to calculate the Entropy

Question

I am new to R and unable to calculate the entropy. There is a similar question on stackoverflow with the answer but i wanted to know why this code isn't working. Here is the copy paste data from the same question.

One of the answer mentions, "The part I think you are missing is the calculation of the class frequencies and you will get your answer", but how do i fix this. I tried most of the options but still i don't get any output. It just runs without any errors.

info <- function(CLASS.FREQ){
      freq.class <- CLASS.FREQ
      info <- 0
      for(i in 1:length(freq.class)){
        if(freq.class[[i]] != 0){ # zero check in class
          entropy <- -sum(freq.class[[i]] * log2(freq.class[[i]]))  #I calculate the entropy for each class i here
        }else{ 
          entropy <- 0
        } 
        info <- info + entropy # sum up entropy from all classes
      }
      return(info)
    }

Dataset as below,

buys <- c("no", "no", "yes", "yes", "yes", "no", "yes", "no", "yes", "yes", "yes", "yes", "yes", "no")

credit <- c("fair", "excellent", "fair", "fair", "fair", "excellent", "excellent", "fair", "fair", "fair", "excellent", "excellent", "fair", "excellent")

student <- c("no", "no", "no","no", "yes", "yes", "yes", "no", "yes", "yes", "yes", "no", "yes", "no")

income <- c("high", "high", "high", "medium", "low", "low", "low", "medium", "low", "medium", "medium", "medium", "high", "medium")

age <- c(25, 27, 35, 41, 48, 42, 36, 29, 26, 45, 23, 33, 37, 44)

we change the age from categorical to numeric

Cheers, Jack

Please add a link to the similar question so that we have all the context. — Thierry, Oct 07 '15 at 13:48
Here is the link, http://stackoverflow.com/questions/27254550/calculating-entropy ... — jack, Oct 07 '15 at 13:49

score 1 · Accepted Answer · answered Oct 07 '15 at 16:37

You need to calculate the propertion of "no" and "yes" in "buys", the proportion of "fair" and "excellent" in "credit", and so on. Here is one way to do it:

data <- list(
  buys = c("no", "no", "yes", "yes", "yes", "no", "yes", "no", "yes", "yes", "yes", "yes", "yes", "no"),
  credit = c("fair", "excellent", "fair", "fair", "fair", "excellent", "excellent", "fair", "fair", "fair", "excellent", "excellent", "fair", "excellent"),
  student = c("no", "no", "no","no", "yes", "yes", "yes", "no", "yes", "yes", "yes", "no", "yes", "no"),
  income = c("high", "high", "high", "medium", "low", "low", "low", "medium", "low", "medium", "medium", "medium", "high", "medium"),
  age = c(25, 27, 35, 41, 48, 42, 36, 29, 26, 45, 23, 33, 37, 44) 
  )

freq <- lapply( data, function(x){rowMeans(outer(unique(x),x,"=="))})

.

> freq
$buys
[1] 0.3571429 0.6428571

$credit
[1] 0.5714286 0.4285714

$student
[1] 0.5 0.5

$income
[1] 0.2857143 0.4285714 0.2857143

$age
 [1] 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857 0.07142857
[14] 0.07142857

Such a proportion can never be 0. So change if(freq.class[[i]] != 0){ # zero check in class to if(length(freq.class[[i]]) != 0){ # zero check in class:

info <- function(CLASS.FREQ){
  freq.class <- CLASS.FREQ
  info <- 0
  for(i in 1:length(freq.class)){
    if(length(freq.class[[i]]) != 0){ # zero check in class
      entropy <- -sum(freq.class[[i]] * log2(freq.class[[i]]))  #I calculate the entropy for each class i here
    }else{ 
      entropy <- 0
    } 
    info <- info + entropy # sum up entropy from all classes
  }
  return(info)
}

.

> info(freq)
[1] 8.289526
> info(freq$buys)
[1] 0.940286
> info(freq$age)
[1] 3.807355
>

to calculate the Entropy

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