javascript - D3: What is a Bisector? -

i'm looking making charts d3, , stumbled upon d3.bisector. however, don't understand or documentation.

almost examples find in web use date array, similar example in official documentation:

var data = [   {date: new date(2011,  1, 1), value: 0.5},   {date: new date(2011,  2, 1), value: 0.6},   {date: new date(2011,  3, 1), value: 0.7},   {date: new date(2011,  4, 1), value: 0.8} ];  var bisect = d3.bisector(function(d) { return d.date; }).right; 

so bisector do, besides picking date object array elements? *.right return?

and of use if have simple 1-dimensional array, var data = [3, 6, 2, 7, 5, 4, 8]?

thanks enlightening me.

the underlying idea behind bisect this:

consider array mention - var data = [3, 6, 2, 7, 5, 4, 8]

you want insert new value let's 3.5 data array , want know how 'partition' it. in other words, want know index of 3.5 if inserted when data array sorted.

   var data = [3, 6, 2, 7, 5, 4, 8]     //sorted data    [2, 3, 4, 5, 6, 7, 8]    //you want insert 3.5     sorted array after insertion of 3.5 should like:    [2, 3, 3.5, 4, 5, 6, 7, 8]     index of 3.5 in sorted data array "2". 

there situations want know how insertion of element 'bisects' or 'divides' array. in case, want first sort array , call binary search find out correct position insertion of element.

bisectleft , bisectright take care clarify anomaly in situation want enter element exists in array. let's want enter 3 array. there 2 situations:

   3* -> new element entered      [2, 3*, 3, 4, 5, 6, 7, 8] -> entered @ "1" (array still sorted)      [2, 3, 3*, 4, 5, 6, 7, 8] -> entered @ "2" (array still sorted) 

so depending upon how take care of ambiguity, can enter element 'left' or 'right' of existing element. docs (mark emphasis):

the returned insertion point partitions array 2 halves v < x v in array.slice(lo, i) left side , v >= x v in array.slice(i, hi) right side.

in bisectleft 1 suitable index, duplicate entries will on right of index , situation opposite in bisecright.

now know how bisectleft , bisectright work, bisector allows define custom comparator or accessor function parition values or make sense of < , > on objects well.

so piece of code:

  var bisect = d3.bisector(function(d) { return d.date; }).right;    var bisect = d3.bisector(function(a, b) { return a.date - b.date; }).right; 

just specifies use bisectright option , return suitable index insertion of element assuming array sorted(in ascending order).

so if build on example , assuming bisector named bisect. , did:

 bisect(data, 3); //it return 2. 

i hope clarifies things , gets started in right direction.