# Map list to bin numbers

Does WL have the equivalent of Matlab's discretize or NumPy's digitize? I.e., a function that takes a length-N list and a list of bin edges and returns a length-N list of bin numbers, mapping each list item to its bin number?

• HistogramList seems similar. This could also be done efficiently with GroupBy and some easy little Compile-d selection determiner. Or maybe hit it first with Sort then write something that only checks the next bin up. Again, can be easily Compile-d. – b3m2a1 Apr 8 '19 at 23:07
• I need it to work like a map (in terms of the order of the items in the resulting list). Of course it is possible to write something ... – Alan Apr 9 '19 at 0:13
• Related: 140577 – Carl Woll Apr 9 '19 at 3:50
• Did you try BinCounts? I guess it is what you need. – Rom38 Apr 9 '19 at 4:52
• @Rom38 You probably meant BinLists, right? – Henrik Schumacher Apr 9 '19 at 5:55

Here's a version based on Nearest:

digitize[edges_] := DigitizeFunction[edges, Nearest[edges -> "Index"]]
digitize[data_, edges_] := digitize[edges][data]

DigitizeFunction[edges_, nf_NearestFunction][data_] := With[{init = nf[data][[All, 1]]},
init + UnitStep[data - edges[[init]]] - 1
]


For example:

SeedRandom
data = RandomReal[10, 10]
digitize[data, {2, 4, 5, 7, 8}]


{8.17389, 1.1142, 7.89526, 1.87803, 2.41361, 0.657388, 5.42247, 2.31155, 3.96006, 7.00474}

{5, 0, 4, 0, 1, 0, 3, 1, 1, 4}

Note that I broke up the definition of digitize into two pieces, so that if you do this for multiple data sets with the same edges list, you only need to compute the nearest function once.

This is a very quick-n-dirty, but may serve as a simple example.

This creates a piecewise function following the first definition in Matlab's discretize documentation, then applies that to the data.

disc[data_, edges_] := Module[{e = Partition[edges, 2, 1], p, l},
l = Length@e;
p=Piecewise[Append[Table[{i, e[[i, 1]] <= x < e[[i, 2]]}, {i, l - 1}]
, {l,e[[l, 1]] <= x <= e[[l, 2]]}]
, "NaN"];
Table[p, {x, data}]];


From the first example in the above referenced documentation:

data={1, 1, 2, 3, 6, 5, 8, 10, 4, 4};
edges={2, 4, 6, 8, 10};

disc[data,edges]


{NaN,NaN,1,1,3,2,4,4,2,2}

I'm sure there are more efficient/elegant solutions, and will revisit as time permits.

You may also use Interpolation with InterpolationOrder -> 0. However, employing Nearest as Carl Woll did will usually be much faster.

First, we prepare the interplating function.

m = 20;
binboundaries = Join[{-1.}, Sort[RandomReal[{-1, 1}, m - 1]], {1.}];

f = Interpolation[Transpose[{binboundaries, Range[0, m]}], InterpolationOrder -> 0];


Now you can apply it to lists of values as follows:

vals = RandomReal[{-1, 1}, 1000];
Round[f[vals]]