freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-python/6821ebf8237de8297eaee798.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
6821ebf8237de8297eaee798	Challenge 22: Tribonacci Sequence	29	challenge-22

--description--

The Tribonacci sequence is a series of numbers where each number is the sum of the three preceding ones. When starting with 0, 0 and 1, the first 10 numbers in the sequence are 0, 0, 1, 1, 2, 4, 7, 13, 24, 44.

Given an array containing the first three numbers of a Tribonacci sequence, and an integer representing the length of the sequence, return an array containing the sequence of the given length.

• Your function should handle sequences of any length greater than or equal to zero.
• If the length is zero, return an empty array.
• Note that the starting numbers are part of the sequence.

--hints--

tribonacci_sequence([0, 0, 1], 20) should return [0, 0, 1, 1, 2, 4, 7, 13, 24, 44, 81, 149, 274, 504, 927, 1705, 3136, 5768, 10609, 19513].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(tribonacci_sequence([0, 0, 1], 20), [0, 0, 1, 1, 2, 4, 7, 13, 24, 44, 81, 149, 274, 504, 927, 1705, 3136, 5768, 10609, 19513])`)
}})

tribonacci_sequence([21, 32, 43], 1) should return [21].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(tribonacci_sequence([21, 32, 43], 1), [21])`)
}})

tribonacci_sequence([0, 0, 1], 0) should return [].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(tribonacci_sequence([0, 0, 1], 0), [])`)
}})

tribonacci_sequence([10, 20, 30], 2) should return [10, 20].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(tribonacci_sequence([10, 20, 30], 2), [10, 20])`)
}})

tribonacci_sequence([10, 20, 30], 3) should return [10, 20, 30].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(tribonacci_sequence([10, 20, 30], 3), [10, 20, 30])`)
}})

tribonacci_sequence([123, 456, 789], 8) should return [123, 456, 789, 1368, 2613, 4770, 8751, 16134].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(tribonacci_sequence([123, 456, 789], 8), [123, 456, 789, 1368, 2613, 4770, 8751, 16134])`)
}})

--seed--

--seed-contents--

def tribonacci_sequence(start_sequence, length):

    return length

--solutions--

def tribonacci_sequence(start_sequence, length):
    if length == 0:
        return []
    if length == 1:
        return [start_sequence[0]]
    if length == 2:
        return [start_sequence[0], start_sequence[1]]
    if length == 3:
        return start_sequence[:]

    sequence = start_sequence[:]
    while len(sequence) < length:
        next_value = sequence[-1] + sequence[-2] + sequence[-3]
        sequence.append(next_value)

    return sequence