替換空值：
foo <- data.frame("day"= c(1, 3, 5, 7), "od" = c(0.1, "#N/A", 0.4, 0.8))
NAs <- foo == "#N/A"
## by replace method
is.na(foo)[NAs] <- TRUE
## or directly
foo[NAs] <- NA
替換負值為0：
方法一：
df[df < 0] = 0
方法二：
df <- data.frame(a=rnorm(1000),b=rnorm(1000))
m <- as.matrix(df)
m[m<0] <- 0
df <- as.data.frame(m)
REF：
http://stackoverflow.com/questions/2767219/r-how-to-replace-elements-of-a-data-frame
http://stackoverflow.com/questions/12835942/fast-replacing-values-in-dataframe-in-r
http://stackoverflow.com/questions/19503266/r-replace-all-particular-values-in-a-data-frame