CODEWITHSUNDEEP
r
user9798936
user9798936

Reputation:

how to count the lower triangle elements in a matrix

Suppose I have a matrix. Suppose I have a list of a lower triangular matrix. How can I count the number of the elements in the matrices?

For example, 

Matrix <- c(0, 4, 3, 1, 2,
            0, 0, 3, 2, 1,
            0, 0, 0, 2, 1,
            0, 0, 0, 0, 1,
            0, 0, 0, 0, 0)
Matrix <- matrix(Matrix, 5, 5)

> Matrix
     [,1] [,2] [,3] [,4] [,5]
[1,]    0    0    0    0    0
[2,]    4    0    0    0    0
[3,]    3    3    0    0    0
[4,]    1    2    2    0    0
[5,]    2    1    1    1    0

How to count the number of the elements in this matrix? This matrix contains 10 elements. How to count it in R?

Upvotes: 5

Views: 7250

Answers (2)

Ronak Shah
Ronak Shah

Reputation: 389205

We can get the lower triangle elements using the function lower.tri and then we can sum over them to count number of elements.

sum(lower.tri(Matrix))
#[1] 10

where 

lower.tri(Matrix) #returns

#      [,1]  [,2]  [,3]  [,4]  [,5]
#[1,] FALSE FALSE FALSE FALSE FALSE
#[2,]  TRUE FALSE FALSE FALSE FALSE
#[3,]  TRUE  TRUE FALSE FALSE FALSE
#[4,]  TRUE  TRUE  TRUE FALSE FALSE
#[5,]  TRUE  TRUE  TRUE  TRUE FALSE

Upvotes: 2

Dom Alvarez
Dom Alvarez

Reputation: 258

For a matrix of size n x n, the number of elements in the lower triangle is

n * (n - 1) / 2

Upvotes: 15

Related Questions