# Question about abs()
**URL:** https://discourse.julialang.org/t/question-about-abs/10198
**Category:** General Usage
**Tags:** integer-overflow
**Created:** [April 6, 2018, 12:01pm UTC](https://discourse.julialang.org/t/question-about-abs/10198 "2018-04-06T12:01:32Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@s920128](https://discourse.julialang.org/u/s920128)
#### Post date: [April 6, 2018, 12:01pm UTC](https://discourse.julialang.org/t/question-about-abs/10198/1 "2018-04-06T12:01:32Z")
I use abs(-9223372036854775808) in Julia
but output is -9223372036854775808
Is this a bug?
---
### Author:  [@swt30](https://discourse.julialang.org/u/swt30)
#### Post date: [April 6, 2018, 12:20pm UTC](https://discourse.julialang.org/t/question-about-abs/10198/2 "2018-04-06T12:20:18Z")
No, this is an integer overflow problem because `-9223372036854775808` is the minimum number representable as a 64-bit signed integer. That is, it is equal to `typemin(Int64)`. The help for `abs` calls this out:
```julia
help?> abs
search: abs abs2 abspath AbstractSet abstract type AbstractChar AbstractDict AbstractFloat AbstractArray
abs(x)
The absolute value of x.
When abs is applied to signed integers, overflow may occur, resulting in the return of a negative value.
This overflow occurs only when abs is applied to the minimum representable value of a signed integer. That
is, when x == typemin(typeof(x)), abs(x) == x < 0, not -x as might be expected.
julia> abs(-3)
3
julia> abs(1 + im)
1.4142135623730951
julia> abs(typemin(Int64))
-9223372036854775808
```
If you are dealing with integers of this magnitude then you should use BigInts:
```julia
julia> abs(big"-9223372036854775808")
9223372036854775808
```
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [April 6, 2018, 12:21pm UTC](https://discourse.julialang.org/t/question-about-abs/10198/3 "2018-04-06T12:21:45Z")
This is a consequence of [two’s complement arithmetic](https://en.wikipedia.org/wiki/Two%27s_complement), but possibly it should throw an `InexactError`.
In general, though, we don’t check for overflow in integer arithmetic for performance reasons:
```julia
julia> Int8(127) + Int8(1)
-128
julia> -Int8(-128)
-128
```
so it makes sense that `abs` is no different. If you are doing integer arithmetic that runs into this, you should generally use a larger integer type. If your numbers are so large that you are overflowing 64-bit integers, then you are probably doing some type of number theory or combinatorics (rather than counting real things like bytes or loop iterations) and you should use `BigInt`.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [April 6, 2018, 12:55pm UTC](https://discourse.julialang.org/t/question-about-abs/10198/4 "2018-04-06T12:55:41Z")
> [@swt30](#):
>
> `help?> abs`
> …
> when `x == typemin(typeof(x))`, `abs(x) == x < 0`, not `-x` as might be expected.
This actually seems inaccurate, because the very same phenomenon occurs with minus as with `abs`:
```julia
julia> x = typemin(Int)
-9223372036854775808
julia> -x
-9223372036854775808
julia> abs(x) == -x == x
true
```
---
### Author:  [@swt30](https://discourse.julialang.org/u/swt30)
#### Post date: [April 6, 2018, 1:16pm UTC](https://discourse.julialang.org/t/question-about-abs/10198/5 "2018-04-06T13:16:17Z")
Yes, another overflow: `x == typemin(T) == typemax(T) + one(T) == -x` for any machine integer type. It depends if you read `-x` in those docs as meaning a machine operation on a machine integer or a mathematical operation on a mathematical integer—could probably be clarified!
---
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [April 6, 2018, 1:42pm UTC](https://discourse.julialang.org/t/question-about-abs/10198/6 "2018-04-06T13:42:37Z")
> [@swt30](#):
>
> If you are dealing with integers of this magnitude then you should use BigInts:
Just in case you can be sure your numbers would always fit in 128 bits, then `Int128` will be much faster and take much less memory than `BigInt`.
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [April 6, 2018, 2:34pm UTC](https://discourse.julialang.org/t/question-about-abs/10198/7 "2018-04-06T14:34:22Z")
> [@stevengj](#):
>
> so it makes sense that `abs` is no different
IMO it would be best if these functions continued to refrain from overflow checks. A situation where some operations (eg `abs`) check, but others (eg `+`, `-`, `^`, …) don’t, is not really useful in practice. Alternatives exist, eg [SaferIntegers.jl checks](https://github.com/JeffreySarnoff/SaferIntegers.jl/blob/f725b679d1b43f8425543e8cf35b6a77f2523e4b/src/int_ops.jl#L32-L36) for this.