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

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.

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.

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

simeonschaub October 16, 2020, 7:38pm 5

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.

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Why is 2^63 negative?

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