# 5 Best Ways to Find a List of Product of All Elements Except the Current Index in Python

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π‘ Problem Formulation: Python developers are often tasked with challenges involving lists and mathematical operations. Consider a case where you’re given a list of numbers and need to find a new list where each element is the product of all original elements except the one at the same index. For instance, given `[1, 2, 3, 4]`, the desired output is `[24, 12, 8, 6]`, since each is the product of the other list elements.

## Method 1: Naive Iterative Solution

The naive method involves two nested loops. The outer loop selects each index, and the inner loop computes the product of the remaining elements. Its simplicity provides ease of understanding.

Here’s an example:

```result = []
arr = [1, 2, 3, 4]
for i in range(len(arr)):
product = 1
for j in range(len(arr)):
if i != j:
product *= arr[j]
result.append(product)
print(result)```

Output:

`[24, 12, 8, 6]`

This code initializes an empty list `result`. For each element in the input list `arr`, it loops through all other elements multiplying them, as long as the indices don’t match. Lastly, it appends the product to `result`.

## Method 2: Using Division

This method calculates the total product of all elements and then divides by the current element. It’s efficient but fails if there’s a zero in the list.

Here’s an example:

```arr = [1, 2, 3, 4]
product_of_all = 1
for num in arr:
product_of_all *= num
result = [product_of_all // i for i in arr]
print(result)```

Output:

`[24, 12, 8, 6]`

This snippet first calculates the product of all numbers in the list. Then, for each number in the list, it computes the individual result as the total product divided by the current number, using list comprehension.

## Method 3: Prefix and Suffix Products

Calculates prefix and suffix products for each index, multiplying them to get the desired output. It is more efficient than the naive approach, without the division’s limitations.

Here’s an example:

```arr = [1, 2, 3, 4]
prefix_products = []
suffix_products = []
product = 1

for num in arr:
prefix_products.append(product)
product *= num

product = 1
for num in reversed(arr):
suffix_products.insert(0, product)
product *= num

result = [prefix_products[i] * suffix_products[i] for i in range(len(arr))]
print(result)```

Output:

`[24, 12, 8, 6]`

The code constructs two lists: one for prefix products and one for suffix products. The final result is calculated by multiplying the corresponding prefix and suffix products for each index using list comprehension.

## Method 4: Improved Prefix and Suffix Products

This method builds on the previous one by combining the prefix and suffix products in a single pass to reduce space complexity.

Here’s an example:

```arr = [1, 2, 3, 4]
result = [1] * len(arr)

product = 1
for i in range(len(arr)):
result[i] *= product
product *= arr[i]

product = 1
for i in range(len(arr) - 1, -1, -1):
result[i] *= product
product *= arr[i]

print(result)```

Output:

`[24, 12, 8, 6]`

This code iterates over the input array once to cumulatively calculate the left hand (prefix) products directly into the result array. Then it iterates in reverse to calculate and multiply the right hand (suffix) products, eliminating the need for separate prefix and suffix arrays.

## Bonus One-Liner Method 5: Using Numpy

Python’s NumPy library’s powerful vectorization capabilities can efficiently solve this problem in a one-liner, but it requires additional library installation.

Here’s an example:

```import numpy as np

arr = np.array([1, 2, 3, 4])
result = np.prod(arr) // arr
print(result.tolist())```

Output:

`[24, 12, 8, 6]`

This snippet uses NumPy’s `np.prod()` to compute the product of all elements. It then uses vectorized division to divide this product by each element in the array, effectively broadcasting the division across the array.

## Summary/Discussion

• Method 1: Naive Iterative Solution. Easy to understand; very inefficient for large lists due to its O(nΒ²) time complexity.
• Method 2: Using Division. More efficient than the naive method but fails when the array contains zeroes, as division by zero is not allowed.
• Method 3: Prefix and Suffix Products. Efficient and works with zeroes; however, it uses additional space for the prefix and suffix arrays.
• Method 4: Improved Prefix and Suffix Products. Space-efficient version, retains the previous methodβs efficiency while reducing space complexity to O(n).
• Method 5: Using Numpy. Highly efficient and succinct, but requires NumPy, an external library, which may not be desirable or available in all environments.