Re: Understanding list and scalar context
Thanks Gil, that helped me to understand the context of the code presented
in the book.
Le 18 sept. 2016 18:02, "Gil Magno" a écrit :
> Hi Khalil
>
> 'say' creates list context. So if 'say' receives the list ('one', 'cd',
> 'qw') it will print 'abcdqw'.
>
> 'reverse' creates list context. So if 'reverse' receives the list ('ab',
> 'cd', 'qw') it will return a list with the same elements, but with its
> order inverted ('qw', 'bc', 'ab').
>
> The catch is that if 'reverse' is forced to be evaluated in scalar
> context, then it will 1) concatenate the elements of the list we give to
> it, 2) treat the concatenated result as a string and 3) invert the string.
>
> To force one expression to be evaluated in scalar context, you should
> give that expression as an argument to the 'scalar' function. So in
> reverse(1, 2, 3); 'reverse' is evaluated in *list* context and the list
> returned is (3, 2, 1) , but in scalar(reverse(1, 2, 3)) 'reverse' is
> evaluated in *scalar* context, so 1, 2, 3 are concatenated to the string
> '123', then the string is inverted to '321' and then it is returned by
> 'reverse'.
>
> Now we come to 'say'. 'say' still creates list context, but the list it
> receives from scalar(reverse(1, 2, 3)) is a list of just one element:
> the element '321'. So 'say' prints '321'.
>
> If you didn't understand something or if it is still confuse, don't
> hesitate to ask further questions. :)
>
> On 18/09/16 12:43, khalil zakaria Zemmoura wrote:
> > Hi everyone,
> >
> > I'm reading the modern perl book and I have some questions to address
> about
> > scalar and list context.
> >
> > Here is the code that I want to understand.
> >
> > while (<>) { chomp; say scalar reverse; }
> >
> > Where I'm struggling is : say scalar reverse;
> >
> > The book says that 'say' impose list context to Its operands. 'reverse'
> > impose list context on to its operands and treat them as a list in list
> > context and a concatenated string in scalar context.
> >
> > The questions are:
> >
> > Is 'say' that is imposing list context in 'reverse' or 'reverse' it self
> > treats it's operands in list context or it's both?
> >
> > The most confusing part:
> > There is 'scalar' before 'reverse', so 'reverse' is evaluated in scalar
> > context!
> > So how can reverse executing in both contexts (list and scalar context)?
> >
> > I'm sure I missed something.
> >
> > It will be great if I have more explanation.
> >
> > Thank you.
> > Regards, Zakaria
> >
>
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Re: Understanding list and scalar context
Hi Khalil
'say' creates list context. So if 'say' receives the list ('one', 'cd',
'qw') it will print 'abcdqw'.
'reverse' creates list context. So if 'reverse' receives the list ('ab',
'cd', 'qw') it will return a list with the same elements, but with its
order inverted ('qw', 'bc', 'ab').
The catch is that if 'reverse' is forced to be evaluated in scalar
context, then it will 1) concatenate the elements of the list we give to
it, 2) treat the concatenated result as a string and 3) invert the string.
To force one expression to be evaluated in scalar context, you should
give that expression as an argument to the 'scalar' function. So in
reverse(1, 2, 3); 'reverse' is evaluated in *list* context and the list
returned is (3, 2, 1) , but in scalar(reverse(1, 2, 3)) 'reverse' is
evaluated in *scalar* context, so 1, 2, 3 are concatenated to the string
'123', then the string is inverted to '321' and then it is returned by
'reverse'.
Now we come to 'say'. 'say' still creates list context, but the list it
receives from scalar(reverse(1, 2, 3)) is a list of just one element:
the element '321'. So 'say' prints '321'.
If you didn't understand something or if it is still confuse, don't
hesitate to ask further questions. :)
On 18/09/16 12:43, khalil zakaria Zemmoura wrote:
> Hi everyone,
>
> I'm reading the modern perl book and I have some questions to address about
> scalar and list context.
>
> Here is the code that I want to understand.
>
> while (<>) { chomp; say scalar reverse; }
>
> Where I'm struggling is : say scalar reverse;
>
> The book says that 'say' impose list context to Its operands. 'reverse'
> impose list context on to its operands and treat them as a list in list
> context and a concatenated string in scalar context.
>
> The questions are:
>
> Is 'say' that is imposing list context in 'reverse' or 'reverse' it self
> treats it's operands in list context or it's both?
>
> The most confusing part:
> There is 'scalar' before 'reverse', so 'reverse' is evaluated in scalar
> context!
> So how can reverse executing in both contexts (list and scalar context)?
>
> I'm sure I missed something.
>
> It will be great if I have more explanation.
>
> Thank you.
> Regards, Zakaria
>
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To unsubscribe, e-mail: beginners-unsubscr...@perl.org
For additional commands, e-mail: beginners-h...@perl.org
http://learn.perl.org/
Understanding list and scalar context
Hi everyone,
I'm reading the modern perl book and I have some questions to address about
scalar and list context.
Here is the code that I want to understand.
while (<>) { chomp; say scalar reverse; }
Where I'm struggling is : say scalar reverse;
The book says that 'say' impose list context to Its operands. 'reverse'
impose list context on to its operands and treat them as a list in list
context and a concatenated string in scalar context.
The questions are:
Is 'say' that is imposing list context in 'reverse' or 'reverse' it self
treats it's operands in list context or it's both?
The most confusing part:
There is 'scalar' before 'reverse', so 'reverse' is evaluated in scalar
context!
So how can reverse executing in both contexts (list and scalar context)?
I'm sure I missed something.
It will be great if I have more explanation.
Thank you.
Regards, Zakaria
