# Syntax of exchanging values of 2 variables seems to be broken in short-circuit evaluation
**URL:** https://discourse.julialang.org/t/syntax-of-exchanging-values-of-2-variables-seems-to-be-broken-in-short-circuit-evaluation/76125
**Category:** General Usage
**Tags:** question
**Created:** [February 9, 2022, 10:44pm UTC](https://discourse.julialang.org/t/syntax-of-exchanging-values-of-2-variables-seems-to-be-broken-in-short-circuit-evaluation/76125 "2022-02-09T22:44:56Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@frankwswang](https://discourse.julialang.org/u/frankwswang)
#### Post date: [February 9, 2022, 10:44pm UTC](https://discourse.julialang.org/t/syntax-of-exchanging-values-of-2-variables-seems-to-be-broken-in-short-circuit-evaluation/76125/1 "2022-02-09T22:44:56Z")
Tested on Julia 1.7.2:
```julia
julia> t1 = rand(4)
4-element Vector{Float64}:
0.8846483727403565
0.5933843798087087
0.23231684762850824
0.6599451690863327
julia> t2 = rand(5)
5-element Vector{Float64}:
0.9708033624024174
0.9204864722716941
0.07792097172573631
0.43291967469455217
0.7960005316340157
julia> t1, t2 = t2, t1
([0.9708033624024174, 0.9204864722716941, 0.07792097172573631, 0.43291967469455217, 0.7960005316340157], [0.8846483727403565, 0.5933843798087087, 0.23231684762850824, 0.6599451690863327])
julia> t1
5-element Vector{Float64}:
0.9708033624024174
0.9204864722716941
0.07792097172573631
0.43291967469455217
0.7960005316340157
julia> t2
4-element Vector{Float64}:
0.8846483727403565
0.5933843798087087
0.23231684762850824
0.6599451690863327
julia> true && (t1, t2 = (t2, t1))
(t1 = [0.9708033624024174, 0.9204864722716941, 0.07792097172573631, 0.43291967469455217, 0.7960005316340157], t2 = ([0.8846483727403565, 0.5933843798087087, 0.23231684762850824, 0.6599451690863327], [0.9708033624024174, 0.9204864722716941, 0.07792097172573631, 0.43291967469455217, 0.7960005316340157]))
julia> t1
5-element Vector{Float64}:
0.9708033624024174
0.9204864722716941
0.07792097172573631
0.43291967469455217
0.7960005316340157
julia> t2
4-element Vector{Float64}:
0.8846483727403565
0.5933843798087087
0.23231684762850824
0.6599451690863327
julia> true && ((t1, t2) = (t2, t1))
([0.8846483727403565, 0.5933843798087087, 0.23231684762850824, 0.6599451690863327], [0.9708033624024174, 0.9204864722716941, 0.07792097172573631, 0.43291967469455217, 0.7960005316340157])
julia> t1
4-element Vector{Float64}:
0.8846483727403565
0.5933843798087087
0.23231684762850824
0.6599451690863327
julia> t2
5-element Vector{Float64}:
0.9708033624024174
0.9204864722716941
0.07792097172573631
0.43291967469455217
0.7960005316340157
```
The values of `t1` and `t2` are not correctly exchanged when it’s part of the short-circuit evaluation unless I put parentheses on both sides of `=`. Moreover, the printed info of `true && (t1, t2 = (t2, t1))` does not even match the output values. In the printed info, it’s suggested that the operation is
```julia
julia> true && (t1; t2 = (t2, t1))
([0.9708033624024174, 0.9204864722716941, 0.07792097172573631, 0.43291967469455217, 0.7960005316340157], [0.8846483727403565, 0.5933843798087087, 0.23231684762850824, 0.6599451690863327])
```
However, in this case the printed info is incorrectly corresponding to `true && (t1, t2 = (t2, t1))`.
I think this is a bug? Thank you!
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [February 9, 2022, 11:05pm UTC](https://discourse.julialang.org/t/syntax-of-exchanging-values-of-2-variables-seems-to-be-broken-in-short-circuit-evaluation/76125/2 "2022-02-09T23:05:27Z")
I tried
```julia
t1 = 1
t2 = 2
t1, t2 = t2, t1
@show t1 t2
true && (t1, t2 = t2, t1)
```
and get the error message
```julia
ERROR: LoadError: syntax: field name "t1" repeated in named tuple
```
which (surprisingly?) indicates that the parser interprets the same construction differently in that context.
---
### Author:  [@frankwswang](https://discourse.julialang.org/u/frankwswang)
#### Post date: [February 9, 2022, 11:18pm UTC](https://discourse.julialang.org/t/syntax-of-exchanging-values-of-2-variables-seems-to-be-broken-in-short-circuit-evaluation/76125/3 "2022-02-09T23:18:27Z")
Yeah, I also noticed this error that’s why I tried `true && (t1, t2 = (t2, t1))` instead. It prompts no error but gives the wrong result.
---
### Author:  [@Jollywatt](https://discourse.julialang.org/u/Jollywatt)
#### Post date: [February 9, 2022, 11:40pm UTC](https://discourse.julialang.org/t/syntax-of-exchanging-values-of-2-variables-seems-to-be-broken-in-short-circuit-evaluation/76125/4 "2022-02-09T23:40:13Z")
This is because of [named tuple syntax](https://discourse.julialang.org/t/named-tuples-construction-unpacking/51074).
Notice `(t1, t2 = (t2, t1))` is short for `(t1 = t1, t2 = (t2, t1))` and results in `NamedTuple{(:t1, :t2)}((t1, (t2, t1)))`.
So you’ll need to disambiguate a (named) tuple literal from a `()`-grouped expression. You could do this like so:
```julia
true && ((t1, t2) = (t2, t1)) # an expression, not a tuple, since no comma
true && begin # explicit block
t1, t2 = t2, t1
end
```
So this isn’t a bug — just confusion about Julia’s (very carefully designed) syntax!
---
### Author:  [@frankwswang](https://discourse.julialang.org/u/frankwswang)
#### Post date: [February 10, 2022, 5:13am UTC](https://discourse.julialang.org/t/syntax-of-exchanging-values-of-2-variables-seems-to-be-broken-in-short-circuit-evaluation/76125/5 "2022-02-10T05:13:01Z")
Thanks a lot! I haven’t got familiar enough with [named Tuple](https://docs.julialang.org/en/v1/manual/functions/#Named-Tuples) syntax.
I think the printed info was a bit confusing here because I first thought `t2` got re-bound to the value of `(t1, t2)`, but it turned out that the evaluation just returned a new named tuple that has a field `.t2`. The original `t2` was never altered. A clearer demonstration:
```julia
julia> t1 = 1
1
julia> t2 = 2
2
julia> t = ( true && (t1, t2 = (t2, t1)) )
(t1 = 1, t2 = (2, 1))
julia> t.t1
1
julia> t.t2
(2, 1)
julia> t1
1
julia> t2
2
```
---
### Author:  [@gustaphe](https://discourse.julialang.org/u/gustaphe)
#### Post date: [February 10, 2022, 6:26am UTC](https://discourse.julialang.org/t/syntax-of-exchanging-values-of-2-variables-seems-to-be-broken-in-short-circuit-evaluation/76125/6 "2022-02-10T06:26:54Z")
For the future, of you choose your examples a bit more carefully, these things will be easier to debug.
```julia
julia> t1, t2 = 1, 2;
julia> true && (t1, t2 = (t2, t1))
(t1=1, t2=(2, 1))
```
This example takes up less horizontal real estate, and doesn’t rely on remembering random arrays of numbers.