r - Converting DF columns to factor is less than straightforward -

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i'm learning r , welcome explanation/pointers error means , why can't assign columns factors:

in r, read.csv() file variable df. class(df) tells me it's "data.frame" columns 1, 2, , 3 non-factors. when try assign columns 1,2,3 factors error:

asfactors <- c(1:3) df[asfactors] <- as.factors(df[asfactors]) # same if use df[,asfactors]  error in sort.list(y) : 'x' must atomic 'sort.list' have called 'sort' on list? 

what sort have me trying change variable type?

instead, seem need use apply() function convert columns factors (but isn't intuitive thing):

df[,asfactors] <- lapply(df[asfactors], factor) 

furthermore, if try convert columns 4,5,6 numeric using lapply new error:

asnumeric <- c(4:6) df[,asnumeric] <- lapply(df[asnumeric], numeric)  error in fun(x[[i]], ...) : invalid 'length' argument 

and if fall on original attempt, get:

df[,asnumeric] <- as.numeric(df[,asnumeric]) error: (list) object cannot coerced type 'double' 

so each variable type seems= need different method of converting columns, or haven't found 1 method applies of them.

to change multiple columns factor, use:

df[,1:3] <- lapply(df[,1:3], factor) 

to change factor numeric, remember use as.numeric(as.character(x)), this:

df[,1:3] <- lapply(df[,1:3], function(x) as.numeric(as.character(x)))