reshape2.Rmd

---
title: "reshape2"
date: 2017-03-01T13:07:39+01:00
draft: false
image: "reshape.jpg"
categories: ["R"]
tags: ["R", "data.table", "dataframes"]
---

## 1. What is reshape2 and why would you use it?

`reshape2` is an R package that let's you change the shape of any dataframe, i.e. to pivot and to "unpivot".

Keep in mind that if your favourite R package for dataframes manipulation is [data.table](http://tomis9.com/data.table), functions *dcast* and *melt* are already in this package and work exactly the same as those in `reshape2`.

## 2. A "Hello World" example

In fact there are only two functions worth mentioning: *dcast*, which is equivalent to MS Excel pivot table, and *melt*, which does the opposite or unpivots a table.

An example dataframe:
```{r}
d <- data.frame(
  account_no = paste(rep(7, 5), 1:5, sep=""),
  Jan = rnorm(5, 10, 1),
  Feb = rnorm(5, 10, 2),
  Mar = rnorm(5, 10, 3)
)
print(d)
```

Transormation into a normalized table (unpivot):
```{r}
dn <- reshape2::melt(
  data = d, 
  id.vars = "account_no", 
  variable.name = "month", 
  value.name = "revenue"
)
print(dn)
```

And back to the previous format using a pivot:
```{r}
reshape2::dcast(
  data = dn, 
  formula = account_no ~ month, 
  value.var = "revenue"
)
```

## 3. Links

A pretty nice and much longer tutorial is available [here](https://seananderson.ca/2013/10/19/reshape/).