CODEWITHSUNDEEP
rmatlabsummatrix-multiplication
Jake Hightower
Jake Hightower

Reputation: 11

Converting matrix multiplication and sum function from Matlab to R

I'm converting a rather complicated set of code from Matlab to R. I have zero experience in Matlab and am a functioning novice in R.

I have a segment of code which reads (in matlab):

dSii=(sum(tao.*Sik,1))'-(sum(m'))'.*Sii-beta.*Sii./N.*(Iii+sum(Iik)');

Which I've simplified and will focus on the first segment (if I can solve the first segment I'm confident I can perform the rest): 

J = (sum(A.*B,1))' - ...

tao (or A) and Sik (or B) are matrices. So my assumption is I'm performing matrix multiplication here (A * B)and summing the resultant column. The '1' is what is throwing me off in that statement. In R, that 1 would likely indicate we're talking about a sum of rows as opposed to columns(indicated by 2). But I can't find any supporting documentation for that kind of Matlab statement.

I was thinking of using a statement like this (but of course, too many '1's and ',')

J<- (apply(A*B, 1), 1, sum)

Thanks for all your help. I searched for other examples here and elsewhere and couldn't find an answer. I'm willing to work for it but this is akin to me studying French (which I don't know) to translate in Spanish (which I'm moderate in) while interpreting the whole process in English. :D

Upvotes: 1

Views: 325

Answers (1)

chepyle
chepyle

Reputation: 996

Because of the different conventions in R and Matlab, the idiosyncrasies have to be learned for each (just like your language analogy!). The Matlab command sum(A.*B,1) means multiply A and B element-wise, so they must be the same shape, and then sum along dimension 1, i.e. add each row together to get the column sums. Dimension 1 is the default so, sum(A.*B) would do the same thing as sum(A.*B,1). Because R treats * as element-wise for matrix multiplication, the following Matlab and R codes will produce the same column of numbers in J:

Matlab: 

A=[[1,2,3];[4,5,6];[7,8,9]];
B=[[10,11,12];[13,14,15];[16,17,18]];
J=sum(A.*B,1)';  %the ' means to transpose the column sums to be a 3x1 matrix

R:

A<-matrix(c(1,2,3,4,5,6,7,8,9),3,byrow=T)
B<-matrix(c(10,11,12,13,14,15,16,17,18),3,byrow=T)
J<-matrix(colSums(A*B)) # no transpose needed here: nrow(J)==3

Upvotes: 1

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