read.csv parameter stringsAsFactors not working?

Question

Based on the documentation read.csv, the parameter stringsAsFactors, when set, should cause quoted data values to be interpreted as factors. Consider the following data file, which we will call test.csv.

"a",b,c 
"1",2,3 
"3",2,3

When I try to read this data using read.csv, it does not appear to parse the first column as a factor.

foo = read.csv("test.csv", stringsAsFactor=T)
is.factor(foo$a)

Output:

[1] FALSE

I tried to use the column name without quotes, but that did not work either. How can I correct this?

As far as I know, `T==TRUE`. Also, where am I missing an `s`? — merlin2011, Nov 21 '13 at 06:36
in the argument name of `stringsAsFactors`, not `stringsAsFactor` — alittleboy, Nov 21 '13 at 06:37
@alittleboy, surprisingly, that `s` makes no difference at all. — merlin2011, Nov 21 '13 at 06:40
Read about argument matching for why that doesn't matter: http://cran.r-project.org/doc/manuals/r-release/R-lang.html#Argument-matching — Thomas, Nov 21 '13 at 06:46
Also, it's worth noting that the default behaviour for read.csv (which is a wrapper for read.table) is to try to coerce characters to numerics. See `? type.covert` which is called by read.csv as it tries to assign an appropriate type to your columns. — Ryan Walker, Nov 21 '13 at 06:50

score 5 · Accepted Answer · answered Nov 21 '13 at 06:36

5

Your example data are coercible to numeric. Try with data that are not so coercible:

foo <- read.csv(text='"a",b,c 
"1",2,3 
"3",2,3 
"a",2,3 ', stringsAsFactors=TRUE)
> foo$a
# [1] 1 3 a
# Levels: 1 3 a

Otherwise use colClasses:

foo <- read.csv(text='"a",b,c 
"1",2,3 
"3",2,3 ', colClasses=c('factor','numeric','numeric'))
> foo$a
# [1] 1 3
# Levels: 1 3

Or you could convert using as.factor after reading the data in.

answered Nov 21 '13 at 06:36

Thomas

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Can I force data that is coercible to numeric to be treated as factors? – merlin2011 Nov 21 '13 at 06:37
@merlin2011: Yes, by overriding the individual column types with the `columnClasses` argument, as Thomas shows. – smci Nov 27 '18 at 23:52

read.csv parameter stringsAsFactors not working?

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