# Flat indices for arrays in R/Rcpp

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Although **3-dimensional** arrays are not the most common object used among
the R projects, which are dominated by `data.frame`

-like objects. However, when
we’re starting to work with **deep learning**, (e.g. using `{keras}`

),
we can run into such objects many times, especially in fields like **time series forecasting** or **NLP**.

The question I’d like to answer in this post is how to find ‘flat’ equivalent of the
three-element index for the **3-dimensional** arrays.

## Problem

Let’s create a sample array to present the problem.

data <- array(0, c(20, 7, 5))

To get a single element, an array can be subscripted in two ways:

**using indices for all the existing dimensions**

data[12, 3, 1] <- 7 data[12, 3, 1] ## [1] 7

**using a single index**

In this approach the array is treated as a **flat vector**, so I named this kind of
indexing **flat index**.

data[123] <- 8 data[123] ## [1] 8

**But how we can easily tranform the first type of indexing into the second one?**

## Solution

The solution generalized to the $n$-dimensional case can be expressed as:

where $x_i$ means i-th index and $d_i$ i-th dimension size. This solution takes into account the 1-based indexing which is used in R.

## Example

Suppose we have an array with the same dimesnions as shown above: $(20, 7, 5)$. We’d like to access an element at index $(11, 3, 2)$.

example <- array(0, c(20, 7, 5)) example[11, 3, 2] <- 7

We calculate the *flat index* according to the aforementioned schema.

flat_idx <- 11 + (3 - 1) * 20 + (2 - 1) * 20 * 7 example[flat_idx] ## [1] 7

## Code snippets

In R code;

#' Get an index you can use access an array element at once #' [x, y, z] = x + (y - 1) * x_dim + (z - 1) * x_dim * y_dim #' [x, y] = x + (y-1) * x_dim #' #' @param dim_sizes Dimensions sizes #' @param dim_indices Indices flat_index <- function(dim_sizes, dim_indices){ dim_indices[1] + sum((dim_indices[-1] - 1) * cumprod(dim_sizes[-length(dim_sizes)])) } # Example 1 arr <- array(0, c(4,5,6,7)) arr[1,2,3,4] <- 777 flat_index(c(4,5,6,7), c(1,2,3,4)) ## [1] 405 which(arr == 777) ## [1] 405 # Example 2 arr2 <- array(0, c(32,10,5)) arr2[12,8,4] <- 777 flat_index( c(32,10,5), c(12,8,4)) ## [1] 1196 which(arr2 == 777) ## [1] 1196

In **Rcpp**, you can use the following code snippet (for 3-dimensional arrays):

// In C++ transformed to the zero-based index int flat_index(int dim1, int dim2, int idx1, int idx2, int idx3){ return idx1 + idx2 * dim1 + idx3 * dim1 * dim2; }

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