# 5 Best Ways to Perform Calculations with Dictionaries in Python

Rate this post

π‘ Problem Formulation: When working with dictionaries in Python, it’s common to need to perform calculations on their values. For instance, you might have a dictionary where the keys are product names and the values are prices, and you wish to calculate the total cost of the products, or find the product with the highest price. This article introduces five different methods to carry out such computations effectively.

## Method 1: Using a For Loop to Sum Values

One of the most straightforward methods to compute over dictionary values is employing a simple for loop to iterate over the dictionary and sum the values. This classic method provides flexibility to include conditions and operate with items individually while accumulating a result.

Here’s an example:

```products_prices = {'apple': 1.00, 'banana': 0.50, 'cherry': 0.75}
total_price = 0
for price in products_prices.values():
total_price += price
print(total_price)```

2.25

The snippet defines a dictionary `products_prices` with several fruits as keys and their prices as values. It then iterates over the values of the dictionary using a for loop, adding each price to the `total_price` variable, which is finally printed to show the sum of all product prices.

## Method 2: Using the builtin `sum()` Function

For tasks that require summing all values in a dictionary, Python’s builtin `sum()` function is efficient and concise. It sums the items of an iterable, in this case, the values of the dictionary, resulting in a single numeric value.

Here’s an example:

```totals = {'x': 8, 'y': 15, 'z': 4}
result = sum(totals.values())
print(result)```

27

This code block shows how one can utilize the `sum()` function on the `values()` method of a dictionary named `totals` to quickly calculate the sum of all values and print it out, which is 27 in this case.

## Method 3: Calculating Weighted Sum with `for` Loop

To calculate a weighted sum where each value in a dictionary should be multiplied by a corresponding weight before summing, a custom for loop can be employed. This allows you to include the logic of weighting each value within the calculation.

Here’s an example:

```weights = {'A': 3, 'B': 5, 'C': 2}
scores = {'A': 92, 'B': 88, 'C': 75}
weighted_sum = sum(weights[key] * scores[key] for key in scores)
print(weighted_sum)```

846

The example introduces two dictionaries, `weights` and `scores`, that hold weights and scores respectively. The calculation involved uses a generator expression within the `sum()` function to multiply each score by its corresponding weight for every key, resulting in the weighted sum.

## Method 4: Using `map()` and `lambda` Functions

When dealing with more complex calculations or needing additional abstraction, Python’s `map()` function combined with `lambda` can be used to apply a calculation to each value in the dictionary. This method can improve readability and maintainability of the code for certain operations.

Here’s an example:

```prices_with_tax = {'book': 12.99, 'pen': 1.99, 'notebook': 4.99}
tax_rate = 0.08
with_tax = dict(map(lambda item: (item[0], item[1] * (1 + tax_rate)), prices_with_tax.items()))
print(with_tax)```

{‘book’: 14.0332, ‘pen’: 2.1488, ‘notebook’: 5.3892}

In this snippet, a dictionary `prices_with_tax` containing prices is provided. A lambda function is used inside the `map()` function to calculate the price with tax for each item. The result is converted back to a dictionary which holds the item names and their prices including tax.

## Bonus One-Liner Method 5: Using Dictionary Comprehensions

Python’s dictionary comprehensions offer a concise and readable way to perform calculations and create new dictionaries. This method is advantageous for its brevity and its direct expression of the transformation from inputs to outputs.

Here’s an example:

```quantities = {'milk': 2, 'bread': 3, 'eggs': 1}
prices = {'milk': 1.50, 'bread': 2.00, 'eggs': 0.50}
total_cost = {item: quantities[item] * prices[item] for item in prices if item in quantities}
print(total_cost)```

{‘milk’: 3.0, ‘bread’: 6.0, ‘eggs’: 0.5}

The code makes use of dictionary comprehension to multiply each item’s price by its quantity, given in the `prices` and `quantities` dictionaries, respectively. It only includes items present in both dictionaries and output the total cost for each product.

## Summary/Discussion

• Method 1: For Loop Summation. Versatile, allows custom logic. Can be verbose for simple tasks.
• Method 2: `sum()` Function. Simple and clean – best for summing values. Not suitable for complex calculations.
• Method 3: Weighted Sum with Loop. Ideal for computations that involve additional factors. Less straightforward than `sum()`.
• Method 4: `map()` and `lambda`. Provides a functional approach. Might be less readable for those not used to functional programming.
• Method 5: Dictionary Comprehension. Quick and concise. Excellent for straightforward transformations with less code but can be less readable for complicated transformations.