Various newbie questions

Hi,

-----Original Message-----
From:	Philippe de Rochambeau [SMTP:philippe at wwphi.net]
Sent:	Wednesday, February 04, 2004 12:33 PM
To:	r-help at stat.math.ethz.ch
Subject:	[R] Various newbie questions

year snow.cover
1970 6.5
1971 12.0
1972 14.9
1973 10.0
1974 10.7
1975 7.9
...

mydata=data.frame(year=c(1970,...),snow.cover=c(6.5,...))

2) How to you retrieve say, snow.cover's second data item? mydata[1][2] 
does not work, neither does mydata[1,2].

year <- 1970:1975
	snow.cover <- c(6.5,12,14.9,10,10.7,7.9)
	mydata <- data.frame(cbind(year,snow.cover))

	# retrieving 2nd column:
	mydata[,2]

	# retrieving 3rd row:
	mydata[3,]

	# snow.cover's second data-item:
	mydata[2,2]
	# or:
	mydata$snow.cover[2]

In a French statistics book, the author provides the following data:

Group A	Number: 35	Mean:27
Group B  Number: 42  Mean:24

and asks: "what is the mean of the group constituted by the reunion of 
the two groups?"

The answer is of course (27 x 35) + (24 x 42) / 77

3) Is there a way to compute this mean in R (apart from doing the above 
operation, of course) if you have two sets of data?

numbers <- c(35,42)
	arithmeans <- c(27,24)

	weighted.mean(arithmeans,numbers)


	Just a general remark: the first thing you should do (as is written
in the R-Posting-Guide) is consulting the available online help of R.
	For example, you could use
	?vector
	?data.frame
	to get an answer for your first question.
	If you don't know actually what you are looking for, you can also
enter a search string and, optionally, the package.
	In the case of question 3, you could have written:
	help.search("mean", package="base")
	and R would have returned several commands - among them also
"weighted.mean"

	Best,
	Roland



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