Vista - Re: Empty output oddity in conjunction with foreach statement

03-27-2007

How about a variable setting that could be used to toggle how $nul get
interpreted?

"Duncan Smith" wrote:

> On Mar 26, 3:06 pm, "Keith Hill" <r_keith_h...@mailhot.nospamIdotcom>
> wrote:
> > The following little nuance of PowerShell is kind of annoying. Say I have a function foo that may not output anything. OTOH it may output a series of say XML objects. I write some code to loop on the output I get from a call to this function e.g.:
> >
> > function foo { }
> >
> > $docs = foo
> > foreach ($doc in $docs) {
> > $_.SelectNodes('//compile')
> >
> > }
> >
> > This errors because foo returns nothing but something needs to be assigned to $docs. That something is $null and with the way the foreach statement is designed it will loop once on that scalar $null and I wind up getting this error:
> >
> > You cannot call a method on a null-valued expression.
> > At line:2 char:19
> > + $_.SelectNodes( <<<< '//compile')
> >
> > Yet this seemingly harmless change causes the loop body to get skipped:
> >
> > foreach ($doc in foo) {
> > $_.SelectNodes('//compile')
> >
> > }
> >
> > In this scenario since the function foo returns 'nothing' as opposed to $null the loop is completely skipped over. My problem with this is that it violates the principle of least surprise (at least for me). You wouldn't think that such a 'harmless' reorg of the code would change its behavior.
> >
> > I understand that having the foreach statement loop once on a scalar value is a good idea. However I wonder if the scalar value $null should be a special case and cause the foreach to skip the loop body.
> >
> > --
> > Keith
>
> There are probably other cases when the $null getting interpreted as
> the integer 0 could be a good thing, if for example you were
> populating an associative array and bumping an index-count.
>
> Could you maybe do something with checking $_ for equivalence with
> $null and use a continue statement at the start of the loop?
>
> Regards,
>
> Duncan.
>
>

03-27-2007	#1 (permalink)
Rob Campbell n/a posts	Re: Empty output oddity in conjunction with foreach statement How about a variable setting that could be used to toggle how $nul get interpreted? "Duncan Smith" wrote: > On Mar 26, 3:06 pm, "Keith Hill" <r_keith_h...@mailhot.nospamIdotcom> > wrote: > > The following little nuance of PowerShell is kind of annoying. Say I have a function foo that may not output anything. OTOH it may output a series of say XML objects. I write some code to loop on the output I get from a call to this function e.g.: > > > > function foo { } > > > > $docs = foo > > foreach ($doc in $docs) { > > $_.SelectNodes('//compile') > > > > } > > > > This errors because foo returns nothing but something needs to be assigned to $docs. That something is $null and with the way the foreach statement is designed it will loop once on that scalar $null and I wind up getting this error: > > > > You cannot call a method on a null-valued expression. > > At line:2 char:19 > > + $_.SelectNodes( <<<< '//compile') > > > > Yet this seemingly harmless change causes the loop body to get skipped: > > > > foreach ($doc in foo) { > > $_.SelectNodes('//compile') > > > > } > > > > In this scenario since the function foo returns 'nothing' as opposed to $null the loop is completely skipped over. My problem with this is that it violates the principle of least surprise (at least for me). You wouldn't think that such a 'harmless' reorg of the code would change its behavior. > > > > I understand that having the foreach statement loop once on a scalar value is a good idea. However I wonder if the scalar value $null should be a special case and cause the foreach to skip the loop body. > > > > -- > > Keith > > There are probably other cases when the $null getting interpreted as > the integer 0 could be a good thing, if for example you were > populating an associative array and bumping an index-count. > > Could you maybe do something with checking $_ for equivalence with > $null and use a continue statement at the start of the loop? > > Regards, > > Duncan. > >
My System Specs

Similar Threads
Thread	Forum
Re: Oddity with ForEach-Object or is it me?	PowerShell
break from foreach statement	PowerShell
Re: Empty output oddity in conjunction with foreach statement	PowerShell
Sorting output from foreach	PowerShell
User Guide p.109 - Incorrect statement about foreach	PowerShell