[R] by() subset by factor gives unexpected results

[Bert Gunter]

... and, of course, by() should not be used at all for this sort of
thing in practice, as the "col" argument can be a vector. See
?plot.default if you you were not aware of this already.

j <- data.frame(x=c(1,2,3), y=c(1,1,1), B=c("red","blue","green"),
                stringsAsFactors = FALSE)

with(j,plot(x,y, col=B, xlim=c(0,4), ylim=c(0,1.2)))


Cheers,
Bert

Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Sat, Aug 5, 2017 at 2:59 AM, Myles English <mylesenglish at gmail.com> wrote:
>
> The answer was (thanks to Mark Leeds) to do with the use of a factor
> instead of a vector.
>
> on [2017-08-05] at 08:57 Myles English writes:
>
>> I am having trouble understanding how the 'by' function works.  Using
>> this bit of code:
>>
>> i <- data.frame(x=c(1,2,3), y=c(0,0,0), B=c("red","blue","blue"))
>> j <- data.frame(x=c(1,2,3), y=c(1,1,1), B=c('red','blue','green'))
>
> The use of I() prevents conversion to a factor:
>
> i <- data.frame(x=c(1,2,3), y=c(0,0,0), B=I(c("red","blue","blue")))
> j <- data.frame(x=c(1,2,3), y=c(1,1,1), B=I(c('red','blue','green')))
>
>> plot(0, 0, type="n", xlim=c(0,4), ylim=c(0,1))
>> by(i, i$B, function(s){ points(s$x, s$y, col=s$B) })
>> by(j, j$B, function(s){ points(s$x, s$y, col=s$B) })
>>
>> I would have expected the point at (1,1) to be coloured red.  When
>> plotted, this row is indeed red:
>>
>>> i[1,]
>>   x y   B
>> 1 1 0 red
>>
>> however, this next point is green on the plot even though I would like
>> it to be red:
>>
>>> j[1,]
>>   x y   B
>> 1 1 1 red
>>
>> How can I achieve that?
>>
>> Myles
>
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