bpa: Basic Probability Assignment Objects
In MuViCP: MultiClass Visualizable Classification using Combination of Projections

Description Usage Arguments Details Value Author(s) References See Also Examples

View source: R/bpa.R

These functions can be used to create, combine and print basic probability assignment objects for classificiation. A Basic Probabilty Assignment is a similar to a probabilty mass function, except that it has an additional mass for the concept for "ignorance".

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bpa(n = 1, setlist = c(1:n, Inf), mlist = c(rep(0, n), 1))
## S3 method for class 'bpa'
print(x, verbose = FALSE, ...)

`n`	The number of distinct values that need to be represented. Usually set to the number of classes in the data.
`setlist`	A subset of 1:`n`, indicating those elements that have positive mass. The special value 'Inf' is used to denote the whole set, which is ignorance in dempster-shafer terms.
`mlist`	The actual masses assigned to the elements in the `setlist`.
`x`	The bpa object to be printed.
`verbose`	If FALSE (default), simply prints out the basic probability assignment. If TRUE, prints a list of all member functions as well.
`...`	Additional arguments to print method. Not Used.

It should be noted that these functions are fairly simplistic, since they were designed to be fast, and work with classification only. In particular, if you have set-valued elements, the combination function will likely give the wrong answer unless the sets are non-intersecting. For the same reason, belief functions have not been implemented either, since for atomic elements, bpa = belief function.

The bpa function returns a list of functions which can be used to query and / or manipulate the create bpa object.

`get.N`	Get the number of distinct values represented in the bpa. Usually set to the number of classes.
`get.setlist`	Get the sets represented in the bpa
`get.full.m`	Get the masses assigned to each of the elements.
`get.focal.elements`	Get only those elements that have a positive mass attached to them. Such elements are called the focal elements of the bpa.
`get.m`	Get the masses attached only to the focal elements, that is the non-zero elements of the mlist.
`get.mass`	Get the masses attached to certain specified elements of the bpa. The elements are specifed as a vector via the argument `s`.
`assign.bpa`	Can be used to re-assign mass to certain specified elements of the bpa. The argument `s` is the same as the setlist, and `m` is the same as mlist.
`get.assigned.class`	Returns a vector of all possible classes, in decreasing order of assigned probabilty. (That is, the first element is the most likely class, and the last element is the least likely class.)
`get.assigned.ratios`	Returns a vector of length `N`-1, each of whose elements is the ratio of the probability assigned to the successive classes. That is, the first element is the ratio of the probability assigned to the most likely class to that assigned to the next most likely class, and so on.
`set.name`	Can be passed a string to name the `bpa` object. (Used by the `bpamat` function; names are set to the row number.)
`get.name`	Returns the name of the `bpa` object. If not assigned, returns `NULL`.
`set.info`	Can be used to store as auxiliary information. (Used internally by the `bpamat` function to store the random seed, or the special character `'C'` if the bpa object resulted from a combination.)
`get.info`	Returns whatever was stored as info. If empty, `NULL` is returned.

Mohit Dayal

Gordon, J. and Shortliffe, E. H. (1984). The dempster-shafer theory of evidence. Rule-Based Expert Systems: The MYCIN Experiments of the Stanford Heuristic Programming Project, 3:832-838. Shafer, G. (1986). The combination of evidence. International Journal of Intelligent Systems, 1(3):155-179.

combine.bpa.bs, combine.bpa.ds, combine.bpa.list.ds, combine.bpa.list.ds

##Empty bpa - All mass is attached to ignorance
b1 <- bpa(3)
b1

##Add a set to this bpa
b1$assign.bpa(s = c(1,2), m = c(0.3,0.4))
print(b1, verbose = TRUE)

##The same thing in a different way - classes can be named
##Note that the print method omits empty classes
b0 <- bpa(3, c('A','B','C', Inf), c(0.3, 0.4, 0, 0.3))
b0

##Another bpa
##Again, class '2' has been omitted
b2 <- bpa(3)
b2$assign.bpa(s = c(1,3), m = c(0.7,0.1))
b2

##Combine
b3 <- combine.bpa.ds(b1,b2)
b3
combine.bpa.bs(b1,b2)

##As a list, should be same answer as above
b4 <- combine.bpa.list.ds(list(b1,b2))
b4
combine.bpa.list.bs(list(b1,b2))

Warning message:
no DISPLAY variable so Tk is not available 
A bpa object. 
Inf 
  1 
A bpa object. 
  1   2 Inf 
0.3 0.4 0.3 
The following functions are available:
 [1] "get.N"               "get.setlist"         "get.full.m"         
 [4] "get.focal.elements"  "get.m"               "get.mass"           
 [7] "assign.bpa"          "get.assigned.class"  "get.assigned.ratios"
[10] "set.name"            "get.name"            "set.info"           
[13] "get.info"           
A bpa object. 
  A   B Inf 
0.3 0.4 0.3 
A bpa object. 
  1   3 Inf 
0.7 0.1 0.2 
A bpa object. 
   1    2    3  Inf 
0.48 0.08 0.03 0.41 
A bpa object. 
        1       Inf 
0.7777778 0.2222222 
A bpa object. 
   1    2    3  Inf 
0.48 0.08 0.03 0.41 
A bpa object. 
        1       Inf 
0.7777778 0.2222222