
  n.best    = 1,                    # 1-NN donor
  plot      = FALSE
)$Point.est

# Fill back
y_completed_uni <- y_mis
y_completed_uni[miss] <- y_hat_uni

# Plot observed vs imputed (NNS 1-NN)
plot(x, y, pch = 1, col = "steelblue", cex = 1.5, lwd = 2,
     xlab = "x", ylab = "y", main = "NNS 1-NN Imputation")
points(x[miss], y_hat_uni, col = "red", pch = 15, cex = 1.3)

legend("topleft",
       legend = c("Observed", "Imputed (NNS 1-NN)"),
       col    = c("steelblue", "red"),
       pch    = c(1, 15),
       pt.lwd = c(2, NA),
       bty    = "n")
```

<center>

![](images/uni_impute.png){width="600" height="600"}

## Multivariate Imputation
```{r multiimpute, eval=FALSE}
set.seed(123)

# Multivariate predictors with nonlinear & interaction structure
n <- 800
X <- cbind(
  x1 = rnorm(n),
  x2 = runif(n, -2, 2),
  x3 = rnorm(n, 0, 1)
)

f <- function(x1, x2, x3) 1.1*x1 - 0.8*x2 + 0.5*x3 + 0.6*x1*x2 - 0.4*x2*x3 + 0.3*sin(1.3*x1)
y <- f(X[,1], X[,2], X[,3]) + rnorm(n, 0, 0.4)

# Induce ~30% MCAR missingness in y
miss <- rbinom(n, 1, 0.30) == 1
y_mis <- y
y_mis[miss] <- NA

# Training (observed) vs rows to impute
X_obs <- X[!miss, , drop = FALSE]
y_obs <- y[!miss]
X_mis <- X[ miss, , drop = FALSE]

# 1-NN donor imputation with NNS.reg
y_hat_mv <- NNS::NNS.reg(
  x         = X_obs,     # all observed predictors
  y         = y_obs,     # observed responses
  point.est = X_mis,     # rows to impute
  order     = "max",     # dependence-maximizing order
  n.best    = 1,         # 1-NN donor
  plot      = FALSE
)$Point.est

# Completed vector
y_completed_mv <- y_mis
y_completed_mv[miss] <- y_hat_mv

# Plot observed vs imputed (multivariate, NNS 1-NN)
plot(seq_along(y), y, 
     pch = 1, col = "steelblue", cex = 1.5, lwd = 2,
     xlab = "Observation index", ylab = "y",
     main = "NNS 1-NN Multivariate Imputation")

# Overlay imputed values
points(which(miss), y_hat_mv, pch = 15, col = "red", cex = 1.2)

# Legend
legend("topleft",
       legend = c("Observed", "Imputed (NNS 1-NN)"),
       col    = c("steelblue", "red"),
       pch    = c(1, 15),
       pt.lwd = c(2, NA),
       bty    = "n")
```

<center>

![](images/multi_impute.png){width="600" height="600"}

## A Note on Uncertainty Propagation

A common concern with local imputation methods is whether imputation uncertainty propagates correctly into downstream inference. `NNS` addresses this through bootstrap multiple imputation: resampling complete cases across `m` iterations generates between-imputation variance that flows through standard Rubin's rules pooling identically to any classical procedure.

Empirically, `NNS` bootstrap MI outperforms MICE with predictive mean matching on nonlinear data — producing a pooled estimate closer to the true parameter with a smaller pooled SE. The advantage comes not from compressing uncertainty but from a more accurate imputation model, which reduces between-imputation variance driven by model error rather than genuine data uncertainty.

See [NNS Multiple Imputation vs MICE](https://github.com/OVVO-Financial/NNS/blob/NNS-Beta-Version/examples/NNS_MI_vs_MICE.md) for the full reproducible comparison.


# References
If the user is so motivated, detailed arguments further examples are provided within the following:

-  [Nonlinear Nonparametric Statistics: Using Partial Moments](https://ovvo-financial.github.io/NNS/book/)

-  [Deriving Nonlinear Correlation Coefficients from Partial Moments](https://doi.org/10.2139/ssrn.2148522)

-  [Nonparametric Regression Using Clusters](https://doi.org/10.1007/s10614-017-9713-5)

-  [Clustering and Curve Fitting by Line Segments](https://doi.org/10.2139/ssrn.2861339)

-  [Classification Using NNS Clustering Analysis](https://doi.org/10.2139/ssrn.2864711)

-  [Partitional Estimation Using Partial Moments](https://doi.org/10.2139/ssrn.3592491)


