CODEWITHSUNDEEP
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Xarrus
Xarrus

Reputation: 85

Double for loop in R creating a matrix

I would like to create a 5x5 matrix in R using two for loops. I have 5 numbers p_j and q_i with i and j in {1,2,3,4,5}. I would like to create a matrix with where the element in (j,i) is given by p_j * q_j. j-th row and i-th column. So, first of all I would like to create an empty matrix m and then filling the matrix using the for loops.

for (i in 1:5){ for (j in 1:5){ } }

Upvotes: 1

Views: 5514

Answers (2)

G5W
G5W

Reputation: 37641

I will walk through two solutions that I proposed. Both of these solutions avoid any explicit looping. Mostly in R, if it is easy to avoid a loop, you probably should. First, let's get some example data. 

set.seed(2017)
p = sample(5)
q = sample(5)
p
[1] 5 3 2 1 4
q
[1] 4 1 2 5 3

Here p and q are randomly generated. The set.seed part is so that we both get the same "random" numbers.

Solution 1 - matrix multiplication

p %*% t(q)
     [,1] [,2] [,3] [,4] [,5]
[1,]   20    5   10   25   15
[2,]   12    3    6   15    9
[3,]    8    2    4   10    6
[4,]    4    1    2    5    3
[5,]   16    4    8   20   12

%*% is the way to specify matrix multiplication in R.
p %*% t(q) multiplies the 5x1 matrix p by the 1x5 matrix t(q), the transpose of q, resulting in the 5x5 matrix with the desired answer.

Solution 2 - outer

outer(p,q, `*`)
     [,1] [,2] [,3] [,4] [,5]
[1,]   20    5   10   25   15
[2,]   12    3    6   15    9
[3,]    8    2    4   10    6
[4,]    4    1    2    5    3
[5,]   16    4    8   20   12

The function outer in r, creates the "outer product" of two vectors - that is, it takes all combinations of an element of p and an element of q and combines them using the function that you supplied, in this case *, exactly the calculation that you are asking for. Actually, this could have been written more succinctly as outer(p,q) because the default function to use to combine p & q is *.

Upvotes: 4

Julian Zucker
Julian Zucker

Reputation: 564

Assuming you have variables named "p_1", "p_2" in the working environment:

mymatrix <- matrix(nrow = 5, ncol = 5)
for (i in 1:5) { 
  for (j in 1:5) { 
    mymatrix[i, j] <- get(paste("p_", j, sep="")) * 
                      get(paste("q_", i, sep=""))
  }
}

Upvotes: 2

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