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fix: sub-interval midpoint formula in ternary search (#15070)

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* fix: sub-interval midpoint formula in ternary search

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
This commit is contained in:
Kanika Katare
2026-08-25 16:08:42 +02:00
committed by GitHub
co-authored by pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
parent f5988cc097
commit acd4b34b0e
1 changed files with 21 additions and 9 deletions
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searches/ternary_search.py
+21 -9
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@@ -82,6 +82,11 @@ def ite_ternary_search(array: list[int], target: int) -> int:
-1
>>> ite_ternary_search([.1, .4 , -.1], .1)
0
>>> test_list_large = list(range(100))
>>> ite_ternary_search(test_list_large, 65)
65
>>> ite_ternary_search(test_list_large, 105)
-1
"""
left = 0
@@ -90,8 +95,8 @@ def ite_ternary_search(array: list[int], target: int) -> int:
if right - left < precision:
return lin_search(left, right, array, target)
one_third = (left + right) // 3 + 1
two_third = 2 * (left + right) // 3 + 1
one_third = left + (right - left) // 3
two_third = right - (right - left) // 3
if array[one_third] == target:
return one_third
@@ -99,13 +104,13 @@ def ite_ternary_search(array: list[int], target: int) -> int:
return two_third
elif target < array[one_third]:
right = one_third - 1
right = one_third
elif array[two_third] < target:
left = two_third + 1
else:
left = one_third + 1
right = two_third - 1
right = two_third
return -1
@@ -133,12 +138,19 @@ def rec_ternary_search(left: int, right: int, array: list[int], target: int) ->
-1
>>> rec_ternary_search(0, 3, [.1, .4 , -.1], .1)
0
>>> test_list_large = list(range(100))
>>> rec_ternary_search(0, len(test_list_large), test_list_large, 65)
65
>>> rec_ternary_search(20, 80, test_list_large, 65)
65
>>> rec_ternary_search(20, 80, test_list_large, 15)
-1
"""
if left < right:
if right - left < precision:
return lin_search(left, right, array, target)
one_third = (left + right) // 3 + 1
two_third = 2 * (left + right) // 3 + 1
one_third = left + (right - left) // 3
two_third = right - (right - left) // 3
if array[one_third] == target:
return one_third
@@ -146,11 +158,11 @@ def rec_ternary_search(left: int, right: int, array: list[int], target: int) ->
return two_third
elif target < array[one_third]:
return rec_ternary_search(left, one_third - 1, array, target)
return rec_ternary_search(left, one_third, array, target)
elif array[two_third] < target:
return rec_ternary_search(two_third + 1, right, array, target)
else:
return rec_ternary_search(one_third + 1, two_third - 1, array, target)
return rec_ternary_search(one_third + 1, two_third, array, target)
else:
return -1
@@ -165,7 +177,7 @@ if __name__ == "__main__":
assert collection == sorted(collection), f"List must be ordered.\n{collection}."
target = int(input("Enter the number to be found in the list:\n").strip())
result1 = ite_ternary_search(collection, target)
result2 = rec_ternary_search(0, len(collection) - 1, collection, target)
result2 = rec_ternary_search(0, len(collection), collection, target)
if result2 != -1:
print(f"Iterative search: {target} found at positions: {result1}")
print(f"Recursive search: {target} found at positions: {result2}")