Update influenza-like-illness-analysis.Rmd

module3/exo1/influenza-like-illness-analysis.Rmd

@@ -21,7 +21,17 @@ knitr::opts_chunk$set(echo = TRUE)
 
 ## Data preprocessing
 
-The data on the incidence of influenza-like illness are available from the Web site of the [Réseau Sentinelles](http://www.sentiweb.fr/). 
+The data on the incidence of influenza-like illness are available from the Web site of the [Réseau Sentinelles](http://www.sentiweb.fr/). We download them as a file in CSV format, in which each line corresponds to a week in the observation period. Only the complete dataset, starting in 1984 and ending with a recent week, is available for download. The URL is:	The data on the incidence of influenza-like illness are available from the Web site of the [Réseau Sentinelles](http://www.sentiweb.fr/). 
+```{r}	
+data_url = "http://www.sentiweb.fr/datasets/incidence-PAY-3.csv"	
+```	
+In order to protect us in case the Réseau Sentinelles Web server disappears or is modified, we make a local copy of this dataset that we store together with our analysis. It is unnecessary and even risky to download the data at each execution, because in case of a malfunction we might be replacing our file by a corrupted version. Therefore we download the data only if no local copy exists.	
+```{r}	
+data_file = "incidence-PAY-3.csv"	
+if (!file.exists(data_file)) {	
+    download.file(data_url, data_file, method="auto")	
+}	
+```
 
 This is the documentation of the data from [the download site](https://ns.sentiweb.fr/incidence/csv-schema-v1.json):
 
@@ -42,7 +52,7 @@ This is the documentation of the data from [the download site](https://ns.sentiw
 
 The first line of the CSV file is a comment, which we ignore with `skip=1`.
 ```{r}
-data = read.csv("incidence-PAY-3.csv", skip=1,na.strings = "-")
+data = read.csv(data_file, skip=1,na.strings = "-")
 ```
 
 Let's have a look at what we got: