删除某些列值与另一列不匹配的行（全部在同一数据框中）

Question

I'm attempting to remove all of the rows (cases) within a data frame in which a certain column's value does not match another column value.

The data frame bilat_total contains these 10 columns/variables:

bilat_total[,c("year", "importer1", "importer2", "flow1", 
                              "flow2", "country", "imports", "exports", "bi_tot", 
                              "mother")]

Thus the table's head is:

year   importer1       importer2  flow1  flow2     country
6  2009 Afghanistan          Bhutan     NA     NA Afghanistan
11 2009 Afghanistan Solomon Islands     NA     NA Afghanistan
12 2009 Afghanistan           India 516.13 120.70 Afghanistan
13 2009 Afghanistan           Japan 124.21   0.46 Afghanistan
15 2009 Afghanistan        Maldives     NA     NA Afghanistan
19 2009 Afghanistan      Bangladesh   4.56   1.09 Afghanistan

   imports exports       bi_tot         mother
6  6689.35  448.25           NA United Kingdom
11 6689.35  448.25           NA United Kingdom
12 6689.35  448.25 1.804361e-02 United Kingdom
13 6689.35  448.25 6.876602e-05 United Kingdom
15 6689.35  448.25           NA United Kingdom
19 6689.35  448.25 1.629456e-04 United Kingdom

I've attempted to remove all the cases in which importer2 do not match mother by making a subset:

subset(bilat_total, importer2 == mother)

But each time I do, I get the error:

Error in Ops.factor(importer2, mother) : level sets of factors are different

How would I go about dropping all the rows/cases in which importer 2 and mother don't match?

Answer 1

The error may be because the columns are factor class. We can convert the columns to character class and then compare to subset the rows.

subset(bilat_total, as.character(importer2) == as.character(mother))

Based on the data example showed

subset(bilat_total, importer2 == mother)
# Error in Ops.factor(importer2, mother) : 
#  level sets of factors are different