Evidently, I'm overlooking something simple.  I'm trying to used order with data.frame.   


For example: 

t = data.frame(x = c(11,12,14), y = c(19,20,21), z = c(10,9,7))
t[order(z), ]
Error in order(z) : object 'z' not found

Thank you for any insights and advice provided. 
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