stevenbeales
12/27/2018 - 3:05 AM

## Binary Search

With a sorted data-set, we can take advantage of the ordering to make a sort which is more efficient than going element by element.

Binary search requires a sorted data-set. We then take the following steps:

1. Check the middle value of the dataset.
2. If this value matches our target we can return the index.
3. If the middle value is less than our target
4. Start at step 1 using the right half of the list.
5. If the middle value is greater than our target
6. Start at step 1 using the left half of the list.

We eventually run out of values in the list, or find the target value.

In each iteration, we are cutting the list in half. The time complexity is O(log N).

``````# In-place binary search example

def binary_search(sorted_list, left_pointer, right_pointer, target):
# this condition indicates we've reached an empty "sub-list"
if left_pointer >= right_pointer:

# We calculate the middle index from the pointers now
mid_idx = (left_pointer + right_pointer) // 2
mid_val = sorted_list[mid_idx]

if mid_val == target:
return mid_idx
if mid_val > target:
# we reduce the sub-list by passing in a new right_pointer
return binary_search(sorted_list, left_pointer, mid_idx, target)
if mid_val < target:
# we reduce the sub-list by passing in a new left_pointer
return binary_search(sorted_list, mid_idx + 1, right_pointer, target)

values = [77, 80, 102, 123, 288, 300, 540]
start_of_values = 0
end_of_values = len(values)
result = binary_search(values, start_of_values, end_of_values, 288)

print("element {0} is located at index {1}".format(288, result))

-------------------------------------------------------------------------------------------------------

# Iterative version

def binary_search(sorted_list, target):
left_pointer = 0
right_pointer = len(sorted_list)

# fill in the condition for the while loop
while left_pointer < right_pointer:
# calculate the middle index using the two pointers
mid_idx = (right_pointer + left_pointer) // 2
mid_val = sorted_list[mid_idx]
if mid_val == target:
return mid_idx
if target < mid_val:
# set the right_pointer to the appropriate value
right_pointer = mid_idx
if target > mid_val:
# set the left_pointer to the appropriate value
left_pointer = mid_idx + 1

return "Value not in list"

# test cases
print(binary_search([5,6,7,8,9], 9))
print(binary_search([5,6,7,8,9], 10))
print(binary_search([5,6,7,8,9], 8))
print(binary_search([5,6,7,8,9], 4))
print(binary_search([5,6,7,8,9], 6))``````