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I have following problem: 

myvec <- c(1:3) mymat <- as.matrix(cbind(a = 6:15, b = 16:25, c= 26:35)) mymat a b c [1,] 6 16 26 [2,] 7 17 27 [3,] 8 18 28 [4,] 9 19 29 [5,] 10 20 30 [6,] 11 21 31 [7,] 12 22 32 [8,] 13 23 33 [9,] 14 24 34 [10,] 15 25 35

I want to multiply the mymat with myvec and construct new vector such that 

sum(6*1, 16*2, 26*3) sum(7*1, 17*2, 27*3) .................... sum(15*1, 25*2, 35*3)

Sorry, this is simple question that I do not know...

Edit: typo corrected

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3 Answers 3

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56

The %*% operator in R does matrix multiplication:

> mymat %*% myvec [,1] [1,] 116 [2,] 122 ... [10,] 170
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3

An alternative, but longer way can be this one:

rowSums(t(apply(mymat, 1, function(x) myvec*x)),na.rm=T)

Is the only way that I found that can ignore NA's inside the matrix.

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1

Matrices are vectors in column major order:

 colSums( t(mymat) * myvec )

(Edited after hopefully reading question correctly this time.)

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3 Comments

Why the c( ) around the entire expression?
Maybe not needed? The idea was so it ends up as a vector. But that idea seems excessively cautious. I'll drop it.
If the matrix is stored in column major order, then you need to transpose it first before applying a row-wise multiplication. colSums(t(mymat) * myvec)

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