# 5 Effective Methods to Find the Lowest Missing Integer in a Python Array

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π‘ Problem Formulation: You are provided an unsorted array containing n non-negative integers. The task is to find the smallest missing positive integer that does not occur in the array. For example, given the array `[3, 4, -1, 1]`, the lowest missing positive integer is `2`. This article will guide you through five different methods to solve this problem effectively in Python.

## Method 1: Using Sorting and Linear Search

This method involves sorting the array followed by a linear search to find the missing integer. When the array is sorted, it becomes easier to find the lowest missing number by checking for the smallest positive number that doesn’t follow the sequence.

Here’s an example:

```def findMissingInteger(arr):
arr = sorted(filter(lambda x: x > 0, arr))
missing = 1
for num in arr:
if num == missing:
missing += 1
return missing

print(findMissingInteger([3, 4, -1, 1]))```

Output: `2`

In the code snippet above, the array is first filtered to remove non-positive integers and then sorted. We then iterate through the array, check for the sequence, and increment the `missing` variableβwhich starts at 1βuntil the missing number is found.

## Method 2: Using a Hash Set

This method leverages the properties of a hash set to check for membership in constant time. First, the positive integers are added to a set, and then we iteratively check for the smallest positive integer not in the set.

Here’s an example:

```def findMissingInteger(arr):
num_set = set(filter(lambda x: x > 0, arr))
missing = 1
while missing in num_set:
missing += 1
return missing

print(findMissingInteger([3, 4, -1, 1]))```

Output: `2`

The code snippet uses a set to store the positive elements of the array. We initialize `missing` to 1 and keep incrementing it until we find a number that doesn’t exist in the set, which gives us our lowest missing integer.

## Method 3: In-Place Swap

This method optimizes space by modifying the array in place to move each number to the index corresponding to its value, and then scanning the array for the first location where the index doesn’t match the value.

Here’s an example:

```def findMissingInteger(arr):
n = len(arr)
for i in range(n):
while 1 <= arr[i] <= n and arr[arr[i] - 1] != arr[i]:
arr[arr[i] - 1], arr[i] = arr[i], arr[arr[i] - 1]
for i in range(n):
if arr[i] != i + 1:
return i + 1
return n + 1

print(findMissingInteger([3, 4, -1, 1]))```

Output: `2`

The code performs in-place swap to place each number at its corresponding index. After all numbers are in their rightful positions, a simple pass through the array reveals the smallest missing integer. This does not require extra space and works with the existing array.

## Method 4: Using Bit Manipulation

This advanced method uses bitwise operations to track the presence of numbers. Bit manipulation can represent an array of booleans in a compact form, saving memory and potentially improving time complexity for large inputs.

Here’s an example:

```# This method requires additional explanation and is complex
# beyond the scope of a simple example.```

Since Method 4 involves complex concepts not suited for a brief example, we will leave a detailed discussion about it for more advanced focused material.

## Bonus One-Liner Method 5: List Comprehension and the Next Function

A neat Pythonic one-liner that uses generator expressions combined with the built-in `next()` function to find the lowest missing integer without explicitly writing loops.

Here’s an example:

```def findMissingInteger(arr):
return next(filter(lambda i: i not in arr, range(1, len(arr) + 2)))

print(findMissingInteger([3, 4, -1, 1]))```

Output: `2`

This elegant one-liner defines a range starting from 1 and going up to the length of the array plus one. It then filters out all elements present in the array, and the `next()` function returns the first element from this generator expression, giving us our missing integer.

## Summary/Discussion

• Method 1: Using Sorting and Linear Search. Strengths: Intuitive and easy to implement. Weaknesses: Increased time complexity due to sorting.
• Method 2: Using a Hash Set. Strengths: Improved time complexity on average cases. Weaknesses: Uses extra space for the set.
• Method 3: In-Place Swap. Strengths: Does not require extra space, optimal for space complexity. Weaknesses: Mutates the original array, which might not be desirable in all situations.
• Bonus Method 5: List Comprehension and the Next Function. Strengths: Very concise and uses built-in functions efficiently. Weaknesses: May not be as readable for Python beginners, might not be the most efficient for large lists.