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MediumDSAAsked in 20245 min read

Count Occurrences of Each Element in an Array (Frequency Counter)

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The problem

Problem, as an interviewer would state it: "Count how many times each distinct element appears in an array. Discuss edge cases, runtime, and alternative approaches."

Examples:

Code
count(["a", "b", "a", "c", "a", "b"]); // a: 3, b: 2, c: 1

Clarifying questions expected:

  • Should 1 and "1" be counted separately? What about NaN?
  • Are the elements guaranteed to be safe strings, or could one be the word constructor?
  • Do you also want the results sorted by frequency, or just the counts?

Code / implementation expected: Yes — real, direct proof of the correct counter and of exactly how the naive object counter goes wrong.

  • js-coding
  • utility
  • algorithm

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Solution

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5 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: a Map-based frequency counter is correct, while a plain-object counter corrupts the word constructor and merges 1 with the string 1
Practice

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