[Python-ideas] Respectively and its unpacking sentence

[Mirmojtaba Gharibi]

Yes, I'm aware sequence unpacking.
There is an overlap like you mentioned, but there are things that can't be
done with sequence unpacking, but can be done here.

For example, let's say you're given two lists that are not necessarily
numbers, so you can't use numpy, but you want to apply some component-wise
operator between each component. This is something you can't do with
sequence unpacking or with numpy. For example:

$StudentFullName = $FirstName + " " + $LastName

So, in effect, I think one big part of is component wise operations.

Another thing that can't be achieved with sequence unpacking is:
f($x)
i.e. applying f for each component of x.

About your question above, it's not ambiguous here either:
 a; b = x1;a + x2;5
is exactly "Equivalent" to
a = x1+x2
b = a + 5

Also, there is a difference in style in sequence unpacking, and here.
In sequence unpacking, you have to pair up the right variables and repeat
the operator, for example:
x,y,z = x1+x2 , y1+y2, z1+z2
Here you don't have to repeat it and pair up the right variables, i.e.
x;y;z = x1;y1;z1 + x2;y2;z2
It's I think good that you (kind of) don't break the encapsulation-ish
thing we have for the three values here. Also, you don't risk, making a
mistake in the operator for one of the values by centralizing the operator
use. For example you could make the mistake:
x,y,z = x1+x2, y1-y2, z1+z2

Also there are all sort of other things that are less of a motivation for
me but that cannot be done with sequence unpacking.
For instance:
add ; prod = a +;* y  (This one I'm not sure how can be achieved without
ambiguity)
x;y = f;g (a;b)


On Wed, Jan 27, 2016 at 12:57 AM, Sjoerd Job Postmus <sjoerdjob at sjec.nl>
wrote:

> On Wed, Jan 27, 2016 at 12:25:05AM -0500, Mirmojtaba Gharibi wrote:
> > Hello,
> >
> > I'm thinking of this idea that we have a pseudo-operator called
> > "Respectively" and shown maybe with ;
>
> Hopefully, you're already aware of sequence unpacking? Search for
> 'unpacking' at https://docs.python.org/2/tutorial/datastructures.html .
> Unfortunately, it does not have its own section I can directly link to.
>
>     x, y = 3, 5
>
> would give the same result as
>
>     x = 3
>     y = 5
>
> But it's more robust, as it can also deal with things like
>
>     x, y = y + 1, x + 4
> >
> > Some examples first:
> >
> > a;b;c = x1;y1;z1 + x2;y2;z2
> > is equivalent to
> > a=x1+x2
> > b=y1+y2
> > c=z1+z2
>
> So what would happen with the following?
>
>     a; b = x1;a + x2;5
>
> >
> > So it means for each position in the statement, do something like
> > respectively. It's like what I call a vertical expansion, i.e. running
> > statements one by one.
> > Then there is another unpacking operator which maybe we can show with $
> > sign and it operates on lists and tuples and creates the "Respectively"
> > version of them.
> > So for instance,
> > vec=[]*10
> > $vec = $u + $v
> > will add two 10-dimensional vectors to each other and put the result in
> vec.
> >
> > I think this is a syntax that can make many things more concise plus it
> > makes component wise operation on a list done one by one easy.
> >
> > For example, we can calculate the inner product between two vectors like
> > follows (inner product is the sum of component wise multiplication of two
> > vectors):
> >
> > innerProduct =0
> > innerProduct += $a * $b
> >
> > which is equivalent to
> > innerProduct=0
> > for i in range(len(a)):
> > ...innerProduct += a[i]+b[i]
> >
>
> From what I can see, it would be very beneficial for you to look into
> numpy: http://www.numpy.org/ . It already provides inner product, sums
> of arrays and such. I myself am not very familiar with it, but I think
> it provides what you need.
>
> >
> > For example, let's say we want to apply a function to all element in a
> > list, we can do:
> > f($a)
> >
> > The $ and ; take precedence over anything except ().
> >
> > Also, an important thing is that whenever, we don't have the respectively
> > operator, such as for example in the statement above on the left hand
> side,
> > we basically use the same variable or value or operator for each
> statement
> > or you can equivalently think we have repeated that whole thing with
> ;;;;.
> > Such as:
> > s=0
> > s;s;s += a;b;c; * d;e;f
> > which result in s being a*d+b,c*e+d*f
> >
> > Also, I didn't spot (at least for now any ambiguity).
> > For example one might think what if we do this recursively, such as in:
> > x;y;z + (a;b;c);(d;e;f);(g;h;i)
> > using the formula above this is equivalent to
> > (x;x;x);(y;y;y);(z;z;z)+(a;b;c);(d;e;f);(g;h;i)
> > if we apply print on the statement above, the result will be:
> > x+a
> > x+b
> > x+c
> > y+d
> > y+e
> > y+f
> > z+g
> > z+h
> > z+i
> >
> > Beware that in all of these ; or $ does not create a new list. Rather,
> they
> > are like creating new lines in the program and executing those lines one
> by
> > one( in the case of $, to be more accurate, we create for loops).
> >
> > I'll appreciate your time and looking forward to hearing your thoughts.
>
> Again, probably you should use numpy. I'm not really sure it warrants a
> change to the language, because it seems like it would really only be
> beneficial to those working with matrices. Numpy already supports it,
> and I'm suspecting that the use case for `a;b = c;d + e;f` can already
> be satisfied by `a, b = c + e, d + f`, and it already has clearly
> documented semantics and still works fine when one of the names on the
> left also appears on the right: First all the calculations on the right
> are performed, then they are assigned to the names on the left.
>
> >
> > Cheers,
> > Moj
>
> Kind regards,
> Sjoerd Job
>
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