Problems tagged with "recursion"

Suppose binary_search has been called on an array arr with a target t that is not necessarily in the array, and with start and stop indices start and stop, respectively (remember, our convention is that the start index is included, while the stop index is excluded from the search). Which of the following must be true? Mark all which apply. You may assume that stop - start\(\geq 1\).

Note: remember that any statement about an empty array is considered to be automatically true. Also, you cannot assume that this call to binary_search is the root call (it could be a recursive call).

Suppose binary_search is called on a sorted array of size \(n\). In total, how many recursive calls to binary_search are made in the worst case?

Consider the code below which claims to compute the most common element in the array, returning a pair: the element along with the number of times it appears.

import math

def most_common(arr, start, stop):
    """Attempts to compute the most common element in arr[start:stop]."""
    if stop - start == 1:
        return (arr[start], 1)

    middle = math.floor((start + stop) / 2)

    left_value, left_count = most_common(arr, start, middle)
    right_value, right_count = most_common(arr, middle, stop)

    if left_count > right_count:
        return (left_value, left_count)
    else:
        return (right_value, right_count)

You may assume that the function is always called on a non-empty array, and with start = 0 and stop = len(arr). Will this code always return the correct answer (the most common element)?

Consider the code for mergesort and merge where a new print-statement has been added to merge:

def mergesort(arr):
    """Sort array in-place."""
    if len(arr) > 1:
        middle = math.floor(len(arr) / 2)
        left = arr[:middle]
        right = arr[middle:]
        mergesort(left)
        mergesort(right)
        merge(left, right, arr)

def merge(left, right, out):
    """Merge sorted arrays, store in out."""
    left.append(float('inf'))
    right.append(float('inf'))
    left_ix = 0
    right_ix = 0

    for ix in range(len(out)):
        print("Here!") # <---- the newly-added line
        if left[left_ix] < right[right_ix]:
            out[ix] = left[left_ix]
            left_ix += 1
        else:
            out[ix] = right[right_ix]
            right_ix += 1

Suppose mergesort is called on an array of size \(n\). In total, how many times will "Here!" be printed?

Suppose we modify binary_search so that instead of using the floor operation to find the middle element of the array, we use the ceiling operation (recall that the ceiling of a real number \(x\) is the smallest integer that is \(\geq x\)). The full code is shown below for convenience:

import math

def new_binary_search(arr, start, stop, target):
    if stop <= start:
        return None

    middle = math.ceil((start + stop) / 2)

    if arr[middle] == target:
        return middle
    elif arr[middle] < target:
        return new_binary_search(arr, middle + 1, stop, target)
    else:
        return new_binary_search(arr, start, middle, target)

You may assume that arr will be a sorted list containing at least one number, and that the initial call to new_binary_search will be with start = 0 and stop = len(arr).