List Processing and In-Place Mutation Notes for GATE DA: Concepts, Formulas, Worked Examples & Practice

    List Processing and In-Place Mutation notes for GATE DA: 11 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

    Chapter Roadmap: List Processing and In-Place Mutation

    Chapter Roadmap

    Chapter journey

    1. Recursive List Processing and In-Place Mutation
      Current topic. High-importance tracing.
      • Single-index recursive scans
      • Adjacent-swap counting
      • Two-pointer reversal
    2. Chapter mastery
      • Track exact list state after every swap
      • Decide if call is scan or reversal
      • Predict final return value and list
    Weight hint: Core tracing topic. Small code pieces can carry good marks through careful step-by-step execution.

    Topic Hero: Recursion Meets Mutable Lists

    Topic Hero: Recursion Meets Mutable Lists

    A Python list is mutable. If a recursive function changes the list, the changed list is visible to all deeper recursive calls.

    Two core shapes appear repeatedly:

    Shape Indices used Typical operation
    Single-index scan i Compare and possibly swap L[i] and L[i+1]
    Two-pointer reversal s1, s2 Swap D[s1] and D[s2], then move inward
    Key idea: The list is shared, but the index variables are local to each recursive call.

    The List Is Shared, Indices Are Local

    The List Is Shared, Indices Are Local

    In-place mutation

    def swap_positions(L, i, j):
L[i], L[j] = L[j], L[i]

    This changes the actual list object. There is no new list being created.

    What this means for recursion

    • If call 1 swaps elements, call 2 sees the swapped list.
    • If call 2 swaps elements, call 3 sees the newer list.
    • Changing i, j, s1, or s2 inside one call does not change those variables in earlier calls.

    Python swap detail

    L[i], L[j] = L[j], L[i]

    The right-hand side is evaluated first using the old list values. Then both assignments happen together. This avoids the classic mistake of overwriting one value before saving the other.

    Single-Index Recursive Scan Pattern

    Single-Index Recursive Scan Pattern

    Basic shape

    def scan(L, i=0):
if i >= len(L) - 1:
    return 0
if L[i] > L[i + 1]:
    L[i], L[i + 1] = L[i + 1], L[i]
    return 1 + scan(L, i + 1)
return scan(L, i + 1)

    Why the base condition is i >= len(L) - 1

    The function compares L[i] with L[i + 1].

    If len(L) = n, the last valid comparison is between indices n - 2 and n - 1. Therefore recursion should stop when i reaches n - 1.

    Empty and single-element lists

    • If len(L) = 0, then len(L) - 1 = -1, and 0 >= -1 is true.
    • If len(L) = 1, then len(L) - 1 = 0, and 0 >= 0 is true.

    Both cases safely return the base value.

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    List Processing and In-Place Mutation Notes for GATE DA: Concepts, Formulas, Worked Examples & Practice

    List Processing and In-Place Mutation notes for GATE DA: 11 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

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