Crossfilter - Minimum Value by Day

Question

How do I find the minimum value per day? The dimension being the Days, the group the minimum amount. Using the following data as an example -

var data = [
  {date: "2011-11-14T10:17:54Z", amount: 10 },
  {date: "2011-11-14T12:20:19Z", amount: 1 },
  {date: "2011-11-14T14:20:19Z", amount: 0 },
  {date: "2011-11-15T06:30:43Z", amount: 10 },
  {date: "2011-11-15T10:30:43Z", amount: 10 },
  {date: "2011-11-15T16:28:54Z", amount: 100 },
  {date: "2011-11-16T18:48:46Z", amount: 100 },
  {date: "2011-11-16T20:53:41Z", amount: 11 },
  {date: "2011-11-16T22:54:06Z", amount: 10 },
];

So the results should be -

  {date: "2011-11-14T14:20:19Z", amount: 0 },
  {date: "2011-11-15T06:30:43Z", amount: 10 },
  {date: "2011-11-15T10:30:43Z", amount: 10 },
  {date: "2011-11-16T22:54:06Z", amount: 10 },

I'm going round in circles trying to work this out and I'm now completely confused so any help would be appreciated!

Thanks.

Answer 1

The following will get your your absolute minimum. Won't work with filtering though!

var data = [
  {date: "2011-11-14T10:17:54Z", amount: 10 },
  {date: "2011-11-14T12:20:19Z", amount: 1 },
  {date: "2011-11-14T14:20:19Z", amount: 0 },
  {date: "2011-11-15T06:30:43Z", amount: 10 },
  {date: "2011-11-15T10:30:43Z", amount: 10 },
  {date: "2011-11-15T16:28:54Z", amount: 100 },
  {date: "2011-11-16T18:48:46Z", amount: 100 },
  {date: "2011-11-16T20:53:41Z", amount: 11 },
  {date: "2011-11-16T22:54:06Z", amount: 10 },
];

var ndx = crossfilter(data);
var dayDim = ndx.dimension(function (d) { return "" + d.date.substr(8,2); });
var minGroup = dayDim.group().reduce(
  function (p, v) { if(v.amount < p || p === null) p = v.amount; return p; },
  function (p, v) { return p; },
  function () { return null; }
);

console.table(minGroup.top(Infinity));

If you want filtering to work, you need to keep a sorted array of all the values found in each array and then add/remove values as data is filtered and unfiltered. Then use this sorted array to figure out your minimum. Something like the following (untested):

var data = [
  {date: "2011-11-14T10:17:54Z", amount: 10 },
  {date: "2011-11-14T12:20:19Z", amount: 1 },
  {date: "2011-11-14T14:20:19Z", amount: 0 },
  {date: "2011-11-15T06:30:43Z", amount: 10 },
  {date: "2011-11-15T10:30:43Z", amount: 10 },
  {date: "2011-11-15T16:28:54Z", amount: 100 },
  {date: "2011-11-16T18:48:46Z", amount: 100 },
  {date: "2011-11-16T20:53:41Z", amount: 11 },
  {date: "2011-11-16T22:54:06Z", amount: 10 },
];

var ndx = crossfilter(data);
var dayDim = ndx.dimension(function (d) { return "" + d.date.substr(8,2); });
var minGroup = dayDim.group().reduce(
  function (p, v) { 
    if(v.amount < p.min || p.min === null) {
      p.min = v.amount;
    }
    p.values.push(v.amount);
    return p;
  },
  function (p, v) {
    p.values.splice(p.values.indexOf(v.amount),1);
    if(v.amount === p.min) {
      p.values.sort(function(a,b) { return a < b ? -1 : 1; });
      p.min = p.values[0];
    } 
    return p;
  },
  function () { return { min: null, values: [] }; }
);

console.log(minGroup.top(Infinity));