How to use matrix algebra in R to create new column?

Question

I have a dataframe with multiple columns. I have another dataframe with two columns, factor and coefficient. I want to create a new column in the initial dataframe (mydata) that is the sum of multiplying each element in each row of mydata(a:e) by the coefficients (a:e) in df. The result for the first row in the newcol should be 64 (1*1 + 2*2 + 3*3 + 4*4 + 7*5). Ideally, I would be able to somehow replicate this 20+ times with different coefficients.

mydata <- data.frame(a = 1:10, b = 2:11, c = 3:12, d = 4:13, d_1 = 5:14, d_2 = 6:15, d_3 = 7:16, e = 8:17)
df <- data.frame(factor = c('a','b','c','d','e'), coefficient = 1:5)

mydata$newcol <- mydata[,c("a","b","c","d","e")] %*% df$coefficient
mydata$newcol2 <- mydata[,c("a","b","c","d_1","e")] %*% df$coefficient

Any advice would be helpful!

Answer 1

We can use sweep here, subset mydata based on factor column in df and multiply it with coefficient for each element and then take rowSums to calculate the sum.

mydata$newcol <- rowSums(sweep(mydata[as.character(df$factor)], 2,df$coefficient, `*`))

mydata
#    a  b  c  d d_1 d_2 d_3  e newcol
#1   1  2  3  4   5   6   7  8     70
#2   2  3  4  5   6   7   8  9     85
#3   3  4  5  6   7   8   9 10    100
#4   4  5  6  7   8   9  10 11    115
#5   5  6  7  8   9  10  11 12    130
#6   6  7  8  9  10  11  12 13    145
#7   7  8  9 10  11  12  13 14    160
#8   8  9 10 11  12  13  14 15    175
#9   9 10 11 12  13  14  15 16    190
#10 10 11 12 13  14  15  16 17    205

Or we can also transpose mydata and multiply the coefficient and get colSums .

colSums(t(mydata[as.character(df$factor)]) *  df$coefficient)