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Hello Python

Find the First Matching Index

Practice ExerciseIntermediateMediumArrayLinear Search

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Problem

Implement first_index(values, target). Return the first matching index or -1 when target is absent. Do not use list.index.

Starter code

def first_index(values, target):
    pass
Test cases

first-duplicate

{
  "args": [
    [
      4,
      2,
      4,
      7
    ],
    4
  ]
}

Expected: 0

missing

{
  "args": [
    [
      1,
      3
    ],
    2
  ]
}

Expected: -1

Wizard outline
  1. Step 1: Define absence

    Return -1 for an empty search space. The sentinel is part of the public contract and anchors the loop fallback.

  2. Step 2: Inspect the current candidate

    Recognize a match at the first position. Checking before moving preserves the earliest-index invariant.

  3. Step 3: Complete the monotone scan

    Find the first match anywhere or prove absence. Enumerating in increasing index order makes the first returned match minimal.

Footguns and prerequisites
  • Returning the last match violates the first-index contract.
  • Calling list.index hides absence handling behind an exception.
  • functions
  • lists and tuples
  • control flow
Reviewed references
Recommended approach and implementation

Enumerate values from left to right and return on the first equality.

Why it works: Indexes are visited in increasing order. Therefore the first equality is the smallest matching index; reaching the end proves absence.

def first_index(values, target):
    for index, value in enumerate(values):
        if value == target:
            return index
    return -1