FAQ: Binary Search: Python - Recursive Binary Search: The Recursive Steps

This community-built FAQ covers the “Recursive Binary Search: The Recursive Steps” exercise from the lesson “Binary Search: Python”.

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I am not understanding how the result is calculated in the recursive code for (result + mid_idx + 1). Mid_idx is fixed at half of the sorted_list. 1 is added to the zero based index. Result should always be 1, as the last iteration of binary_search? So if Result is always 1, then a right hand value would not work, because (1 + Mid_idx + 1) < index. Unless result is being stored somehow dynamically? Thank you.

Users may find the course on recursion helpful. It helped me see that mid_idx fluctuates with each function call.

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I don’t understand why we have to save the recursive call to result:

result = binary_search(right_half, target)
    if result == "value not found":
      return result
    return result + mid_idx + 1

Why this:
result == binary_search(right_half, target)
?
Why do I save the recursive call to a variable?

Why this:
return result + mid_idx + 1
?
Why do I return result(I expect that) but with an extra mid_idx and 1?

Why doesn’t this save it to a variable and return an extra mid_idx and 1:

if mid_val > target:
    left_half = sorted_list[:mid_idx]
    return binary_search(left_half, target)

?

1 Like

Its because when you are meeting the condition of using the right half of your original list, you are taking the a slice of that list starting from the mid index of the original list plus one index. Now the right half of your list is a new list with different indeces for the same values seen in the original list. For example, you are searching for 5 in the list [0,1,2,3,4,5,6,7] which is at index 6. Going through the function you will need use recursion to slice the list and take the right half [4,5,6,7] but now your target value of 5 is at index 2. To reference the original list, you would have to take this into consideration so saving it to result and adding mid_idx+1 will give you the reference to the original list

can someone answer this???

ok thanks its starting to make sense, from the lesson, result returns 0 after recursive call on the right half, then the mid_idx is 2, so its (0 + 2 + 1) = 3, and index of 16 in original list is 3.

To anyone who still doesnt get it, first experiment returning without adding result to mid_idx + 1, you’ll see you only get 0. But thats not the index of 16 in the original list.