melt() 使用時 measure variable 的型態要一致

使用 melt() 來 reshape 資料時， measure variable 的型態要一致，否則會出現警告。

例如有以下的資料：

	# Parts of Codes are from https://www.r-bloggers.com/reshape-and-aggregate-data-with-the-r-package-reshape2/
	require(reshape2)
	x <- data.frame(subject = c("John", "Mary"),
	SN = c(1,2),
	time = c(1,1),
	age = c(33,NA),
	weight = c(90, NA),
	height = c(2,2))
	melt(x,id.vars = "SN")
	str(x)

view raw melt_trap.r hosted with ❤ by GitHub

資料中，subject 欄位為 factor。若使用 subject 為 id.var 進行 melt，則會出現以下警告訊息：

因為，第一個欄位為 Factor 型態，melt() 把後面其他的欄位也視為 Factor，造成轉換時的錯誤。


Reference:
[1] thiagogm, 2013. Reshape and aggregate data with the R package reshape2, www.r-bloggers.com.