CODEWITHSUNDEEP
rif-statement
Adam Warner
Adam Warner

Reputation: 1354

Extracting Location and Value of Vector

So I want to extract both the location and the value of a vector. Here is the example code: 

x <- matrix(1:96, nrow = 8)
x
y <- c(0,8,0,0,0,7,0,0,0)
y
which(y > 0)

This gives and output of

[1] 2 6

I want to then find the values of x[2,8] and x[6,7]. I have tried to do: 

test <- ifelse(y > 0, x[which(y > 0 ),y], 0)

But that only produces 

[1]  0 62  0  0  0 62  0  0  0

Which is actually x[6,8]. There is probably a relatively easy way to do this in R and I am missing something fairly simple.

Upvotes: 2

Views: 79

Answers (1)

akrun
akrun

Reputation: 886938

We create a row/column numeric index based on which(y>0) and y[y>0], cbind it and use that to extract the values in 'x'

 x1 <- x[cbind(which(y>0), y[y>0])]
 x1 
 #[1] 58 54

If we want to replace the non-zero elements in 'y', use the condition y>0 and replace those 'y' values with 'x1'.

 y[y>0] <- x1
 y
 #[1]  0 58  0  0  0 54  0  0  0

Or if we don't need to change the initial 'y' vector

 y1 <-  replace(y, which(y > 0), x1)
 y1
 #[1]  0 58  0  0  0 54  0  0  0

Upvotes: 2

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