如何在一个矩阵中定位满足条件的元素以识别第二个矩阵中的元素

Question

I want to identify the positions of elements in one matrix that meet a condition to then apply those positions to another matrix and find the means of those.

my_vector_1<-c(1,2,1,4,1,1,7,8,)
my_matrix_1<-matrix(data=my_vector_1, nrow=3, ncol=3)

my_vector_2<-c(2,4,6,8,10,11,12,13,14)
my_matrix_2<-matrix(data=my_vector_2, nrow=3, ncol=3)

First locate the positions of my_matrix_1==1 in the first matrix to find...

[1,1]
[2,2]
[3,1]
[3,2]

Then find the mean of the elements in the second matrix that are in the positions identified above...

7.25    #mean of 2, 10, 6, 11 in my_matrix_2

Answer 1

You could subset my_matrix_2 where my_matrix_1 has value 1 and take mean of those values.

mean(my_matrix_2[my_matrix_1  == 1])
#[1] 7.25

Answer 2

We can use arr.ind to find the row/column position

ind <- which(my_matrix_1 == 1, arr.ind = TRUE)
ind
#     row col
#[1,]   1   1
#[2,]   3   1
#[3,]   2   2
#[4,]   3   2


mean(my_matrix_2[ind])
#7.25

---

Another way to do this would be

mean(my_matrix_2 * NA^(my_matrix_1 != 1), na.rm = TRUE)