d3-comparator

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Multi-dimensional comparator function generator for D3

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# d3.comparator

A [D3](http://d3js.org) generator for comparator functions. It can be used to easily sort arrays of objects by one or multiple dimensions.

If you want to sort by a single dimension, you end up writing comparators like `function cmp(a, b) { return a.value - b.value; }` or `function cmp(a, b) { return d3.ascending(a.value, b.value); }`. For sorting by two or more dimensions it gets ugly pretty fast. d3.comparator provides a simple API for generating those comparators.

## API

d3.comparator()

Constructs a new comparator with the default return value 0. I.e. using it will not change sort order.

comparator(a, b)

The comparator can be used with the array sort method.

comparator.order(cmp, [accessor])

Adds a dimension to the comparator. The return value of accessor will be compared with cmp (which has to be a comparator function itself, e.g. d3.ascending). If accessor isn't specified, an identity function `function(d) { return d; }` is used.

Dimensions are compared in order. As soon as cmp returns something other than `0`, subsequent dimensions are ignored.

Example using a single dimension:

```javascript
var cmp = d3.comparator().order(d3.ascending, function(d) { return d.value; });
```

Example using two dimensions, roughly equivalent to SQL `ORDER BY year DESC, value ASC`:

```javascript
var cmp = d3.comparator()
  .order(d3.descending, function(d) { return d.year; })
  .order(d3.ascending, function(d) { return d.value; });
```

Note: These are just comparator functions. To acually sort an array use

```javascript
someArray.sort(cmp);
```

## Author

Jeremy Stucki, [Interactive Things](http://interactivethings.com)

## License

BSD, see LICENSE.txt

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