diff --git a/README.org b/README.org
index 8b6a45f..be90303 100755
--- a/README.org
+++ b/README.org
@@ -16,3 +16,7 @@ data using the R programming environment
 ** Reading data
 
  * [[./script/extractpdf.R][Reading pdf files]]
+
+** Iteration
+
+ * [[./script/iteration.R][Lapply, apply and for loop: brief introduction]]
diff --git a/script/iteration.R b/script/iteration.R
new file mode 100755
index 0000000..2a555b5
--- /dev/null
+++ b/script/iteration.R
@@ -0,0 +1,52 @@
+#' ---
+#' title: "Iteration in R"
+#' date: "2021-12-04"
+#' author: "Jose https://ajuda.multifarm.top"
+#' output:
+#'   html_document:
+#'    code_folding: show
+#'    toc: yes
+#'    toc_float:
+#'      smooth_scroll: true
+#'    df_print: paged
+#'    highlight: zenburn
+#' ---
+
+#' remove objects
+#'
+objects()
+
+rm(list = ls())
+
+#' data
+
+data(sleep)
+
+str(sleep)
+
+#' ## for loop
+#'
+#' for( var in seq ) expr
+#' Where 'var' us a name and 'seq' is a vector and 'expr' an expression
+
+for(j in names(sleep)) print(class(names(sleep)))
+
+sapply(sleep, class)
+
+lapply(sleep, class)
+
+## The language has other iteration operators (while() and repeat), and
+## the usual conditional operators (if ... else).
+
+#' Check package 'SoDA'
+
+#' ## References
+#'
+#' John M. Chambers. Software for Data Analysis: Programming with R.
+## @book{chambers2008software,
+##   title={Software for data analysis: programming with R},
+##   author={Chambers, John M},
+##   volume={2},
+##   year={2008},
+##   publisher={Springer}
+## }