List Exercises (Part 3)

Table of contents

1. Exercise 1 (Code Reading)
2. Exercise 2 (Code Reading)
3. Exercise 3 (Code Reading)
4. Exercise 4 (Debugging)
5. Exercise 5 (Debugging)

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Exercise 1 (Code Reading)

def mystery(nums):
    c = [0] * 3
    for n in nums:
        c[n] += 1
    print(c)

What does the above code print in each of the following cases?

1. If nums = [0, 0]
2. If nums = [1, 1]
3. If nums = [2, 2]
4. If nums = [0, 1, 2, 0, 1, 2, ...] (300 elements)

Solution

1. [2, 0, 0]
2. [0, 2, 0]
3. [0, 0, 2]
4. [100, 100, 100]

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Exercise 2 (Code Reading)

Read and understand the following function, then suggest suitable name for it.

def mystery(a):
    N = len(a)
    for i in range(1, N):
        if a[i] > a[0]:
            temp = a[i]
            a[i] = a[0]
            a[0] = temp
    print(a)

Solution

The function finds the maximum element in the list `a` and places it at the front (index 0) of the list. A suitable name would be move_max_to_front(a).

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Exercise 3 (Code Reading)

Read and understand the following function, then suggest suitable name for it.

def mystery(nums, n):
    N = len(nums)
    for i in range(N-1, -1, -1):
        if nums[i] == n:
            return i
    return -1

Solution

The function searches for the last occurrence of the integer n in the list nums and returns its index. If n is not found, it returns -1. A suitable name would be find_last_index(nums, n).

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Exercise 4 (Debugging)

Find and fix the bug in the following function that is supposed to find the index of the first occurrence of target in the list nums.

def find_index(nums, target):
    for i in range(len(nums)):
        if nums[i] == target:
            pos = i
            break

    return pos

Solution. The variable pos is not initialized before the loop, and if the target is not found, it will raise an error.

def find_index(nums, target):
    for i in range(len(nums)):
        if nums[i] == target:
            return i
    return -1

Alternatively, we can initialize pos to -1 before the loop:

def find_index(nums, target):
    pos = -1
    for i in range(len(nums)):
        if nums[i] == target:
            pos = i
            break
    return pos

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Exercise 5 (Debugging)

Find and fix the bug in the following code, which is supposed to merge two lists by alternating their elements.

def merge_lists(a, b):
    merged = []
    for i in range(len(a)):
        merged.append(a[i])
        merged.append(b[i])
    return merged

Solution. The bug is that the code assumes both lists a and b have the same length. If they don’t, it will raise an IndexError. We should iterate up to the length of the shorter list and then append the remaining elements from the longer list.

def merge_lists(a, b):
    merged = []
    if len(a) < len(b):
        shorter = a
        longer = b
    else:
        shorter = b
        longer = a

    for i in range(len(shorter)):
        merged.append(a[i])
        merged.append(b[i])

    merged.extend(longer[len(shorter):])
    return merged