arrays - Explain how the phi coefficient function works in Eloquent Javascript? - Stack Overflow

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I'm currently reading Eloquent Javascript and so far it's been a good read, but I'm stuck on this one function that he's put up, for calculating the phi-coeffcient. This is the code.

There's obviously quite a bit of context for it, and I can't copy/paste everything off the book, so if someone who's actually read the book could explain this to me, it would be awesome!

What I don't get is, what is being referred to when he says "table[3]", or "table[0]"? I understand the phi coefficient formula:

ϕ = (n11n00 - n10n01) / (√ n1•n0•n•1n•0)

But I don't get (at all) how he's translated that into JS. What exactly is happening in this code?

function phi(table) {
  return (table[3] * table[0] - table[2] * table[1]) /
    Math.sqrt((table[2] + table[3]) *
              (table[0] + table[1]) *
              (table[1] + table[3]) *
              (table[0] + table[2]));
}

I'm currently reading Eloquent Javascript and so far it's been a good read, but I'm stuck on this one function that he's put up, for calculating the phi-coeffcient. This is the code.

There's obviously quite a bit of context for it, and I can't copy/paste everything off the book, so if someone who's actually read the book could explain this to me, it would be awesome!

What I don't get is, what is being referred to when he says "table[3]", or "table[0]"? I understand the phi coefficient formula:

ϕ = (n11n00 - n10n01) / (√ n1•n0•n•1n•0)

But I don't get (at all) how he's translated that into JS. What exactly is happening in this code?

function phi(table) {
  return (table[3] * table[0] - table[2] * table[1]) /
    Math.sqrt((table[2] + table[3]) *
              (table[0] + table[1]) *
              (table[1] + table[3]) *
              (table[0] + table[2]));
}

• javascript
• arrays
• math

Share Improve this question Follow edited May 24, 2017 at 13:33 asked Oct 12, 2014 at 9:03 user3696530user3696530

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1 Answer 1

Sorted by: Reset to default 9

Try to visualize the process on each step, and substitute, like an equation:

┌──────────────┬──────────────┐
│ label: n00   │ label: n01   │
│ count: 76    │ count: 9     │
│ no squirrel, │ no squirrel, │
│ no pizza     │ pizza        │
├──────────────┼──────────────┤
│ label: n10   │ label: n11   │
│ count: 4     │ count: 1     │
│ squirrel,    │ squirrel,    │
│ no pizza     │ pizza        │
└──────────────┴──────────────┘

table = [n00, n01, n10, n11]

n00 = table[0] = 76
n10 = table[1] = 4
n01 = table[2] = 9
n11 = table[3] = 1

n1• = n10 + n11 = table[2] + table[3] = 9  + 1 = 10
n0• = n00 + n01 = table[0] + table[1] = 76 + 4 = 80
n•1 = n01 + n11 = table[1] + table[3] = 4  + 1 = 5
n•0 = n00 + n10 = table[0] + table[2] = 76 + 9 = 85

//pseudo code
phi = function(table) {
  return (n11 * n00 - n10 * n01) /
    Math.sqrt(n1• * n0• * n•1 * n•0)
}

//JavaScript code
phi = function(table) {
  return (table[3] * table[0] - table[2] * table[1]) /
    Math.sqrt((table[2] + table[3]) *
              (table[0] + table[1]) *
              (table[1] + table[3]) *
              (table[0] + table[2]))
}

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