R data selection using sapply not working

Question

I have following code for getting the pollutant mean. The data for the program are in folder specdata with names "001.csv", "002.csv", ... to "332.csv" . The names of any one data gives following names:

[1] "Date"    "sulfate" "nitrate" "ID"

In the code below, I have to calculate the mean of pollutant nitrate or sulfate . I think the code is correct. But data$pollutant is giving

NULL

Error in pollutantmean("specdata", "sulfate", 23) : attempt to apply non-function

The code is supposed to call in following way:

pollutantmean("specdata", "nitrate", 23)

What am I doing wrong here??

pollutantmean <- function(directory, pollutant, id = 1:332) {
  f <- function(num){
    if(num>=0 & num<=9){
      fname <- paste('specdata/00',as.character(num),".csv",sep="")
    }
    else if (num>=10 & num <=99){
      fname <- paste('specdata/0',as.character(num),".csv",sep="")
    }
    else{
      fname <- paste('specdata/',as.character(num),".csv",sep="")
    }
    data <- read.csv(fname)
    data <- data[complete.cases(data),]
    return(mean(data$pollutant))
  }
  results <- sapply(id, f)
  return(results)

}

Not related to your question, but useful hint: run `sprintf("%03.f", 1:322)`. — tonytonov, May 16 '14 at 06:07
ah thanks! this could have saved me in looping above. I learning R. This is good and shortcut method. Thanks — Mahadeva, May 16 '14 at 06:10
Minor point: you also hard-code `"specdata/"` and never use `directory`. — r2evans, May 16 '14 at 14:04

score 0 · Accepted Answer · answered May 16 '14 at 06:14

0

The error you have is most probably caused by calling data$pollutant. You have pollutant="nitrate", which is a character string, in that case reference by $ is not going to work. Use [] instead, here's a minimal example:

df <- data.frame(hello=1:3, world=5)
name <- "hello"
df$name
NULL
df[name]
#  hello
#1     1
#2     2
#3     3

answered May 16 '14 at 06:14

tonytonov

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Thanks, this was the problem. But check that df$"hello" still gives the data. So, I was getting the confusion. – Mahadeva May 16 '14 at 06:20
Please tell me how df$"hello" gives the answer but df$name doesn't – Mahadeva May 16 '14 at 06:24
`df$name` is interpreted as "search column names for something like 'name'", so there is no actual lookup for variable `name`. See [this](http://stackoverflow.com/questions/18222286/select-a-specific-column-using-the-dollar-sign-in-r/18228613#18228613) answer for more details. – tonytonov May 16 '14 at 06:30

R data selection using sapply not working

1 Answers1