Re: Why 'statement may have no effect' lint warning here?

[Manuel Collado]

El 12/04/2021 a las 23:15, Arkadiusz Drabczyk escribió:
> > > The following code produces 'statement may have no effect' when run
> > > with --lint:
> > >
> > > BEGIN {
> > >      var = 1
> > >      var == 1 && var = 2
> > >      print "var ==", var
> ...
> > The intent for that
> > warning is soemthing like:
>
> > BEGIN {
> >         var = 1
> >         var == 2        # no effect
> > }
>
> I see why lint would warn me in that case but I still don't understand
> that part:
> ...
> nothing is changed, that is value returned by == is not saved
> anywhere but it's an expression, right? == operator returns either 0
> or 1. That
>
>      var = 1
>      var == 2 && var = 3
>      print "var ==", var
>
> would print 1 because var == 2 is not true and `var = 3' would not be
> executed. So `var == 2' has an effect because it effects the other
> side of &&.

The lint warning is 'statement may have no effect'. The key part is the
'may' word. Just a hint that in some cases what the programmer wrote was
not what (s)he wanted.

A more elaborate example:

$ cat no-effect.awk
BEGIN {
    var = "abcd"
    sub("a", "x", var) && sub("c", "y", var)
    print "var ==", var

    var = "abcd"
    sub("e", "x", var) && sub("c", "y", var)
    print "var ==", var
}

$ gawk --lint -f no-effect.awk
gawk: no-effect.awk:3: warning: statement may have no effect
gawk: no-effect.awk:7: warning: statement may have no effect
var == xbyd
var == abcd

At first sight the && expression looks like if both substitutions
should be made in all cases, but in fact they aren't. So a lint warning
makes sense.

HTH.
--
Manuel Collado - http://mcollado.z15.es