Eloquent Javascript 2nd. Chapter 4. Computing Correlation. Final Analysis - [Variable Recursion & in operator] Pt 2 -

Eloquent Javascript 2nd. Chapter 4. Computing Correlation. Final Analysis - [Variable Recursion & in operator] Pt 2 -

i'm working through end of first illustration in chapter 4 eloquent javascript. here total piece of code (it's lastly piece have questions regarding attached first portion reference).

var journal = []; function addentry(events, diditurnintoasquirrel) { journal.push({ events: events, squirrel: diditurnintoasquirrel }); function phi(table) { homecoming (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])); } function hasevent(event, entry) { homecoming entry.events.indexof(event) != -1; } function tablefor(event, journal) { var table = [0, 0, 0, 0]; (var = 0; < journal.length; i++) { var entry = journal[i], index = 0; if (hasevent(event, entry)) index += 1; if (entry.squirrel) index += 2; table[index] += 1; } homecoming table; } function gathercorrelations(journal) { var phis = {}; (var entry = 0; entry < journal.length; entry++) { var events = journal[entry].events; (var = 0; < events.length; i++) { var event = events[i]; if (!(event in phis)) phis[event] = phi(tablefor(event, journal)); } } homecoming phis; } var correlations = gathercorrelations(journal); console.log(correlations.pizza);

my questions are:

what purpose of .events in

var events = journal[entry].events;

does phone call on recursion? if why? couldn't have had journal[entry] , function run calling on entry tablefor function? phone call addentry function (where events variable established) in of import way?

what purpose of (!(event in phis)).

i read : if event in phis true flip not true , trigger necessary phi calculation. wouldn't create more sense eliminate ! (does not equal) or piece of code altogether? if have loop won't function run on it's on until max length of journal , stop?

var events = journal[entry].events; getting events object object @ index entry in array journal , assigning temporary variable called events.

this done convenience don't have maintain referring journal[entry].events. example, later on has line:

var event = events[i];

which become:

var event = journal[entry].events[i];

without assignment temporary variable.

if(!(event in phis)) it's testing see if object phis not have property name contained in variable event. if object doesn't have property, adds next line:

phis[event] = phi(tablefor(event, journal));

see in operator

here's simple snippet of code help understand in operator:

var foo = {}; console.log("bar" in foo); // logs "false" because foo doesn't have "bar" property foo.bar = 1; // or foo["bar"] = 1; console.log("bar" in foo); // logs "true" because foo has "bar" property

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