Crossfilter reduce functions

Question

I want to understand how the reduce functions are used in Crossfilter. Namely, the

reduceAdd(p,v){...}
reduceRemove(p,v){...}

and

reduceInitial(p,v){...}

in

group.reduce(reduceAdd, reduceRemove, reduceInitial);

From Crossfilter's API reference, I understand that the p and v arguments represent the group and dimension value respectively.

From what I understand, the return value of the reduce functions determine what the groupValue should be changed to after the element is added or removed from the group. Is this correct?

Also, what is the reduceInitial function for?

Gordon Gordon · Accepted Answer · 2017-12-17T19:01:51

That is correct, if you substitute "bin value" for "groupValue" in what you wrote.

A group is made of an array of bins; each bin is a key-value pair. For every group, all rows of the data supplied to crossfilter will "fall into" one bin or another. The reduce functions determine what happens when a row falls into a bin, or is removed from it because the filters changed.

Crossfilter determines which bin any row falls into by using using the dimension value accessor and the group value function.

When are reduction functions called?

When a crossfilter initializes a new group, it adds all the currently matching rows of the data to all the groups. The group determines a key for each row by using the dimension value accessor and the group value function. Then it looks up the bin using that key, and applies the reduceAdd function to the previous bin value and the row data to produce the new bin value.

When any filter on any dimension of the crossfilter changes value, some rows will stop matching and some rows will start matching the new set of filters. The rows that stop matching are removed from the matching bin using the reduceRemove function, and the rows that start matching are added using the reduceAdd function.

When a row is added, some groups may not already have a bin which matches the key for that row. At that point a new bin must be initialized and at that point the group calls reduceInitial to get the user-specified blank value for the bins of that group.

Crossfilter's group.reduce compared to Array.reduce

The reduceAdd and reduceRemove functions are similar to the functions you would pass to Javascript's Array.reduce function. The first parameter p takes the previous value of the bin, and the second paramter v takes the current row data being considered.

In contrast to Array.reduce, in group.reduce

values can be removed as well as added
the initial value is produced by the reduceInitial function instead of being passed to reduce
it doesn't perform the aggregation immediately; instead you are supplying functions that will get called whenever filters change or data is added or removed

Crossfilter reduce functions

1 Answers

When are reduction functions called?

Crossfilter's group.reduce compared to Array.reduce