subexpressions

[Sergey Dorofeev]

Hello all!

Please help, is there way to use sub-expressions in lambda?
For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
lambda x: sin(x*x)+cos(x* x)
How to make x*x to be evaluated once?

[Peter Otten]

Re: subexpressions

Sergey Dorofeev wrote:

Please help, is there way to use sub-expressions in lambda?
For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
lambda x: sin(x*x)+cos(x* x)
How to make x*x to be evaluated once?

>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]][0])(.5) == sin(.5*.5) +

cos(.5*.5)
True

The real answer is of course: Use a function.

Peter

[Sergey Dorofeev]

Re: subexpressions


"Peter Otten" <__peter__@web. dewrote in message
news:f3ok60$vp7 $03$1@news.t-online.com...

Sergey Dorofeev wrote:
>

>Please help, is there way to use sub-expressions in lambda?
>For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
>lambda x: sin(x*x)+cos(x* x)
>How to make x*x to be evaluated once?

>

>>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]][0])(.5) == sin(.5*.5) +

cos(.5*.5)
True
>
The real answer is of course: Use a function.

But what about something like

lambda x: sin(y)+cos(y) where y=x*x

?
May be this could be a PEP? If there is no straight way to do this.

[Peter Otten]

Re: subexpressions

Sergey Dorofeev wrote:

"Peter Otten" <__peter__@web. dewrote in message
news:f3ok60$vp7 $03$1@news.t-online.com...

>Sergey Dorofeev wrote:
>>

>>Please help, is there way to use sub-expressions in lambda?
>>For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
>>lambda x: sin(x*x)+cos(x* x)
>>How to make x*x to be evaluated once?

>>

>>>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]][0])(.5) == sin(.5*.5) +

>cos(.5*.5)
>True
>>
>The real answer is of course: Use a function.

>
But what about something like
>
lambda x: sin(y)+cos(y) where y=x*x
>
?
May be this could be a PEP? If there is no straight way to do this.

def f(x):
y = x*x
return sin(y) + cos(y)

What is not straightforward about that?

Peter

[Sergey Dorofeev]

Re: subexpressions


"Peter Otten" <__peter__@web. dewrote in message
news:f3oo0p$c7c $03$1@news.t-online.com...

>>>Please help, is there way to use sub-expressions in lambda?
>>>For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
>>>lambda x: sin(x*x)+cos(x* x)
>>>How to make x*x to be evaluated once?
>>>
>>>>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]][0])(.5) == sin(.5*.5)
>>>>>+
>>cos(.5*.5)
>>True
>>>
>>The real answer is of course: Use a function.

>>
>But what about something like
>>
>lambda x: sin(y)+cos(y) where y=x*x
>>
>?
>May be this could be a PEP? If there is no straight way to do this.

>
def f(x):
y = x*x
return sin(y) + cos(y)
>
What is not straightforward about that?

This code is needed once in a map, so I don't want 3+ extra lines.
Solution seemed so simple...
I always considered python as languague, where simple things do not require
extensive coding.
Moreover, this construction is common thing in functional programming.

[Peter Otten]

Re: subexpressions

Sergey Dorofeev wrote:

>
"Peter Otten" <__peter__@web. dewrote in message
news:f3oo0p$c7c $03$1@news.t-online.com...

>>>>Please help, is there way to use sub-expressions in lambda?
>>>>For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
>>>>lambda x: sin(x*x)+cos(x* x)
>>>>How to make x*x to be evaluated once?
>>>>
>>>>>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]][0])(.5) == sin(.5*.5)
>>>>>>+
>>>cos(.5*.5)
>>>True
>>>>
>>>The real answer is of course: Use a function.
>>>
>>But what about something like
>>>
>>lambda x: sin(y)+cos(y) where y=x*x
>>>
>>?
>>May be this could be a PEP? If there is no straight way to do this.

>>
>def f(x):
> y = x*x
> return sin(y) + cos(y)
>>
>What is not straightforward about that?

>
This code is needed once in a map,

Perhaps you like [sin(y)+cos(y) for y in (x*x for x in items)] then.

so I don't want 3+ extra lines.

What syntax would you suggest for a lambda enhanced to cover your use case?
I suppose you will end up with roughly the same number of characters, all
crammed in one line -- or broken into lines at a random position as it
happens with overambitious list comprehensions.

Solution seemed so simple...

It /is/ simple.

I always considered python as languague, where simple things do not
require extensive coding.

In Python, when conciseness and readability compete, readability tends to
win (with the inline if...else as a notable exception).

Moreover, this construction is common thing in functional programming.

I can write Haskell in any language :-)

Peter

[Sergey Dorofeev]

Re: subexpressions


"Peter Otten" <__peter__@web. dewrote in message
news:f3oret$sit $03$1@news.t-online.com...

What syntax would you suggest for a lambda enhanced to cover your use
case?
I suppose you will end up with roughly the same number of characters, all
crammed in one line -- or broken into lines at a random position as it
happens with overambitious list comprehensions.

Agree, this argument is strong.

[A.T.Hofkamp]

Re: subexpressions

On 2007-06-01, Sergey Dorofeev <sergey@fidoman .ruwrote:

Hello all!
>
Please help, is there way to use sub-expressions in lambda?
For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
lambda x: sin(x*x)+cos(x* x)
How to make x*x to be evaluated once?

lambda x: (lambda y: sin(y) + cos(y))(x*x)

Albert

[Steve Holden]

Re: subexpressions

Sergey Dorofeev wrote:

"Peter Otten" <__peter__@web. dewrote in message
news:f3ok60$vp7 $03$1@news.t-online.com...

>Sergey Dorofeev wrote:
>>

>>Please help, is there way to use sub-expressions in lambda?
>>For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
>>lambda x: sin(x*x)+cos(x* x)
>>How to make x*x to be evaluated once?
>>>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]][0])(.5) == sin(.5*.5) +

>cos(.5*.5)
>True
>>
>The real answer is of course: Use a function.

>
But what about something like
>
lambda x: sin(y)+cos(y) where y=x*x
>
?
May be this could be a PEP? If there is no straight way to do this.
>
>

Or maybe it could be made a part of some other language. When
straightforward mechanisms (in rhis case, function definitins) exist to
avoid repeated computations it's very unlikely that such mangled
constructions will be made a part of Python.

If it *were* considered, you should at least change the "where" to
"for", and extend it to unpacking assignment to allow

lambda x, y: (sin(xx+yy) + cos(xx+yy) for xx, yy = x*x, y*y

regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
------------------ Asciimercial ---------------------
Get on the web: Blog, lens and tag your way to fame!!
holdenweb.blogs pot.com squidoo.com/pythonology
tagged items: del.icio.us/steve.holden/python
All these services currently offer free registration!
-------------- Thank You for Reading ----------------

[Steve Holden]

Re: subexpressions

Sergey Dorofeev wrote:

"Peter Otten" <__peter__@web. dewrote in message
news:f3oo0p$c7c $03$1@news.t-online.com...

>>>>Please help, is there way to use sub-expressions in lambda?
>>>>For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
>>>>lambda x: sin(x*x)+cos(x* x)
>>>>How to make x*x to be evaluated once?
>>>>>>(lambda x: [sin(x2) + cos(x2) for x2 in [x*x]][0])(.5) == sin(.5*.5)
>>>>>>+
>>>cos(.5*.5)
>>>True
>>>>
>>>The real answer is of course: Use a function.
>>But what about something like
>>>
>>lambda x: sin(y)+cos(y) where y=x*x
>>>
>>?
>>May be this could be a PEP? If there is no straight way to do this.

>def f(x):
> y = x*x
> return sin(y) + cos(y)
>>
>What is not straightforward about that?

>
This code is needed once in a map, so I don't want 3+ extra lines.
Solution seemed so simple...
I always considered python as languague, where simple things do not require
extensive coding.
Moreover, this construction is common thing in functional programming.
>
>

Stop thinking of three lines as "extensive coding" and your problem
disappears immediately.

regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
------------------ Asciimercial ---------------------
Get on the web: Blog, lens and tag your way to fame!!
holdenweb.blogs pot.com squidoo.com/pythonology
tagged items: del.icio.us/steve.holden/python
All these services currently offer free registration!
-------------- Thank You for Reading ----------------

[Bruno Desthuilliers]

Re: subexpressions

Steve Holden a écrit :
(snip)

Stop thinking of three lines as "extensive coding" and your problem
disappears immediately.

Lol !
+1 QOTW

[Paul Boddie]

Re: subexpressions

On 1 Jun, 12:55, Steve Howell <showel...@yaho o.comwrote:

>
FWIW there's the possibility that even without a
subexpression syntax, some Python implementations
would detect the duplication of x*x and optimize that
for you. It would have to know that x*x had no side
effects, which I think is a safe assumption even in a
dynamic language like Python.

On the basis of you believing that x is one of the built-in numeric
types, yes, but how does the compiler know that?

Paul

[Terry Reedy]

Re: subexpressions


"Sergey Dorofeev" <sergey@fidoman .ruwrote in message
news:f3oj68$1pb 8$1@news.rtcomm .ru...
| How to make x*x to be evaluated once?

Addendum to the answers already posted: In Python,

lambda params: expression

is an inline abbreviation for

def <lambda>(params ): return expression

except that there is no external binding of the otherwise illegal
..func_name '<lambda>'. The resulting function objects are otherwise
identical.

After years of discussion, Guido has decided to leave lambda alone for 3.0.
It will not be neither expanded, nor removed, nor renamed.

Terry Jan Reedy

[Kay Schluehr]

Re: subexpressions

On Jun 1, 9:51 am, "Sergey Dorofeev" <ser...@fidoman .ruwrote:

Hello all!
>
Please help, is there way to use sub-expressions in lambda?
For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
lambda x: sin(x*x)+cos(x* x)
How to make x*x to be evaluated once?

lambda x: (lambda y=x*x: math.sin(y)+mat h.cos(y))()

Kay

[Steven Bethard]

Re: subexpressions

Sergey Dorofeev wrote:

Please help, is there way to use sub-expressions in lambda?
For example, if I want to calculate sin(x^2)+cos(x^ 2) I must code:
lambda x: sin(x*x)+cos(x* x)

[and later]

This code is needed once in a map,

Peter Otten wrote:

Perhaps you like [sin(y)+cos(y) for y in (x*x for x in items)] then.

Just wanted to emphasize this suggestion so that it doesn't get lost in
the flood of lambda recommendations . If your code really looks like::

map(lambda x: sin(x * x) + cos(x * x), items)

you should be using a list comprehension instead. Using map() here is
not only more obscure and more verbose, but slower than::

[sin(x * x) + cos(x * x) for x in items]

From there, it's a simple nested generator comprehension to pull out
the subexpression:

[sin(y) + cos(y) for y in (x * x for x in items)]

If you aren't yet familiar with list and generator comprehensions, you
should take a few minutes to look at some of your uses of map() and
filter and see if you can simplify them using comprehensions instead.

STeVe