Functional Programming Grammar Question

JavaScript
1

Hi!
I am working through the excellent Site Point book, JavaScript, Novice to Ninja.
Chapter 11, Functional Programming
I have been trying to tease out how exactly the syntax works…and I have created this little file:


//  const square = (x) => x * x

//  const mean = (array) => sum(array) / array.length
//  const variance = (array) => sum(array, square) / array.length - square(mean(array))
// const hypoteneuse = (a, b) => Math.sqrt(square(a) + square(b))

 // const areaC = (d) => (3.14 + square(d))
  // const stdDev = (array) => Math.sqrt(sum(square(array - array)) / (array.length - 1))

  function sum (array, callback) {
    if (callback) {
      array = array.map(callback)
    }
    return array.reduce((a, b) => a + b)
  }
  console.log(sum([4, 8, 9, 7, 6, 9]))

When I try to use the function for standard deviation (const stdDev) by uncommenting the appropriate functions and doing
console.log(stdDev[4, 8, 9, 7, 6, 9])) - I get
“array.reduce is not a function” as an error message

What am I missing?

Thank you!!!

2

How are you running this? I ask, because I just ran that in Chrome in the console, and it gave me the answer 43.

3

Ah! I am running in FF developer Edition.
Didn’t think to try in another browser.

4

That wouldn’t make a difference… the .reduce() method is even supported by IE9+. Rather, that error message means that you’re passing something other than an array to the sum() function. Now in this line

const stdDev = (array) => Math.sqrt(sum(square(array - array)) / (array.length - 1))

you are subtracting array from array, but subtraction only works for numbers; so the result you are passing to square() is NaN (and NaN * NaN is still NaN).

1 Like
5

I ignored the lines of code above the sum() function, as it was commented out. I just picked up the rest of the uncommented lines and ran that.

6

Thank you!!
I had deduced that it had to be something in the stdDev function that I had tried writing on my own…
And I had had the thought that it probably was something to do with array - array…
Dang!!!
Happy!!!
Thank you!!!
Just thank you. I think I have taught myself quite a lot quite quickly and it’s just going to take some (a lot) of practice before I can easily remember enough —
Anyway, thank you again!

2 Likes
7

Ah – yeah. Sorry about that.
The only issue I have been having is with the new function I tried to write “stdDev”
Sorry if that wasn’t clear.

1 Like
8

Um?

const square = (x) => x * x

//  const mean = (array) => sum(array) / array.length
//  const variance = (array) => sum(array, square) / array.length - square(mean(array))
// const hypoteneuse = (a, b) => Math.sqrt(square(a) + square(b))

 // const areaC = (d) => (3.14 + square(d))
   const stdDev = (array) => Math.sqrt(sum(square((sum(array)) - (sum(array)))) / (array.length - 1))

   function sum (array, callback) {
     if (callback) {
       array = array.map(callback)
     }
     return array.reduce((a, b) => a + b)
   }
   console.log(stdDev([4, 8, 9, 7, 6, 9]))
  // console.log(typeof )

Is it okay to ask…
Why does this not “work”

The attempt is to use sum() to sum the arrays then subtract them…

I can see that things are getting pretty convoluted in my code :slight_smile:
But still…can’t follow why it wouldn’t “work”

The plot thickens…

const subSum = (array) => (sum(array)) - (sum(array))

This does "work" as intended...changing the array into a number ... which makes sense -- also completely losing track of the idea of standard deviation...but I am really just practicing ...

Anyway...thanks in advance
9

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