I've looked at some answers on matrix multiplication (e.g. R: matrix by vector multiplication) but still can't find an answer to my question:
I sell four types of fruit, with per-item prices in this vector:
prices <- c(2.25, 0.42, 0.85, 1.24)
names(prices) <- c("pomegranate", "banana", "apple", "mango")
> prices
pomegranate banana apple mango
2.25 0.42 0.85 1.24
The following matrix gives the number of each item sold on each day of the week:
vv <- c(43, 87, 48, 90, 99, 60, 1, 62, 62, 107, 34, 10, 130, 15, 5, 124, 124, 101, 22, 104)
M <- matrix(vv, nrow=4, ncol=5, dimnames=list(c("banana", "mango", "pomegranate", "apple"), c("M", "Tu", "W", "Th", "F")))
> M
M Tu W Th F
banana 43 99 62 130 124
mango 87 60 107 15 101
pomegranate 48 1 34 5 22
apple 90 62 10 124 104
Notice that the rows of the matrix are in a different order than the prices vector.
I want to generate a vector of my revenue for each day, i.e.:
> dailyrevenue
M Tu W Th F
310.44 170.93 243.72 189.85 315.22
Is there any quick way to tell R to multiply each price by its corresponding row name? The standard forms of multiplication would assume that each banana costs $2.25 rather than the correct amount, $0.42.
To get what I want, I can use the following code:
OrderOfPrices <- order(match(names(prices), rownames(M))) # per https://stackoverflow.com/a/2117080/8436923
r.prices <- prices[OrderOfPrices]
dailyrevenue <- colSums(r.prices * M)
I also wouldn't mind using %*% or crossprod (returning a 1-row matrix rather than a vector) if I could avoid calling the order function.
You can use subsetting to create a prices vector in the correct order:
pricesm <- prices[rownames(M)]
pricesm
#banana mango pomegranate apple
# 0.42 1.24 2.25 0.85
rev <- pricesm %*% M
rev
# M Tu W Th F
#[1,] 310.44 170.93 243.72 189.85 315.22