R number of items to replace is not a multiple of replacement length / results however correct

which() can return a vector if there are multiple matches. For example:

which((1:12)%%2 == 0) # which are even?

Is matrix$col_b[i] unique? The results may still look correct. Notice what happens in this case:

x <- 1:2
x[1] <- 3:4
x

Also, 1:b-1 does not give you the numbers from 1 to b - 1 but the number from 1 to b, all minus 1:

b <- 10
1:b-1

You need parentheses to force the subtraction first: 1:(b - 1).