Last updated on September 26, 2019

Factorial is a number accompanied by an exclamation mark. For example, four factorial is written as 4! which simply means 4*3*2*1 = 24.

The mathematical formula is as follow

1 |
n! = n(n−1)...2×1 |

Now if we are given number **5** in our case, it will look like this

1 2 3 |
5! = 5(5-1)(5-2)(5-3)(5-4) = 5 * 4 * 3 * 2 * 1 = 120 |

**Code Example**

Some of the steps taken below are:

- Create a function that takes in a
`number`

as a parameter - Creating a variable of
`total`

by setting it at 0 - Create a for loop starting from 1 with condition that if
`total`

is not equal to 0 – here we want our total to be 1. - If it’s 1, it will go into the
`if`

statement and calculate it accordingly

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func factorial(number: Int) -> Int { var total = 0 for x in 1...number{ if total != 0 { total = total * x } else { total = 1 } } return total } factorial(number: 5) |

I’ll give you an idea how the program actually works

**Loop count > Total value > Programs > Calculation**

- For 1 -> Total is 0 -> Else -> Total is 1
- For 2 -> Total is 1 -> If -> 1 * 2 = 2
- For 3 -> Total is 2 -> If -> 2 * 3 = 6
- For 4 -> Total is 6 -> If -> 6 * 4 = 24
- For 5 -> Total is 24 -> If -> 24 * 5 = 120

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