Generating equations from factors in R [duplicate] - r

Question

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Google Docs exports spreadsheet values with commas. read.csv() in R treats these as factors instead of numeric
(2 answers)
Closed 8 years ago.
I am fairly new to R, and I am trying to create a new column, which is one column minus another column. For example:
price <- c("$10.00", "$7.15", "$8.75", "12.00", "9.20")
quantity <- c(5, 6, 7, 8, 9)
price <- as.factor(price)
quantity <- as.factor(quantity)
df <- data.frame(price, quantity)
In my actual data set, all the columns imported as factors. When I try to create the new column I get this:
diff <- price - quantity
In Ops.factor(price, quantity): - not meaningful for factors
I have tried to coerce the data to numeric using as.numeric(df), as.numeric(levels(df)), as.numeric(levels(df))[df], and setting stringsAsFactors to false, but the data gets converted to NAs. Data.matrix changes the values. Is there another way to get the above equation to work? Thanks!

Answer 1

You should avoid "" and $ in price column and avoid converting them to factors if you want to do math operations on them:
price <- c(10.00, 7.15, 8.75, 12.00, 9.20)
quantity <- c(5, 6, 7, 8, 9)
df <- data.frame(price, quantity)
df$diff <- price - quantity
df
price quantity diff
1 10.00 5 5.00
2 7.15 6 1.15
3 8.75 7 1.75
4 12.00 8 4.00
5 9.20 9 0.20

Answer 2

Try:
as.numeric(gsub("^\\$","", price))-as.numeric(as.character(quantity))
#[1] 5.00 1.15 1.75 4.00 0.20
Or from df
df$diff <- Reduce(`-`,lapply(df, function(x) as.numeric(gsub("^\\$","",x))))
df$diff
#[1] 5.00 1.15 1.75 4.00 0.20

Answer 3

If you're stuck with factor columns, you could add a new diff column with within() and some type coercion
> within(df, {
diff <- as.numeric(gsub("[$]", "", price)) -
as.numeric(as.character(quantity))
})
# price quantity diff
# 1 $10.00 5 5.00
# 2 $7.15 6 1.15
# 3 $8.75 7 1.75
# 4 12.00 8 4.00
# 5 9.20 9 0.20
You may also consider going back and re-reading the data into R. It's simple, and will make things a little easier. Here's how you could do it and get the desired result that way.
Create a data file: This won't be necessary for you, since you can just read the original file again.
> write.table(df, "df.txt")
Read the data into R, remove the $ sign, and calculate the difference:
> df2 <- read.table("df.txt", stringsAsFactors = FALSE)
> df2$price <- as.numeric(gsub("[$]", "", df2$price))
> with(df2, { price - quantity })
# [1] 5.00 1.15 1.75 4.00 0.20