Counting the number of values in each interval

BinCounts[aRRay, {0, 0.4, 0.1}]

HistogramList[aRRay, {0, .4, .1}][[2]]

{1, 2, 0, 4}

This is slower than BinCounts for regularly spaced bins:

SeedRandom[43]; aRRay = Sort@RandomReal[1, 1000000];
BinCounts[aRRay, {0, 1., .1}] // Timing

{1.093812, {99869, 99884, 100051, 100351, 99540, 99971, 99695, 99930, 100337, 100372}}

HistogramList[aRRay, {0, 1., .1}][[2]] // Timing

{1.527600, {99869, 99884, 100051, 100351, 99540, 99971, 99695, 99930, 100337, 100372}}

Slightly faster for irregular bins:

bins = Flatten[{0, Sort[RandomReal[1, 50]], 1}];
BinCounts[aRRay, {bins}] // AbsoluteTiming // First

1.667586

HistogramList[aRRay, {bins}][[2]] // AbsoluteTiming // First

1.606415

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