Julia vs Python implementation for https://projecteuler.net/problem=2. This was very strange to me and I don't know where I am messing up in Julia

Question

Python implementation

import numpy as np

# Function to generate the Fibonacci series
def fibonacci_fun(fib_series_fun, num_terms_fun):

    # Initialize the first value in series
    first_num_fun = 1

    # Append the first value to series
    fib_series_fun.append(first_num_fun)

    # Initialize the value preceeding the first number; technically always 0 but 1 in this problem
    num_bef_start_fun = 1

    # Add the first_num to the num_bef_start value and append it
    next_num_fun = first_num_fun + num_bef_start_fun
    fib_series_fun.append(next_num_fun)

    # While we do not have num_terms number of elements in Fibonacci series, repeat
    while len(fib_series_fun) != num_terms_fun:

        # Add the previous 2 numbers of the series to itself
        new_num_fun = fib_series_fun[len(fib_series_fun)-2] + fib_series_fun[len(fib_series_fun)-1];
        
        fib_series_fun.append(new_num_fun)

    return fib_series_fun

# Main function
def main():

    # Define the number of terms in Fibonacci series
    num_terms = 25

    # Define empty array of Int16 to store Fibonacci sereis values
    fib_series = []

    # Run the fibonacci_fun to get the Fibonacci series
    fib_series = fibonacci_fun(fib_series, num_terms)
    print(fib_series)

    fib_series = np.array(fib_series)

    # Values less than 4 million
    values_less_than = fib_series[fib_series < 4000000]

    # Get only the even values in the Fibonacci series
    even_values_less_than_fib_series = values_less_than[values_less_than % 2 == 0]
    print(even_values_less_than_fib_series)

    sum_of_fib_series = sum(even_values_less_than_fib_series)
    print("The sum of even values in the Fibonacci series is %d.\n" %sum_of_fib_series)

    print("Process finished!")

main()

Julia Implementation

using Printf

# Function to generate the Fibonacci series
function fibonacci_fun(fib_series_fun, num_terms_fun)

    # Initialize the first value in series
    first_num_fun = 1

    # Append the first value to series
    append!(fib_series_fun, first_num_fun)

    # Initialize the value preceeding the first number; technically always 0 but 1 in this problem
    num_bef_start_fun = 1

    # Add the first_num to the num_bef_start value and append it
    next_num_fun = first_num_fun + num_bef_start_fun
    append!(fib_series_fun, next_num_fun)

    # While we do not have num_terms number of elements in Fibonacci series, repeat
    while length(fib_series_fun) != num_terms_fun

        # Add the previous 2 numbers of the series to itself
        new_num_fun = fib_series_fun[length(fib_series_fun)-1] + fib_series_fun[length(fib_series_fun)];
        
        append!(fib_series_fun, new_num_fun)
    end

    return fib_series_fun

end

# Main function
function main()

    # Define the number of terms in Fibonacci series
    num_terms = 25

    # Define empty array of Int16 to store Fibonacci sereis values
    global fib_series = Int16[]

    # Run the fibonacci_fun to get the Fibonacci series
    fib_series = fibonacci_fun(fib_series, num_terms)
    println(fib_series)

    # Values less than 4 million
    values_less_than = fib_series[fib_series .< 4000000]

    # Get only the even values in the Fibonacci series
    even_values_less_than_fib_series = values_less_than[values_less_than .% 2 .== 0]
    println(even_values_less_than_fib_series)

    sum_of_fib_series = sum(even_values_less_than_fib_series)
    @printf("The sum of even values in the Fibonacci series is %d.\n", sum_of_fib_series)

    println("Process finished!")

end

main()

Python Implementation Output:

[1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393]

[    2     8    34   144   610  2584 10946 46368]

The sum of even values in the Fibonacci series is 60696.

Julia Implementation Output:

Int16[1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, -19168, 9489, -9679]

Int16[2, 8, 34, 144, 610, 2584, 10946, -19168]

The sum of even values in the Fibonacci series is -4840. Process finished!

Its because you are using 16 bit signed integers in your Julia implementation. The highest positive interger value for that type is 32,767, anything above that will be seen as a negative int — DrBwts

Jonathan Fogel Jonathan Fogel · Accepted Answer · 2022-07-28T14:46:17

TLDR: You are using a number that exceeds the upper limit of Int16, which causes it to "loop back through" with the remainder, returning a negative number.

Julia has Concrete Typing(like Int16), and Abstract Typing(like Int). A general rule is that you should use the Concrete typed versions for data storage, as its more efficient(which is what it is being used for in your code- so that's good). The difference being that Int will act normally no matter the input size, and Int16 limits itself to a certain number of bits, so it has bounds. The bounds for Int16 are (-2^15 , 2^15 - 1).

The values from 46368 and up are above the bounds, so they act wonky.

So you could solve your problem by changing:

global fib_series = Int64[]

or, if you wanted to:

global fib_series = Int[]

Julia vs Python implementation for https://projecteuler.net/problem=2. This was very strange to me and I don't know where I am messing up in Julia

1 Answers