# 5 Best Ways to Compute n+nn+nnn in Python

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π‘ Problem Formulation: We need to devise a Python program that prompts the user to input a single digit number, `n`, and calculates the value of `n + nn + nnn`. For instance, if the input is `5`, the output should be `5 + 55 + 555 = 615`.

## Method 1: Using String Manipulation

This method involves converting the input digit into a string, concatenating it to form ‘nn’ and ‘nnn’, and then converting these concatenated strings back into integers for the final computation. This method is straightforward and utilizes basic typecasting and string manipulation techniques.

Here’s an example:

```num = input("Enter a digit: ")
n = int(num)
nn = int(num*2)
nnn = int(num*3)
result = n + nn + nnn
print("The result is:", result)```

Output: The result is: 615

This Python snippet takes the input from the user and creates variables for `n`, `nn`, and `nnn` by repeating the string representation of `n` before casting them back to integers. The final calculation simply adds these integers together.

## Method 2: Arithmetic Approach

The arithmetic approach utilizes mathematics to compute nn and nnn by multiplying n with appropriate powers of 10. This avoids string operations and works with numbers directly, possibly offering a marginal performance benefit over string manipulation.

Here’s an example:

```n = int(input("Enter a digit: "))
nn = n * 11    # Equivalent to n*10 + n
nnn = n * 111  # Equivalent to n*100 + n*10 + n
result = n + nn + nnn
print("The result is:", result)```

Output: The result is: 615

This method simply multiplies the input number `n` by 11 and 111 to get `nn` and `nnn` respectively and then adds them up. It’s a neat arithmetic trick that eliminates the need for string handling.

## Method 3: Using a Loop

By using a loop, we can dynamically construct the numbers nn and nnn regardless of the digit’s length. This method is more robust and can handle inputs with more than one digit.

Here’s an example:

```n = input("Enter a number: ")
total_sum = 0
for i in range(1, 4):
total_sum += int(n * i)
print("The result is:", total_sum)```

Output: The result is: 615

In this example, we loop through a range of numbers from 1 to 3. With each iteration, we multiply the string `n` by the iterator `i` to create ‘n’, ‘nn’, and ‘nnn’, which we then convert to integers and sum up.

## Method 4: List Comprehension and Sum

Python’s list comprehensions provide a compact and expressive way to handle transformations and computations. Here, we use list comprehension to generate the ‘nn’ and ‘nnn’ numbers in a list, which we then sum using Python’s built-in `sum()` function.

Here’s an example:

```n = input("Enter a number: ")
result = sum(int(n * i) for i in range(1, 4))
print("The result is:", result)```

Output: The result is: 615

This one-liner uses a list comprehension inside the `sum()` function to perform the multiplication and summing in a single, concise statement. It’s a very Pythonic approach, showcasing the language’s ability for writing compact code.

## Bonus One-Liner Method 5: Using f-Strings and eval

f-Strings in Python 3.6+ allow us to embed expressions inside string literals, which we can combine with the `eval()` function to compute the formula directly. This method is creative but should be used with caution due to security risks associated with `eval()`.

Here’s an example:

```n = input("Enter a number: ")
result = eval(f"{n}+{n*2}+{n*3}")
print("The result is:", result)```

Output: The result is: 615

This snippet creates an f-String with the expression `n+nn+nnn` and directly evaluates it using `eval()`. While this method is incredibly concise, it’s generally a bad practice to use `eval()` with user inputs due to potential security issues.

## Summary/Discussion

• Method 1: String Manipulation. Simple and easy to understand. Inefficient for large numbers; converting between types may be slow.
• Method 2: Arithmetic Approach. Fast and concise. Limited to input as a single digit for this specific formula.
• Method 3: Using a Loop. Robust and handles multi-digit inputs. Slightly more complex than other methods.
• Method 4: List Comprehension and Sum. Very Pythonic and compact. Can be less intuitive for beginners.
• Method 5: Using f-Strings and eval. Extremely concise. Poses a security risk and is not recommended for untrusted input.