CODEWITHSUNDEEP
r
flamenco
flamenco

Reputation: 2840

Extract the first and last non-zero row and column from a binary matrix in R

The idea is to create a box(draw the lines) around an image which is represented as a binary matrix. For that I need to extract the first and last non-zero row and column on each of the four sides. For exemplification, here is a snippet that creates the figure bellow:

R = 50; b = 10
x = 1:(3*R); y = x; x.cent = R; y.cent = R; r = R-b
A = outer(x, y, function(x,y) sqrt(((x-x.cent)^2)+((y-y.cent)^2)))
circle <- ifelse(A<r, 1, 0)

enter image description here

The circle is for exemplification purpose. In reality it can be any shape. True, it can be done by slicing each side and look for each row/column for non-zero values but I'm looking for an elegant way to do it to be computationally fast.

Upvotes: 1

Views: 48

Answers (1)

Darren Tsai
Darren Tsai

Reputation: 35594

Use rowSums() and colSums() to identify the indices of non-zero row/column sums and then use range() to exract the first and last values.

range(which(rowSums(circle) != 0))
# [1] 11 89

range(which(colSums(circle) != 0))
# [1] 11 89

Upvotes: 3

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