Creating New Variables in R that relate to - r

Question

I have 7 different variable in an excel spreadsheet that I have imported into R. They each are columns with a size of 3331. They are:
'Tribe' - there are 8 of them
'Month' - when the sampling was carried out
'Year' - the year when the sampling was carried out
'ID" - an identifier for each snail
'Weight' - weight of a snail in grams
'Length' - length of a snail shell in millimetres
'Width' - width of a snail shell in millimetres
This is a case where 8 different tribes have been asked to record data on a suspected endangered species of snail to see if they are getting rarer, or changing in size or weight.
This happened at different frequencies between 1993 and 1998.
I would like to know how to be able to create a new variables to the data so that if I entered names(Snails) # then it would list the 7 given variables plus any added variable that I have.
The dataset is limited to the point where I would like to add new variables. Such as, knowing the counts per month of snails in any given month.
This would rely on me using - Tribe,Month,Year and ID. Where if an ID (snail identifier) was were listed according to the rates in any given month then I would be able to sum them to see if there are any changes in counts. I have tried:
count=c(Tribe,Year,Month,ID)
count
But, after doing things like that, R just has a large list of that is 4X the size of the dataset. I would like to be able to create a given new variable that is of column size n=3331.
Or maybe I would like to create a simpler variable so I can see if a tribe collected at any given month. I don't know how I can do this.
I have looked at other forums and searched but, there is nothing that I can see that helps me in my case. I appreciate any help. Thanks

Answer 1

I'm guessing you need to organise your variables in a single structure, such as a data.frame.
See ?data.frame for the help file.
To get you started, you could do something like:
snails <- data.frame(Tribe,Year,Month,ID)
snails
# or for just the first few rows
head(snails)
Then this would have your data looking similar to your Excel file like:
Tribe Year Month ID
1 1 1 1 a
2 2 2 2 b
3 3 3 3 c
<<etc>>
Then if you do names(snails) it will list out your column names.
You could possibly avoid some of this mucking about by just importing your Excel file either directly from Excel, or saving as a csv (comma separated values) file first and then using read.csv("name_of_your_file.csv")
See http://www.statmethods.net/input/importingdata.html for some more specifics on this.
To tabulate your data, you can do things like...
table(snails$Tribe)
...to see the number of snail records collected by each tribe. Or...
table(snails$Tribe,snails$Year)
...to see the trends in each tribe by each year. The $ character will let you access the named variable (column) inside a data.frame in the same way you are currently using the free floating variables. This might seem like more work initially, but it will pay off greatly when you need to do some more involved analysis.
Take for example if you want to only analyse the weights from tribe "1", you could do:
snails$Weight[snails$Tribe==1]
# mean of these weights
mean(snails$Weight[snails$Tribe==1])
There are a lot more things I could explain but you would probably be better served by reading an excellent website like Quick-R here: http://www.statmethods.net/management/index.html to get you doing some more advanced analysis and plotting.