R编程：缓存矩阵的逆

Question

I am following a Data Science course on Coursera and I have a question regarding one of the assignments where I have to inverse a Matrix and then cache that result.

Basically I have been googling away and I found the answer but there are parts of the answer that I do not yet understand. For this reason I don't want to submit my assignment yet since I don't want to submit anything that I do not fully understand.

The part that I do not understand from the code below is the part where setInverse is defined. where does the 'function(inverse) inv' come from? especially the 'inverse' was never defined?

After this a list is returned which does not make much sense to me as well?

If someone could take the time to explain this function to me I would be very grateful!

    makeCacheMatrix <- function(x = matrix()) {
    inv <- NULL
    set <- function(y) {
        x <<- y
        inv <<- NULL
    }
    get <- function() x
    setInverse <- function(inverse) inv <<- inverse
    getInverse <- function() inv
    list(set = set,
         get = get,
         setInverse = setInverse,
         getInverse = getInverse)
}


## Write a short comment describing this function

cacheSolve <- function(x, ...) {
    ## Return a matrix that is the inverse of 'x'
    inv <- x$getInverse()
    if (!is.null(inv)) {
        message("getting cached data")
        return(inv)
    }
    mat <- x$get()
    inv <- solve(mat, ...)
    x$setInverse(inv)
    inv
}

Answer 1

I don't know your exact assignment, but I would change your function slightly:

makeCacheMatrix <- function(x = matrix()) {
  inv <- NULL
  set <- function(y) {
    x <<- y
    inv <<- NULL
  }
  get <- function() x
  setInverse <- function() inv <<- solve(x) #calculate the inverse
  getInverse <- function() inv
  list(set = set,
       get = get,
       setInverse = setInverse,
       getInverse = getInverse)
}

You can then use it like this:

funs <- makeCacheMatrix()
funs$set(matrix(1:4, 2))
funs$get()
#     [,1] [,2]
#[1,]    1    3
#[2,]    2    4
funs$setInverse()
funs$getInverse()
#     [,1] [,2]
#[1,]   -2  1.5
#[2,]    1 -0.5

The exercise is probably intended to teach you closures . The point is that x and inv are stored in the enclosing environment of the set , get , setInverse , getInverse functions. That means the environment within which they were defined, ie, the environment created by the makeCacheMatrix() call. See this:

ls(environment(funs$set))
#[1] "get"        "getInverse" "inv"        "set"        "setInverse" "x"

As you see not only are the four functions in this environment, but also the x and inv objects (a consequence of using <<- ). And the get and getInverse functions only fetch these from their enclosing environment.