---
title: "Using apply in R"
output: html_notebook
author: Martin Shepperd
date: 23/10/2020
---

This is an [R Markdown](http://rmarkdown.rstudio.com) Notebook that introduces the apply family of functions for easily and efficiently processing lists.


```{r}
# Initialisation
data(mtcars)
View(mtcars)
```

Using the generic summary function

```{r}
summary(mtcars)
```

If we want to control our analysis more precisely or want something more specific than just applying `summary(mtcars)` we can use the very useful family of **apply() functions**.  This allows to apply a function e.g., `sd()` repeatedly to everything (every vector) in the list.  This can be very convenient e.g., a data frame is a list of vectors so if we give a data frame as an argument to the apply function it will apply the specified function to every vector (ie every column) in the data frame.

Imagine we want the standard deviation (sd) of every variable/column/vector of our data frame.  For this task we can use sapply.

```{r}
# Using sapply which tries to nicely format the result but is otherwise similar to lapply.
sapply(mtcars, sd)
```

What if we want several statistics or something not built in to R?  Then we can use a user-defined function.

```{r}
# Defining a new function
mySummary <- function(x)
# Compute the sd and median of x and return this as a list
{
  summaryList <- list(sd(x), median(x)) 
  names(summaryList) <- c("SD", "Median")
  return(summaryList)
}  
```