Sum of factorials

[maretekent]

Solution Number: {34.rs}

Checklist:

• Created binary in src/bin
• Written Documentation
• Written Tests

What

Find the sum of all numbers which are equal to the sum of the factorial of their digits.
145 is an interesting number, as 1! + 4! + 5! = 1 + 24 + 120 = 145.
Note: as 1! = 1 and 2! = 2 are not sums they are not included.

How

• Read the integers in the range between 10 and 10000
• for every integer break it down and read them like characters with each of them getting the factorial
• then sum the factorials
• When you get the sum compare if its equal to the initial integer before breaking it down

Solution

Nice to have:

I would have loved to iterate over the characters of string and convert and add them using fold

Challenges

1. Rust is statically typed language. It expects types. I found out that the fibonacci of huge numbers I could easily get the error You are attempting to multiply with overflow
2. Comparing the resulting factorial with the number - Parsing the number
3. Writing its test -- perhaps I should have created a function of the entire content inside main
4. No nice rust data structures or techniques used here. I would like to see other people thoughful implementations

Ref

Project Euler problem 34
Issue 34

[mattgathu]

There is a binary file: src/bin/2 that is not required.

Your solution does not test the base case provided:
1! + 4! + 5! = 1 + 24 + 120 = 145.. Add a test for this base case.

Also you need to solve the main question:
Find the sum of all numbers which are equal to the sum of the factorial of their digits.

[mattgathu]

The answer to Find the sum of all numbers which are equal to the sum of the factorial of their digits. is 40730

[mattgathu]

@maretekent maybe we need a test for 1! + 4! + 5! = 1 + 24 + 120 = 145.

A function that takes the number e.g.145 calculates the factorial of its digits and returns their sum e.g.145.

The test for this function will be something like this:

#[test]
fn test_factorial_sum(){
    assert_eq!(factorial_sum(145), 145);
}

We can extrapolate this to an algorithm for finding the sum of all these numbers:

• create a vector to hold numbers
• for each number in the range 0..10K
• if factorial_sum(number) == number, add number to vector
• assert the sum of the vector is 40730 (the expected answer)