Hello,

I am trying to construct a logical expression dynamically, for use in the
subset() function. I am puzzled by problems with the code that follows
below.

Probably there's an easier way to select rows of a data frame according to
some dynamic criteria--and I'd love to hear about it--but I'd also like to
understand what I'm doing incorrectly in constructing the expression in my
code.

Thanks in advance,

tm



# These two expressions print the same, at least.
e1 = expression(name == 'a')
e2 = substitute(expression(cname == 'a'), list(cname=as.symbol('name')))
print (e1)
print (e2)

# But they do not evaluate the same!

# This works.
data = data.frame(name=c('a', 'b', 'c', 'a'), age=c(1,2,3,4))
d1 = subset(data, eval(e1))
print (d1)

# This gives the error:
#     Error in subset.data.frame(data, eval(e2)) :
#       'subset' must evaluate to logical
d2 = subset(data, eval(e2))
print (d2)

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