Vectorization

README.org

@@ -17,6 +17,10 @@ data using the R programming environment
 
  * [[./script/extractpdf.R][Reading pdf files]]
 
+** Vectorization
+
+ * [[./script/vectorization.R][Vectorization]]
+
 ** Iteration
 
  * [[./script/iteration.R][Lapply, apply and for loop: brief introduction]]

script/vectorization.R

@@ -0,0 +1,100 @@
+#' ---
+#' title: "What is vectorization in R?"
+#' date: "2021-11-03"
+#' author: "Jose https://ajuda.multifarm.top"
+#' output:
+#'   html_document:
+#'    code_folding: show
+#'    toc: yes
+#'    toc_float:
+#'      smooth_scroll: true
+#'    df_print: paged
+#'    highlight: zenburn
+#' ---
+
+
+#' One operation that is slow in R, and somewhat slow in all languages, is memory allocation. So one of the slower ways to write a for loop is to resize a vector repeatedly, so that R has to re-allocate memory repeatedly, like this:
+
+j <- 1
+
+system.time(for (i in 1:10) {
+                j[i] = 10
+            })
+
+n <- 1:10
+
+j <- 1
+
+system.time(for (i in seq_along(n)) {
+        j[i] = 10
+    })
+
+fxn <- function(j){
+    for (i in 1:10) {
+        j[i] = 10
+    }
+    return(j)
+}
+
+system.time(fxn(j))
+
+#' Here, in each repetition of the for loop, R has to re-size the vector and re-allocate memory. It has to find the vector in memory, create a new vector that will fit more data, copy the old data over, insert the new data, and erase the old vector. This can get very slow as vectors get big.
+
+#' If one pre-allocates a vector that fits all the values, R doesn’t have to re-allocate memory each iteration, and the results can be much faster. Here’s how you’d do that for the above case:
+
+j <- rep(NA, 10)
+
+system.time(for (i in 1:10) {
+                j[i] = 10
+            })
+
+j <- rep(NA, 10)
+
+system.time(for (i in seq_along(1:10)) {
+                j[i] = 10
+            })
+
+## There are still situations that it may make sense to use for loops instead of vectorized functions, though. These include:
+
+##     Using functions that don’t take vector arguments
+##     Loops where each iteration is dependent on the results of previous iterations
+
+## Note that the second case is tricky. In some cases where the obvious implementation of an algorithm uses a for loop, there’s a vectorized way around it. For instance, here is a good example of implementing a random walk using vectorized code. In these cases, you often want to call functions that are essentially C/FORTRAN implementations of loop operations to avoid the loop in R. Examples of such functions include cumsum (cumulative sums), rle (counting number of repeated value), and ifelse (vectorized if…else statements).
+
+#' ## Using rle
+#' Compute the lengths and values of runs of equal values in a vector
+#  - or the reverse operation.
+
+#' Building data
+
+x <- c("952345172", "alju12", "amou79", "amou91", "baab81", NA)
+
+code <- rep(x, c(5, 10, 10, 20, 2, 7))
+
+df <- data.frame(id = 1:length(code), code)
+
+rle_code <- rle(df$code)
+
+class(rle_code)
+
+attributes(rle_code)
+
+rle_code$values
+
+rle_code$lengths
+
+rle_code$values > 6
+
+rle_code[rle_code$lengths > 6]
+
+rle_code[[1]] > 6
+
+inverse.rle(rle_code)
+
+#' ## Using 'cumsum'
+#'
+
+
+#' ## Using 'ifelse'
+
+?do.call