Changed cachematrix.R for JHU Pgmg Assignment rdpeng#2

Author: sparkl1

cachematrix.R

@@ -1,15 +1,78 @@
 ## Put comments here that give an overall description of what your
-## functions do
+## functions do..................
+## The "makeCacheMatrix()" function creates and initializes a "matrix" cache for subsequent use.
+## The "cacheSolve()" function uses the cached "matrix" as input and updates the matrix inverse
+##      value stored there (if it has been previously computed). 
+##      If there is not previously computed inverse matrix, it computes one and updates the cache.
 
-## Write a short comment describing this function
+## The "set" pieces of these functions provide INPUT in to the cache.
+## The "get" pieces of these functions get OUTPUT from the cache.
 
-makeCacheMatrix <- function(x = matrix()) {
+## TEST RESULTS ARE SHOWN AT END
+
+## Write a short comment describing this function.................
+## This function initializes a matrix value (caches it).
+## It includes a place for the computed inverse matrix, initated to NULL.
 
+makeCacheMatrix <- function(x = matrix()) {
+    matinv <- NULL  ## Initialize the cached matrix inverse value as NULL 
+    ## until it's calculated the first time.
+    set <- function(y) {    ## The "set" variables provide the INPUT to the cache.
+        x <<- y
+        matinv <<- NULL     ## Sets the calculated matrix inverse to NULL 
+        ## (i.e., not yet calculated)
+    }
+    get <- function() x     ## The "get" variables provide the OUTPUT from the cache.
+    setmatinv <- function(matinvin) matinv <<- matinvin
+    getmatinv <- function() matinv
+    list(set = set, get = get,
+         setmatinv = setmatinv,
+         getmatinv = getmatinv)
 }
 
 
-## Write a short comment describing this function
+## Write a short comment describing this function..................
+## The following function computes the matrix inverse from a cached input matrix (which includes 
+## a previously calculated value when the matrix inverse was already calculated). 
+## If a new matrix inverse computation is performed, it is added to this cache. That is, the
+## return value (the matrix inverse) from this function 
+## (another matrix) is used as reference to subsequent computations, and is no longer the
+## initialized NULL value.
+## So long as this computed matrix inverse is not changed ("reinitialized"),
+## the cached value of the matrix inverse is used (instead of being recomputed).
+
 
 cacheSolve <- function(x, ...) {
         ## Return a matrix that is the inverse of 'x'
+    matinv <- x$getmatinv()
+    if(!is.null(matinv)) {
+        message("getting cached data")
+        return(matinv)
+    }
+    data <- x$get()
+    matinv <- solve(data, ...)
+    x$setmatinv(matinv)
+    matinv
 }
+## TEST RESULTS FOLLOW ######################
+## > testmatA
+## [,1] [,2]
+## [1,]    1    4
+## [2,]    3    2
+## > matAtst<-makeCacheMatrix(testmatA)
+## > matAinvtst<-cacheSolve(matAtst)
+## > matAinvtst
+## [,1] [,2]
+## [1,] -0.2  0.4
+## [2,]  0.3 -0.1
+## > matAinvtst<-cacheSolve(matAtst)
+## getting cached data
+## > matAinvtst
+## [,1] [,2]
+## [1,] -0.2  0.4
+## [2,]  0.3 -0.1
+## > matAinvtst %*% testmatA
+## [,1] [,2]
+## [1,]    1    0
+## [2,]    0    1
+## > ## TEST IS GOOD