find the nth octagonal number in swift

main.swift
n * (3*n - 2)
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2 lines

main.swift
func octagonalNumber(atIndex index: Int) -> Int {
    return index * (3 * index - 2)
}

// Example usage
print(octagonalNumber(atIndex: 1)) // Prints 1 (the first octagonal number)
print(octagonalNumber(atIndex: 2)) // Prints 8 (the second octagonal number)
print(octagonalNumber(atIndex: 3)) // Prints 21 (the third octagonal number)
335 chars
9 lines