get mean form a column in a csv file on R - r

Question

I am a very beginner user of R. I am taking the Coursera R programming course and I am stuck in a homework (the pollutant mean homework). The objective of the homework is to obtain means from columns in csv files. The files have four columns. We have 300+ files and each has 1000+ observations. Most of them Are NA. In the csv file I am working with there are only 117 numeric observations. I have been trying stuff like this:
cmydata1 <- read.csv("/Users/joshuavincent/Documents/specdata/001.csv")
Once I had cmydata1, I tried to get the mean of one of the columns, "nitrate" but I got this:
> mean(cmydata1, "nitrate")
[1] NA
Warning message:
In mean.default(cmydata1, "nitrate") :
argument is not numeric or logical: returning NA
To solve it I created a new list like this:
> cmydata2 <- list(na.omit(cmydata1))
> cmydata2[[1]]
The outcome is the cleaned matrix, no NA anymore
The column names are: "Date" "sulfate" "nitrate" and ID.
However, I still can't get the mean
> mean(cmydata2, "nitrate")
[1] NA
Warning message:
In mean.default(cmydata2, "nitrate") :
argument is not numeric or logical: returning NA
I try to fix it, so I type... and get null
> colnames(cmydata2)
NULL
So, what could I fix to get the mean from that column? (Afterwards I think have to try loops and stuff to finish the homework, but I am going very baby steps towards it)
Note that might help: I have cmydata1 with a table icon in the autofill, while cmydata2 has some shapes, seems like an organigram icon.
Thanks

Answer 1

This is a rather simple question and you should probably just reference other questions that have been asked before. However, to try to answer, you reference columns in dataframes in two main ways listed out below (although there are other ways).
data(mtcars) #calling in some data that is stored in R already
##METHOD 1##
mean(mtcars$mpg,na.rm=T) #the 'na.rm=T' is to remove missing values before calculating the mean
20.09062
##METHOD 2##
mean(mtcars[,'mpg'],na.rm=T)
20.09062