Re: Arithmetic expression: evaluation order bug

[Alain D D Williams]

On Thu, Dec 29, 2022 at 06:23:09PM +0100, Steffen Nurpmeso wrote:
> Hello.
> 
>   Name:         bash
>   Path:         /usr/ports/core
>   Version:      5.2.15
>   Release:      1
> 
>   $ i=10 j=20;echo $(( i += j += i += j ));echo $i,$j
>   60
>   60,50
>   $ i=10 j=20;echo $(( i += j += i += i ));echo $i,$j
>   50
>   50,40

You are modifying something that is used elsewhere in an expression. I am not
surprised that you do not get what you expect; others might expect something
different.

At most there should be a note in the documentation that this sort of thing
leads to undefined behaviour -- that is what happens in other languages.

This is very much not a bug in bash.

C does it differently. I would not be surprised to see different C compilers
produce different results or different results with different levels of
optimisation.

>   $ cat t.c
>   #include <stdio.h>
>   int main(void){
> 
>           int i, j;
> 
>           i = 10, j = 20;
>           i += j += i += j;
>           printf("%d,%d\n", i, j);
> 
>           i = 10, j = 20;
>           i += j += i += i;
>           printf("%d,%d\n", i, j);
> 
>           return 0;
>   }
>   $ tcc -run t.c
>   80,50
>   60,40
> 
> --steffen
> |
> |Der Kragenbaer,                The moon bear,
> |der holt sich munter           he cheerfully and one by one
> |einen nach dem anderen runter  wa.ks himself off
> |(By Robert Gernhardt)
> 

-- 
Alain Williams
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