Long Math Operation result is 0, In Python isn't

[Jogrefer]

Hi everyone
Well, I’m new in this in general and just testing the REPL with random numbers.
Why this is happening and what I’ve to learn previous to declare operations like this one?

In Julia (1.7.0):

n = 123 * 123123567 * 5675675 * 86786 * 456456 * 78678 * 354345345 * 567567 * 3545 * 678678 * 234^23423 * 238^345354
--
0  

To confirm if the result is truly 0 I’ve tested in Python:
In Python (3.10.1)

n
--
30332308162763280714160902682533172328953144277024172702616 

I’ve already test with Real, Int8, Int16, Int64, Float64 before each operation and every print in Julia gives me 0.

[mcabbott]

@Jogrefer

234^23423

The problem is that integers overflow, and thus:

julia> 234^23423
0

julia> 235^23423
-5572363361863899709

Maybe someone knows a better FAQ link, this is one thread about it:

Why is 2^63 negative?

Hi Making my first steps into julia, coming from R and python. Following the getting started guide, I noticed the following: 2^62 # 4611686018427387904 2^63 # <--- This is negative? # -9223372036854775808 2^63 > 1 # false 2^64 # 0 # this is zero, but that makes sense if it is a 64 bit float I’d be interested to understand why? For reference, python and R below: 2^63 > 1 # true 2^63 > 1 # [1] TRUE

[Elrod]

234^23423 * 238^345354

At first glance, you can tell that these numbers do not fit into a 64 bit integer, and hence we’ll only get the lower 64 bits (which will all be zero, as true_answer % (2^64) == 0, which you can tell from the fact that both numbers are even and both exponents are greater than 64).
Multiplying the other numbers by 0 will of course result in an answer of 0.

Convert the numbers to BigInt first.

n = big"123" * 123123567 * 5675675 * 86786 * 456456 * 78678 * 354345345 * 567567 * 3545 * 678678 * big(234)^23423 * big"238"^34535

This is probably enough to be correct (multipications should go left to right, so making the first big should keep all intermediates big, and then we also need the two exponents).
The number is very large.
Far larger than the number you said Python gave.
I’d post it, but it is several times over the character limit.

[trahflow]

Use BigInt, e.g by: big"234"^23423.
Also note, that in python the ^ operator is the xor operator, while in julia it is the exponentiation operator.
So unless you actually want xors, the result you got in python is not the one you expect.
(In python, the exponentiation operator is **)

[Elrod]

@trahflow

Also note, that in python the ^ operator is the xor operator

Ah, that’s why the number was so small in Python.
You can also use ⊻ (type via /xor) in Julia.

Using xor, I now get:

julia> n = big"123" * 123123567 * 5675675 * 86786 * 456456 * 78678 * 354345345 * 567567 * 3545 * 678678 * 234 ⊻ 23423 * 238 ⊻ 34535
30332308162763280714160902682533172328953144277024172521589

[Jogrefer]

Thank You @trahflow @Elrod

At first I’ve tested the BigInt but just at the beginning. Now I write:

n = BigInt(123123 * 3453453 * 564456456 * 234234 * 7567567 * 5345345 * 567567 * (big(5523)^238489) * (big(837218)^2342340))

And I think the result is right (Really big number)

[Elrod]

When you write:

n = BigInt(123123 * 3453453 * 564456456 * 234234 * 7567567 * 5345345 * 567567 * (big(5523)^238489) * (big(837218)^2342340))

Note that

123123 * 3453453 * 564456456 * 234234 * 7567567 * 5345345 * 567567

will be calculated with all of them as Ints, which will overflow.
Additionally,

123123 * 3453453 * 564456456 * 234234 * 7567567 * 5345345 * 567567 * (big(5523)^238489) * (big(837218)^2342340)

will be a BigInt, therefore calling BigInt on the above result will not do anything.
Hence:

n = BigInt(123123) * 3453453 * 564456456 * 234234 * 7567567 * 5345345 * 567567 * (big(5523)^238489) * (big(837218)^2342340)

is what you want.
Making the first of these a BigInt will keep the accumulation in BigInt as we move from left to right.
And, we also still need the exponents to be BigInt as well, hence I left the big on them.

[jondea]

Two packages you may find interesting/useful:

• CheckedArithmetic.jl provides a macro which checks for over/underflows in calculations.

• SafeREPL.jl changes all number literals in the REPL to a different type, BigInts by default, so that you don’t get under/overflow. This will make most calculations slower, but is useful if you care more about correctness than speed.