in R programming, concerning on inverse matrix and its multiplication

Question

this is my solving process from the exercise of [a beginner\'s guide to R]

> Q
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    2    1
[3,]    2             


        

Answer 1

This is not a programming error, this is the result of what's called "floating point arithmetic." rounded to a reasonable length, you will get the identity matrix:

Q2 <- solve(Q)%*%Q
round(Q2, 4)

To learn more on floating point arithmetic go here.