How to create a boxplot of multiple rows

How to create a boxplot of multiple rows

I have a dataframe of 1200 probes (in rows) and two sets of arrays of nine
column each. The first nine columns are named as "positive" and the next
nine as "negative". I want to show that the expression is normal by
choosing 12 random probes using box plot. My code looks as follows:
f<-c(rep("positive", 9), rep("negative", 9))
for(i in seq(from=1, to=1200, by=10)){
boxplot(probes[i]~f,col="lightblue",main="Expression of genes studied
Cells")
}
But I am getting the following error:
Error in model.frame.default(formula = probes[i] ~ f) :
variable lengths differ (found for 'f')
If I use the box plot for a single probe it works fine. I get two boxes ,
one corresponding to "positive" and another to "negative". Thanks