Delete rows with negative values

Question

In R I am trying to delete rows within a dataframe (ants) which have a negative value under the column heading Turbidity. I have tried

ants<-ants[ants$Turbidity<0,]

but it returns the following error:

Warning message:
In Ops.factor(ants$Turbidity, 0) : < not meaningful for factors

Any ideas why this may be? Perhaps I need to make the negative values NA before I then delete all NAs?

Any ideas much appreciated, thank you!

@Joris: result is

str(ants$Turbidity)

num [1:291] 0 0 -0.1 -0.2 -0.2 -0.5 0.1 -0.4 0 -0.2 ...

Answer 1

Marek is right, it's a data problem. Now be careful if you use [as.numeric(ants$Turbidity] , as that one will always be positive. It gives the factor levels (1 to length(ants$Turbidity)), not the numeric factors.

Try this :

tt <- as.numeric(as.character(ants$Turbidity))
which(!is.na(tt))

It will give you a list of indices where the value was not numeric in the first place. This should enable you to first clean up your data.

eg:

> Turbidity <- factor(c(1,2,3,4,5,6,7,8,9,0,"a"))
> tt <- as.numeric(as.character(Turbidity))
Warning message:
NAs introduced by coercion 
> which(is.na(tt))
[1] 11

You shouldn't use the as.numeric(as.character(...)) structure to convert problematic data, as it will generate NA's that will mess with the rest. Eg:

> Turbidity[tt > 5]
[1] 6    7    8    9    <NA>
Levels: 0 1 2 3 4 5 6 7 8 9 a