A function for parsing ounces into a numeric value.

parsing_ounces.R

# Parse a string given in "# #/# oz" into a decimal value.
parse_ounces <- function(val) {
    if (val == "For glass") { return(1) }
    
    if (str_detect(val, "or")) {
        val <- str_remove(val, "or.+$")
    }
    
    val_pieces <- str_remove_all(val, "oz|slices|tsp|dash") %>% 
        str_trim() %>%
        str_split(" ") %>% 
        unlist() %>% 
        str_trim()
    
    val_pieces <- val_pieces[str_length(val_pieces) > 0]

    map_dbl(val_pieces, function(x){
        if (str_detect(x, "/")) {
            x_frac <- str_split(x, "/") %>% unlist() %>% as.numeric()
            return(x_frac[[1]] / x_frac[[2]])
        } else {
            return(as.numeric(x))
        }
    }) %>%
        sum()
}

test_that("The ounces are parsed correctly.", {
    expect_equal(parse_ounces("1 oz"), 1)
    expect_equal(parse_ounces("1/2 oz"), 0.5)
    expect_equal(parse_ounces("1 1/2 oz"), 1.5)
    expect_equal(parse_ounces("For glass"), 1)
    expect_equal(parse_ounces("1/2 or 1"), 0.5)
    expect_equal(parse_ounces("1  3/4 oz"), 1.75)
    expect_equal(parse_ounces("1 dash"), 1)
})

Some editorial decisions were made on what to do with other units including "slices," "tsp," and "dash." Any differences were just ignored for my purposes.