[R] Various newbie questions

[Philippe de Rochambeau]

Hello,

1) What is the difference between a "data frame" (J H Maindonald, Using 
R, p. 12) and a "vector"?

In Using R, the author asks the reader to enter the following data in a 
data frame, which I will call "mydata":

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].

hist(mydata[1,2]) does not work. How would you create a histogram with 
the above data?

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?

4) How do you set class limits in R, for instance

10-20
21-31
etc.

5) How do you determine quartiles in R? Is there a way to determine the 
"semi-inter-quartile deviation" ("écart semi-inter-quartile" in 
French)?

Many thanks!

Philippe de Rochambeau