Re-ordering bars in R's barplot()

you can use ggplot to do this

library("ggplot2")
num <- c(1, 8, 4, 3, 6, 7, 5, 2, 11, 3)
cat <- c(letters[1:10])
data <- data.frame(num, cat)    
ggplot(data,aes(x= reorder(cat,-num),num))+geom_bar(stat ="identity")

The result is as shown below

Using base functions

df <- data[order(data$num,decreasing = TRUE),]
 barplot(df$num,names.arg = df$cat)

I get the following,

num <- c(1, 8, 4, 3, 6, 7, 5, 2, 11, 3)
cat <- c(letters[1:10])
data <- data.frame(num, cat)
barplot(data[order(data[,1],decreasing=TRUE),][,1],names.arg=data[order(data[,1],decreasing=TRUE),][,2])

The above code uses the order() function twice (see comments, below). To avoid doing this the results of the ordered data.frame can be stored in a new data.frame and this can be used to generate the barplot.

num <- c(1, 8, 4, 3, 6, 7, 5, 2, 11, 3)
cat <- c(letters[1:10])
data <- data.frame(num, cat)
data2  <- data[order(data[,1],decreasing=TRUE),]
barplot(data2[,1],names.arg=data2[,2])