Hi Rich, I do not know what u really want, because it seems to me, u want to calculate the mean of all rows, where the chemical is Arsenic??
But try this to get a little more inside: mean(chemdata$quant[chemdata$param=="Arsenic"]) The vector chemdata[chemdata$param=="Arsenic",] is a logical vector, returning TRUE for every row in which the variable param takes the value "Arsenic". Try it in your R editor to see it and understand the R concept! If u now want to get all values of a certain column, given all values have "Arsenic" as param, u just write: chemdata$COLUMNNAME[chemdata$param=="Arsenic"] I do not know if this helps, as it seems to me, that Arsenic only occurs once in your frameā¦.. Good luck Simon On Aug 30, 2011, at 11:00 PM, Rich Shepard wrote: > I don't find how to do what I need to do in Dalgaard or 'R Cookbook', so > I'm asking here. > > I have a data frame with water chemistry data and I want to start > exploring these data. There are three factors (site, date, chemical) > associated with each measurement. The data frame looks like this: > >> summary(chemdata) > site_id.sample_date.param.quant > BC-0.5|1996-04-19|Arsenic|0.01 : 1 > BC-0.5|1996-04-19|Calcium|76.56 : 1 > BC-0.5|1996-04-19|Chloride|12 : 1 > BC-0.5|1996-04-19|Magnesium|43.23 : 1 > BC-0.5|1996-04-19|Sulfate|175 : 1 > BC-0.5|1996-04-19|Total Dissolved Solids|460: 1 > (Other) :14880 > > I want first to calculate (and plot) descriptive stats by chemical, > ignoring site and date and telling R to ignore missing data. (Incorporating > those factors will occur later.) What I have not been able to figure out is > how to specify the command to, for example, calculate mean and sd for > Arsenic. My floundering and thrashing includes attempts like these: > >> mean(chemdata.param="Arsenic") > Error in is.numeric(x) : 'x' is missing >> mean(chemdata.quant, param="Arsenic") > Error in mean(chemdata.quant, param = "Arsenic") : > object 'chemdata.quant' not found >> mean(chemdata$quant, param="Arsenic") > [1] NA > Warning message: > In mean.default(chemdata$quant, param = "Arsenic") : > argument is not numeric or logical: returning NA > > As a newcomer to R I've done a lot of reading, yet all the examples use > nicely structured data to illustrate the point being made. I need to work > with my data and learn how to specify columns and write correct commands for > the analyses I need. Please point me in the right direction. > > Rich > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.