R: Remove character observations in a variable

I have a a variable in a dataframe whose observations are a mix of numeric and character values (due to faulty data entry). How can I subset开发者_开发技巧 in only the observations which are numeric? Suppose the values of filename$varname are (1, 2, 1, 5, 3, a, 3, d, 1), I would like subset out "a" and "d" and keep only the rest of the values which are numeric.

---

You can make use of the fact that as.numeric will convert character strings to NA whilst keeping numeric data:

x <- c(1, 2, 1, 5, 3, "a", 3, "d", 1)
as.numeric(x)

[1]  1  2  1  5  3 NA  3 NA  1
Warning message:
NAs introduced by coercion 

Now use is.na to test for NA values and exclude these using vector subsetting:

y <- as.numeric(x)
y[!is.na(y)]
[1] 1 2 1 5 3 3 1

---

Without a reproducible example it is hard to see what your data actually looks like. For instance, is the column of your data frame a factor or just strings? If it is just strings then Andrie's answer works (just use as.numeric()), and if the data is a factor you first need to convert that to strings with as.character(x):

as.numeric(as.character(filename$varname))

You will get some NAs but that is absolutely fine as those values are indeed missing.

EDIT: To clarify abit more. You have a data frame, so you don't want to take values out of the data frame as then it wouldn't be a dataframe anymore (equal rows). You want to correctly assign NA for missing values instead as most statistical functions in R can handle them.