Why cannot order a factor in data frame in R? - r

Question

I have a data frame (named 'mdf') which includes two columns. The basic information is below:
> head(mdf); tail(mdf)
Country Rank
1 ABW 161
2 AFG 105
3 AGO 60
4 ALB 125
5 ARE 32
6 ARG 26
Country Rank
184 WSM 181
185 YEM 90
186 ZAF 28
187 ZAR 112
188 ZMB 104
189 ZWE 134
> str(mdf)
'data.frame': 189 obs. of 2 variables:
$ Country: Factor w/ 229 levels "","ABW","ADO",..: 2 4 5 6 7 8 9 11 12 13 ...
$ Rank : Factor w/ 195 levels "",".. Not available. ",..: 72 10 149 32 118 111 41 84 26 112 ...
My purpose is to rearrange it by ordering 'Rank' variable, but the result is:
> mdf[order(mdf$Rank),]
Country Rank
178 USA 1
78 IND 10
153 SLV 100
170 TTO 101
43 CYP 102
54 EST 103
188 ZMB 104
2 AFG 105
175 UGA 106
130 NPL 107
73 HND 108
60 GAB 109
31 CAN 11
67 GNQ 110
As you see, it is not what I need (i.e. increasing order). How can I do it? Thanks!

Answer 1

To get the answer you are looking for, use:
mdf[order(as.numeric(as.character(mdf$Rank))),]
The reason your original code doesn't work is that your Rank variable is a factor, so it will be sorted by the levels of the factor. For example, if you had a data frame such that:
DF
# x
# 1 2
# 2 22
# 3 11
# 4 1
and order the data
DF[order(DF$x),]
and you look at the levels:
levels(DF$x)
# [1] "1" "2" "11" "22"
We can reorder the levels such that
levels(DF$x) <- relevel(DF$x, ref = '11')
Now,
levels(DF$x)
# [1] "2" "22" "11" "1"
So ordering with the new factor levels we get different results:
DF[order(DF$x),]
To answer your question of why as.numeric doesn't work, it's because each factor level has an associated integer, which you get with as.numeric. If you want the number that is the factor label, you must first convert to a character and then convert to numeric, thus requiring as.numeric(as.character(x))
For example, calling as.numeric(DF$x) gives the integer values for each level, but not the actual label for each level:
# [1] 2 4 3 1
One way to avoid this in the future if you are loading your data frame from a .csv file is to use read.csv(..., stringsAsFactors=FALSE), or also I like the fread function in data.table which uses safer default types.