Scott: /* Read data from a file */

2011-09-16T23:40:07Z

Read data from a file

← Older revision		Revision as of 23:40, 16 September 2011
Line 30:		Line 30:

	Alternate commands:		Alternate commands:
	'''<code>read.csv</code>''' assumes a header row, comma-separated, quoted		* '''<code>read.csv</code>''' assumes a header row, comma-separated, quoted
	'''<code>read.delim</code>''' assumes a header row, tab-delimited, quoted		* '''<code>read.delim</code>''' assumes a header row, tab-delimited, quoted
	'''<code>read.fwf</code>''' works for fixed-width files		* '''<code>read.fwf</code>''' works for fixed-width files
	'''<code>readLines</code>''' gives you one string per line		* '''<code>readLines</code>''' gives you one string per line
	'''<code>scan</code>''' lets you read data directly into a pre-defined data structure		* '''<code>scan</code>''' lets you read data directly into a pre-defined data structure

			To read a file containing a single column of numbers into an array:
			<pre>
			cadence = scan('cadence.txt')
			hist(cadence)
			</pre>

	== Write data to a file ==		== Write data to a file ==

Scott: /* Connect to a database */

2011-02-04T23:23:41Z

Connect to a database

New page

== Save and load native R-format files ==
<pre>
save(top.5.salaries,file="~/top.5.salaries.RData")
load("~/top.5.salaries")
</pre>

== Read data from a file ==
Sample data file:
<pre>
name.last,name.first,team position,salary
"Manning","Peyton","Colts","QB",18700000
"Brady","Tom","Patriots","QB",14626720
"Pepper","Julius","Panthers","DE",14137500
"Palmer","Carson","Bengals","QB",13980000
"Manning","Eli","Giants","QB",12916666
</pre>
Read:
<pre>
top.5.salaries = read.table("top.5.salaries.csv", header=TRUE, sep=",", quote="\"")
</pre>
Options:
* header: default is no header row
* sep: whitespace is default separator
* quote: none is default
* row.names: A character vector containing row names for the returned data frame
* nrows: number of rows to read (default is all)
* strip.white: default is to not strip white space
* blank.lines.skip: default is to skip blank lines
* comment.char: ignores lines that begin with # by default

Alternate commands:
'''<code>read.csv</code>''' assumes a header row, comma-separated, quoted
'''<code>read.delim</code>''' assumes a header row, tab-delimited, quoted
'''<code>read.fwf</code>''' works for fixed-width files
'''<code>readLines</code>''' gives you one string per line
'''<code>scan</code>''' lets you read data directly into a pre-defined data structure

== Write data to a file ==
<pre>
write.table(top.5.salaries, "top.5.salaries.txt")
</pre>
Also '''<code>write.csv</code>''' and '''<code>write.delim</code>'''.

== Read data from a web page ==
<pre>
sp500 = read.csv(paste("http://ichart.finance.yahoo.com/table.csv?",
"s=%5EGSPC&a=03&b=1&c=1999&d=03&e=1&f=2009&g=m&ignore=.csv"))
</pre>

== Connect to a database ==
Use the RMySQL package to connect to MySQL: http://rss.acs.unt.edu/Rdoc/library/RMySQL/html/RMySQL-package.html

Install:
<pre>
install.packages("RMySQL")
</pre>

Connect:
<pre>
library(RMySQL)
connection = dbConnect(MySQL(), host="localhost", dbname="sp5", user="barney", password="XXXXX")
</pre>

Import and export data.frames:
<pre>
d = dbReadTable(con, "WL")
dbWriteTable(con, "WL2", a.data.frame) ## table from a data.frame
dbWriteTable(con, "test2", "~/data/test2.csv") ## table from a file
</pre>

Run an arbitrary SQL statement and extract all its output (returns a data.frame):
<pre>
dbGetQuery(con, "select count(*) from a\_table")
dbGetQuery(con, "select * from a\_table")
</pre>

Disconnect
<pre>
dbDisconnect(connection)
</pre>

R I/O - Revision history

Scott: /* Read data from a file */

Scott: /* Connect to a database */