Hi :wave: 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…

In Julia, 2 is an integer literal with 64 bits length. 2^63 overflows into the most signifant bit and thus is negative: julia> bitstring(2) "0000000000000000000000000000000000000000000000000000000000000010" julia> bitstring(2^32) "0000000000000000000000000000000100000000000000000000000000000000" …

Is it possible to have an error when this happens, eg using a “safe” version of Int64? Would this check cost “a lot”?

[image] sylvaticus: using a “safe” version of Int64 Yep, that’s SafeInt64 from SaferIntegers.jl

There is SaferIntegers.jl , if you want checked arithmetic with integers. You can also use functions in Base.Checked directly, like Base.Checked.checked_add, for example.

In R, a few factors differ from Julia: 2 isn’t an integer – it’s a floating point number. You need 2L to get an integer. x^y always produces a floating point number, even for simple cases. > x <- 2L > y <- 4L > typeof(x) [1] "integer" > typeof(y) [1] "integer" > typeof(x^y) [1] "double" There …

Just for completeness: in NumPy, the integer type (np.int64) behaves the same as in Julia.

And we do this for the same reason: this is the behavior of our machines’ integers. It’s the fastest behavior possible. Doing anything else requires extra operations to emulate some different sort of behavior.

Should we add a link to Learn Julia in Y Minutes in the Please read: make it easier to help you - #23 since there is an explicit example about integer overflow within the first 10 lines, this seems to be a recurrent stumbling point for ex-python users? (cc @Tamas_Papp )

I don’t think that PSA should be a link to various docs other than the manual, or target users from specific programming languages. Once you start doing that, it gets too long (it already is) and even fewer people would read it.

Fair, indeed there is Frequently Asked Questions · The Julia Language it’s a bit lengthy for newcomers though… Maybe there should be another PSA gathering links to frequently asked questions that we could at least point to when the questions resurface

Why is 2^63 negative?

New to Julia

Oscar_Smith October 16, 2020, 8:52pm 9

For the how much does this cost question, the answer is anywhere from 10% to 20x. A typical cost is usually in the 2x case, but the added complication can easily prevent massive optimizations.

4 Likes

Topic		Replies	Views
Long Math Operation result is 0, In Python isn't New to Julia question , integer-overflow	7	1186	January 2, 2022
Math errors General Usage	3	712	February 5, 2018
Integer power of large Integer returning very wrong results General Usage base	7	581	May 15, 2023
Multiplication issue New to Julia question	9	457	May 17, 2023
Math operation known issue? a^b General Usage question , special-issue , math , integer-overflow	11	1669	May 6, 2020

Why is 2^63 negative?

Related topics