R: Simplifying code to convert letter grades to numeric grades

Question

I have written a function in R that will convert a data frame containing letter grades into numeric grades. I then use sapply() on each column of the data frame. Is there a simpler way to do this that doesn't require three separate calls of sapply? Is there a way to apply a function to every element of a data frame instead of every row or column?

The source data, "grades", looks like this:

grades <- read.table("Grades.txt", header = TRUE)
head(grades)
  final_exam quiz_avg homework_avg
1          C        A            A
2         C-       B-            A
3         D+       B+            A
4         B+       B+            A
5          F       B+            A
6          B       A-            A

My "convert_grades" function looks like this:

convert_grades <- function(x) {
    if (x == "A+") {
        x <- 4.3
    } else if (x == "A") {
        x <- 4
    } else if (x == "A-") {
        x <- 3.7
    } else if (x == "B+") {
        x <- 3.3
    } else if (x == "B") {
        x <- 3
    } else if (x == "B-") {
        x <- 2.7
    } else if (x == "C+") {
        x <- 2.3
    } else if (x == "C") {
        x <- 2
    } else if (x == "C-") {
        x <- 1.7
    } else if (x == "D+") {
        x <- 1.3
    } else if (x == "D") {
        x <- 1
    } else if (x == "D-") {
        x <- 0.7
    } else if (x == "F") {
        x <- 0
    } else {
        x <- NA
    }
    return(x)
}

My current approach is as follows:

num_grades <- grades
num_grades[, 1] <- sapply(grades[, 1], convert_grades)
num_grades[, 2] <- sapply(grades[, 2], convert_grades)
num_grades[, 3] <- sapply(grades[, 3], convert_grades)

head(num_grades)
   final_exam quiz_avg homework_avg
 1        2.0      4.0            4
 2        1.7      2.7            4
 3        1.3      3.3            4
 4        3.3      3.3            4
 5        0.0      3.3            4
 6        3.0      3.7            4

Answer 1

I would rewrite your convert_grades function as follows:

convert_grades <- function(x) {
  A <- factor(x, levels=c("A+", "A", "A-",
                     "B+", "B", "B-",
                     "C+", "C", "C-",
                     "D+", "D", "D-", "F"))
  values <- c(4.3, 4, 3.7, 
              3.3, 3, 2.7,
              2.3, 2, 1.7,
              1.3, 1, 0.7, 0)
  values[A]
}

Then, I would do the conversion like this:

num_grades <- grades
num_grades[] <- lapply(num_grades, convert_grades)
num_grades
  final_exam quiz_avg homework_avg
1        2.0      4.0            4
2        1.7      2.7            4
3        1.3      3.3            4
4        3.3      3.3            4
5        0.0      3.3            4
6        3.0      3.7            4