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Convert percentage to letter in R

I created an R script to calculate final grades for a class I am teaching and would like to use R to generate the final grade. How can I take a vector of percentages like:

grades <- c(.47, .93, .87, .86, .79, .90)

and convert them to graphs of letters given the appropriate percentage ranges, for example 70-79 = C, 80-89 = B, 90-100 = A?

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grades <- c(.47, .93, .87, .86, .79, .90)
letter.grade <- cut(grades, c(0, 0.7,0.8,0.9,Inf), right=FALSE, labels=letters[4:1]);
letter.grade

As @MrFlick suggests using a cut. The option right = FALSE means that no spacing is included on the right side. For cut to work, just pass your scores and cut points you want as an argument.



Thanks to @jlhoward for suggesting improvement

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Another option is to use findInterval 



LETTERS[4:1][findInterval(grades, c(0,0.7,0.8,0.9))]
#[1] "D" "A" "B" "B" "C" "A"
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