31:T5e4,Percentile annotates a dataset with percentile values from `0` to `1`. The only required argument is an iterable `data`. After that it accepts an optional `opts` object that may have these properties:

`accessor`: A function that returns a sortable value for comparison. (Default: `d => d`)
`groupby`: A function that produces a key to use for grouping data; percentiles will be calculated for each datum _within their group_. If not provided, percentiles will be calculates as though the dataset is one group. Uses [d3.group](https://observablehq.com/@d3/d3-group) to perform the grouping, therefore key equality is determined by the [_SameValueZero_ algorithm](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness#Same-value-zero_equality). (Default: `d => null`)
`cmp`: A function to sort values with; should use the same structure as a [`compareFunction` for `Array.prototype.sort`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort) (Default: `(a, b) => a - b`)
`asProp`: By default `percentile` will return a list of 2-arrays, where the first entry is the original datum and the second entry is the percentile value. If `asProp == true` it will instead assume the provided `data` is an iterable of objects and add a `percentile` key to each data object. (Default: `false`)

### Usage Examples Annotating a simple list:32:[] 2f:{"title":"Array Utilities","cells":[{"id":0,"value":"# Array Utilities","pinned":false,"hidden":false,"mode":"md"},{"id":10,"value":"## Closest\n\n`closest` accepts a `data` array, a `target`, an `accessor` function that should return a value, that can be compared to `target`, extracted from the data point, and a comparison function `cmp` that should return a number representing the distance between two values.\n\nIt returns the data entry for which the result of `accessor` is closest to `value` using the comparison function.\n\nFunction signature:\n\n~~~js\nfunction closest(\n data,\n target,\n accessor = x => x,\n cmp = (a, b) => Math.abs(a - b)\n)\n~~~\n\n### Usage examples\n\nFind the value closest to `2.1` in a list of integers:","pinned":false,"hidden":false,"mode":"md"},{"id":24,"value":"closest([1, 2, 3, 4], 2.1)","pinned":true,"hidden":false,"mode":"ojs"},{"id":189,"value":"Find the item with `d.value` closest to `1.9` in a list of objects:","pinned":false,"hidden":false,"mode":"md"},{"id":28,"value":"closest(\n [\n {key: 'a', value: 1},\n {key: 'b', value: 2},\n {key: 'c', value: 3},\n ],\n 1.9,\n x => x.value\n)","pinned":true,"hidden":false,"mode":"ojs"},{"id":5,"value":"closest = (\n data,\n target,\n accessor = x => x,\n cmp = (a, b) => Math.abs(a - b)\n) => {\n const idx = d3.bisectLeft(data.map(accessor), target)\n if (idx === data.length - 1) return data[idx]\n return cmp(accessor(data[idx - 1]), target) < cmp(accessor(data[idx]), target) ? data[idx - 1] : data[idx]\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":77,"value":"***\n\n## Percentile","pinned":false,"hidden":false,"mode":"md"},{"id":106,"value":"$31","pinned":false,"hidden":false,"mode":"md"},{"id":99,"value":"percentile([1, 2, 3, 4, 5, 6])","pinned":true,"hidden":false,"mode":"ojs"},{"id":83,"value":"percentile = (data, _opts) => {\n const opts = {\n accessor: d => d,\n groupby: d => null,\n cmp: (a, b) => a - b,\n asProp: false,\n ..._opts\n }\n // Group data using the groupby function\n const grouped = Array.from(d3.group(data, opts.groupby))\n\n // This reducer iterates over each group and annotates\n // the datapoints with percentiles, then pops them back\n // into the accumulator, producing a flat array of datapoints\n // with percentiles\n const output = grouped.reduce((acc, [key, group]) => {\n return acc.concat(\n group.sort(\n // Sort to ensure data is properly ordered\n (a, b) => opts.cmp(opts.accessor(a), opts.accessor(b))\n ).map(\n // Annotate data with a calculated percentile\n (d, i, arr) => opts.asProp ? { ...d, percentile: i / (arr.length - 1) } : [d, i / (arr.length - 1)]\n )\n )\n }, [])\n\n // Sort the output before returning it\n return output.sort(\n (a, b) => opts.cmp(opts.accessor(a), opts.accessor(b))\n )\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":15,"value":"***\n\n## Appendix","pinned":false,"hidden":false,"mode":"md"},{"id":7,"value":"d3 = require('d3@6')","pinned":true,"hidden":false,"mode":"ojs"}],"files":"$W32","stdlib":"1","titling":"auto"}