Python IF THEN chain equivalence

[jzakiya]

I'm translating a program in Python that has this IF Then chain


IF x1 < limit: --- do a ---
IF x2 < limit: --- do b ---
IF x3 < limit: --- do c ---
.-----
------
IF x10 < limt: --- do j ---
THEN
THEN
-----
THEN
THEN
THEN

In other words, as long as 'xi' is less than 'limit' keep going
down the chain, and when 'xi' isn't less than 'limit' jump to end of
chain a continue.

Is this the equivalence in Python?

IF x1 < limit:
--- do a ---
elif x2 < limit:
--- do b ---
----
----
elif x10 < limit:
--- do j ---

[Diez B. Roggisch]

Re: Python IF THEN chain equivalence

jzakiya schrieb:

I'm translating a program in Python that has this IF Then chain
>
>
IF x1 < limit: --- do a ---
IF x2 < limit: --- do b ---
IF x3 < limit: --- do c ---
.-----
------
IF x10 < limt: --- do j ---
THEN
THEN
-----
THEN
THEN
THEN
>
In other words, as long as 'xi' is less than 'limit' keep going
down the chain, and when 'xi' isn't less than 'limit' jump to end of
chain a continue.
>
Is this the equivalence in Python?
>
IF x1 < limit:
--- do a ---
elif x2 < limit:
--- do b ---
----
----
elif x10 < limit:
--- do j ---

Of course not. After "do a", it would stop.

You need to use

if x1 < limit:
do a
if x2 < limit:
do b
...


Alternatively, and depending on the nature of "do X", you can do

for x, todo in ((x1, do_a), (x2, do_b), ...):
if x < limit:
todo()
else:
break

This implies though that the "dos" are pure functions without (variable)
arguments.

Diez

[Alan Baljeu]

Re: Python IF THEN chain equivalence

I think you should rethink your post. The first case you posted makes no sense in any language I know. Also, a whole lot of nested IF's is a bad idea in any language. In Python, you will end up with code indented 40+ characters if you keep going.



----- Original Message ----
From: jzakiya <jzakiya@mail.c om>
To: python-list@python.org
Sent: Thursday, November 13, 2008 5:06:53 PM
Subject: Python IF THEN chain equivalence

I'm translating a program in Python that has this IF Then chain


IF x1 < limit: --- do a ---
IF x2 < limit: --- do b ---
IF x3 < limit: --- do c ---
.-----
------
IF x10 < limt: --- do j ---
THEN
THEN
-----
THEN
THEN
THEN

In other words, as long as 'xi' is less than 'limit' keep going
down the chain, and when 'xi' isn't less than 'limit' jump to end of
chain a continue.

Is this the equivalence in Python?

IF x1 < limit:
--- do a ---
elif x2 < limit:
--- do b ---
----
----
elif x10 < limit:
--- do j ---


--

http://mail.python.org/mailman/listinfo/python-list

_______________ _______________ _______________ _______________ ______
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[jzakiya]

Re: Python IF THEN chain equivalence

On Nov 13, 5:21 pm, Alan Baljeu <alanbal...@yah oo.comwrote:

I think you should rethink your post. The first case you posted makes no sense in any language I know.  Also, a whole lot of nested IF's is a bad idea in any language.  In Python, you will end up with code indented 40+ characters if you keep going.
>
----- Original Message ----
From: jzakiya <jzak...@mail.c om>
To: python-l...@python.org
Sent: Thursday, November 13, 2008 5:06:53 PM
Subject: Python IF THEN chain equivalence
>
I'm translating a program in Python that has this IF Then chain
>
IF  x1 < limit:   --- do a ---
    IF  x2 < limit:  --- do b ---
        IF x3 < limit:  --- do c ---
                       .-----
                        ------
                    IF  x10 < limt: --- do j ---
                    THEN
                 THEN
              -----
          THEN
     THEN
THEN
>
In other words, as long as    'xi' is less than 'limit' keep going
down the chain, and when 'xi' isn't less than 'limit' jump to end of
chain a continue.
>
Is this the equivalence in Python?
>
IF  x1 < limit:
        --- do a  ---
elif x2 < limit:
        --- do b ---
----
----
elif x10 < limit:
       --- do j ---
>
--http://mail.python.org/mailman/listinfo/python-list
>
      _______________ _______________ _______________ _______________ ______
Ask a question on any topic and get answers from real people. Go to Yahoo! Answers and share what you know athttp://ca.answers.yaho o.com
>
>

In the code the 'xi's and 'limit' are variables and the --- do letters
---
phrases are simply writes to any array: an_array[xi]=0

Actually, the code makes perfectly good sense, and is a necessity of
the algorithm I'm implementing, and works perfectly good in Forth, and
can be
written quite nicely within a normal page width.

I was just hoping I could perform the equivalent chain in Python
without
having to grossly indent the source code past the normal width of a
printed page.
But if that's the only way to do it in Python, then so be it.

[Grant Edwards]

Re: Python IF THEN chain equivalence

On 2008-11-13, jzakiya <jzakiya@mail.c omwrote:

I'm translating a program in Python that has this IF Then chain
>
>
IF x1 < limit: --- do a ---
IF x2 < limit: --- do b ---
IF x3 < limit: --- do c ---
.-----
------
IF x10 < limt: --- do j ---
THEN
THEN
-----
THEN
THEN
THEN

The placement of the THEN statements makes absolutely no sense
in any language I've ever seen.

In other words, as long as 'xi' is less than 'limit' keep going
down the chain, and when 'xi' isn't less than 'limit' jump to end of
chain a continue.
>
Is this the equivalence in Python?
>
IF x1 < limit:
--- do a ---
elif x2 < limit:
--- do b ---
----
----
elif x10 < limit:
--- do j ---

No. That's not the same at all.


Here's one solution:

while True:
if x1 limit: break
do a
if x2 limit: break
do b
if x3 limit: break
do c
...
if x10 limit: break
do j
break

--
Grant Edwards grante Yow! Eisenhower!! Your
at mimeograph machine upsets
visi.com my stomach!!

[Grant Edwards]

Re: Python IF THEN chain equivalence

On 2008-11-13, Grant Edwards <invalid@invali dwrote:

On 2008-11-13, jzakiya <jzakiya@mail.c omwrote:

>I'm translating a program in Python that has this IF Then chain
>>
>>
>IF x1 < limit: --- do a ---
> IF x2 < limit: --- do b ---
> IF x3 < limit: --- do c ---
> .-----
> ------
> IF x10 < limt: --- do j ---
> THEN
> THEN
> -----
> THEN
> THEN
>THEN

>
The placement of the THEN statements makes absolutely no sense
in any language I've ever seen.
>

>In other words, as long as 'xi' is less than 'limit' keep going
>down the chain, and when 'xi' isn't less than 'limit' jump to end of
>chain a continue.
>>
>Is this the equivalence in Python?
>>
> IF x1 < limit:
> --- do a ---
> elif x2 < limit:
> --- do b ---
> ----
> ----
> elif x10 < limit:
> --- do j ---

>
No. That's not the same at all.
>
>
Here's one solution:
>
while True:
if x1 limit: break
do a
if x2 limit: break
do b
if x3 limit: break
do c
...
if x10 limit: break
do j
break

Oops. I botched the case where xN == limit. Each of the tests
should be xN >= limit.

--
Grant Edwards grante Yow! ... I see TOILET
at SEATS ...
visi.com

[M.-A. Lemburg]

Re: Python IF THEN chain equivalence

On 2008-11-13 23:31, jzakiya wrote:

On Nov 13, 5:21 pm, Alan Baljeu <alanbal...@yah oo.comwrote:

>I think you should rethink your post. The first case you posted makes no sense in any language I know. Also, a whole lot of nested IF's is a bad idea in any language. In Python, you will end up with code indented 40+ characters if you keep going.
>>
>----- Original Message ----
>From: jzakiya <jzak...@mail.c om>
>To: python-l...@python.org
>Sent: Thursday, November 13, 2008 5:06:53 PM
>Subject: Python IF THEN chain equivalence
>>
>I'm translating a program in Python that has this IF Then chain
>>
>IF x1 < limit: --- do a ---
> IF x2 < limit: --- do b ---
> IF x3 < limit: --- do c ---
> .-----
> ------
> IF x10 < limt: --- do j ---
> THEN
> THEN
> -----
> THEN
> THEN
>THEN
>>
>In other words, as long as 'xi' is less than 'limit' keep going
>down the chain, and when 'xi' isn't less than 'limit' jump to end of
>chain a continue.
>>
>Is this the equivalence in Python?
>>
>IF x1 < limit:
> --- do a ---
>elif x2 < limit:
> --- do b ---
>----
>----
>elif x10 < limit:
> --- do j ---
>>
>--http://mail.python.org/mailman/listinfo/python-list
>>
> _______________ _______________ _______________ _______________ ______
>Ask a question on any topic and get answers from real people. Go to Yahoo! Answers and share what you know athttp://ca.answers.yaho o.com
>>
>>

>
In the code the 'xi's and 'limit' are variables and the --- do letters
---
phrases are simply writes to any array: an_array[xi]=0
>
Actually, the code makes perfectly good sense, and is a necessity of
the algorithm I'm implementing, and works perfectly good in Forth, and
can be
written quite nicely within a normal page width.
>
I was just hoping I could perform the equivalent chain in Python
without
having to grossly indent the source code past the normal width of a
printed page.
But if that's the only way to do it in Python, then so be it.

You should probably consider using a function and then
convert the conditions to define return points:

def do_something(.. .args...):

if x1 >= limit:
return
...do a...
if x2 >= limit:
return
...do b...
etc.

That is provided I understand the flow of control in your
example... it's kind of strange to have THEN mark the *end*
of the then-branch in an if-then-else construct.

--
Marc-Andre Lemburg
eGenix.com

Professional Python Services directly from the Source (#1, Nov 13 2008)

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_______________ _______________ _______________ _______________ ____________
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D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg
Registered at Amtsgericht Duesseldorf: HRB 46611

[jzakiya]

Re: Python IF THEN chain equivalence

On Nov 13, 5:48 pm, "M.-A. Lemburg" <m...@egenix.co mwrote:

On 2008-11-13 23:31, jzakiya wrote:
>
>
>

On Nov 13, 5:21 pm, Alan Baljeu <alanbal...@yah oo.comwrote:

I think you should rethink your post. The first case you posted makes no sense in any language I know.  Also, a whole lot of nested IF's is a bad idea in any language.  In Python, you will end up with code indented 40+ characters if you keep going.

>

----- Original Message ----
From: jzakiya <jzak...@mail.c om>
To: python-l...@python.org
Sent: Thursday, November 13, 2008 5:06:53 PM
Subject: Python IF THEN chain equivalence

>

I'm translating a program in Python that has this IF Then chain

>

IF  x1 < limit:   --- do a ---
    IF  x2 < limit:  --- do b ---
        IF x3 < limit:  --- do c ---
                       .-----
                        ------
                    IF  x10 < limt: --- do j ---
                    THEN
                 THEN
              -----
          THEN
     THEN
THEN

>

In other words, as long as    'xi' is less than 'limit' keep going
down the chain, and when 'xi' isn't less than 'limit' jump to end of
chain a continue.

>

Is this the equivalence in Python?

>

IF  x1 < limit:
        --- do a  ---
elif x2 < limit:
        --- do b ---
----
----
elif x10 < limit:
       --- do j ---

>

--http://mail.python.org/mailman/listinfo/python-list

>

      _______________ _______________ _______________ _______________ ______
Ask a question on any topic and get answers from real people. Go to Yahoo! Answers and share what you know athttp://ca.answers.yaho o.com

>

In the code the 'xi's and 'limit' are variables and the --- do letters
---
phrases are simply writes to any array:   an_array[xi]=0

>

Actually, the code makes perfectly good sense, and is a necessity of
the algorithm I'm implementing, and works perfectly good in Forth, and
can be
written quite nicely within a normal page width.

>

I was just hoping I could perform the equivalent chain in Python
without
having to grossly indent the source code past the normal width of a
printed page.
But if that's the only way to do it in Python, then so be it.

>
You should probably consider using a function and then
convert the conditions to define return points:
>
def do_something(.. .args...):
>
    if x1 >= limit:
        return
    ...do a...
    if x2 >= limit:
        return
    ...do b...
    etc.
>
That is provided I understand the flow of control in your
example... it's kind of strange to have THEN mark the *end*
of the then-branch in an if-then-else construct.
>
--
Marc-Andre Lemburg
eGenix.com
>
Professional Python Services directly from the Source  (#1, Nov 13 2008)>>Python/Zope Consulting and Support ...        http://www.egenix.com/

>mxODBC.Zope.Da tabase.Adapter ...            http://zope.egenix.com/
>mxODBC, mxDateTime, mxTextTools ...        http://python.egenix.com/

>
_______________ _______________ _______________ _______________ ____________
2008-11-12: Released mxODBC Connect 0.9.3      http://python.egenix..com/
>
:::: Try mxODBC.Zope.DA for Windows,Linux,S olaris,MacOSX for free ! ::::
>
   eGenix.com Software, Skills and Services GmbH  Pastor-Loeh-Str.48
    D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg
           Registered at Amtsgericht Duesseldorf: HRB 46611

It's interesting to see people think it's strange to have code that
has multiple nested levels of IF THEN. Apparently you haven't seen
any Forth, assembly, et al code. All you're doing is having the branch
point for each conditional be the end of the chain, otherwise it falls
through to the code after the conditional. This is done all the time
in languages that let you actually manipulate the hardware.

Just as a suggestion :-) a little humility would go a long way toward
being open minded and receptive to different paradigms. I've written
this program I'm doing now in Python in 3 other languages (including
Python, which I'm trying to make more efficient) and I seek to be
flexible in my software linguistic capabilities.

I asked a very narrow question about a very specific language
mechanism, and I know exactly what and why I'm doing what I'm doing.

I'll try some of the suggestions and see if they make the routine
faster in Python.

[M.-A. Lemburg]

Re: Python IF THEN chain equivalence

On 2008-11-14 00:19, jzakiya wrote:

On Nov 13, 5:48 pm, "M.-A. Lemburg" <m...@egenix.co mwrote:

>>>From: jzakiya <jzak...@mail.c om>
>>>To: python-l...@python.org
>>>Sent: Thursday, November 13, 2008 5:06:53 PM
>>>Subject: Python IF THEN chain equivalence
>>>I'm translating a program in Python that has this IF Then chain
>>>IF x1 < limit: --- do a ---
>>> IF x2 < limit: --- do b ---
>>> IF x3 < limit: --- do c ---
>>> .-----
>>> ------
>>> IF x10 < limt: --- do j ---
>>> THEN
>>> THEN
>>> -----
>>> THEN
>>> THEN
>>>THEN

>You should probably consider using a function and then
>convert the conditions to define return points:
>>
>def do_something(.. .args...):
>>
> if x1 >= limit:
> return
> ...do a...
> if x2 >= limit:
> return
> ...do b...
> etc.
>>
>That is provided I understand the flow of control in your
>example... it's kind of strange to have THEN mark the *end*
>of the then-branch in an if-then-else construct.

>
It's interesting to see people think it's strange to have code that
has multiple nested levels of IF THEN.

Not at all, that's fairly common in a lot of languages. It's not
common to mark the end of the then-part using the THEN keyword,
though. You'd normally expect the body of the then-part after the
keyword.

Apparently you haven't seen
any Forth, assembly, et al code. All you're doing is having the branch
point for each conditional be the end of the chain, otherwise it falls
through to the code after the conditional. This is done all the time
in languages that let you actually manipulate the hardware.
>
Just as a suggestion :-) a little humility would go a long way toward
being open minded and receptive to different paradigms.

Without giving any hint as to what the quoted snippet
of code is written in, how do you expect people to make
any sense of it ? Especially when using an RPN stack oriented
language in a Python forum.

There's a reason why we hide Python byte code running on the
VM stack machine from Python users ;-)

--
Marc-Andre Lemburg
eGenix.com

Professional Python Services directly from the Source (#1, Nov 14 2008)

>>Python/Zope Consulting and Support ... http://www.egenix.com/
>>mxODBC.Zope.D atabase.Adapter ... http://zope.egenix.com/
>>mxODBC, mxDateTime, mxTextTools ... http://python.egenix.com/

_______________ _______________ _______________ _______________ ____________
2008-11-12: Released mxODBC Connect 0.9.3 http://python.egenix.com/

:::: Try mxODBC.Zope.DA for Windows,Linux,S olaris,MacOSX for free ! ::::


eGenix.com Software, Skills and Services GmbH Pastor-Loeh-Str.48
D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg
Registered at Amtsgericht Duesseldorf: HRB 46611

[bearophileHUGS@lycos.com]

Re: Python IF THEN chain equivalence

jzakiya:

I asked a very narrow question about a very specific language
mechanism, and I know exactly what and why I'm doing what I'm doing.

You are of course free to use Python as you want. And probably some of
the answers weren't fully polite.
But it's interesting to see why they have given such answers. While
your code can be fit for Forth or assembly, it may be unfit, in its
current form, for the usual practices of Python programming (or C#/
Java programming, etc). In high-level languages large trees of very
nested ifs are seen as error-prone, not much readable, brittle... So
some alternative solutions are often used (even if they may be a
little slower).
If you need speed you may use Psyco too.

Bye,
bearophile

[Mensanator]

Re: Python IF THEN chain equivalence

On Nov 13, 4:39 pm, Grant Edwards <invalid@invali dwrote:

On 2008-11-13, jzakiya <jzak...@mail.c omwrote:
>

I'm translating a program in Python that has this IF Then chain

>

IF  x1 < limit:   --- do a ---
    IF  x2 < limit:  --- do b ---
        IF x3 < limit:  --- do c ---
                       .-----
                        ------
                    IF  x10 < limt: --- do j ---
                    THEN
                 THEN
              -----
          THEN
     THEN
THEN

>
The placement of the THEN statements makes absolutely no sense
in any language I've ever seen.

It looks like Visual Basic.

IF a THEN
do_a
IF b THEN
do_b
IF c THEN
do_c
END IF
END IF
END IF

Words different, but same structure.

>

In other words, as long as    'xi' is less than 'limit' keep going
down the chain, and when 'xi' isn't less than 'limit' jump to end of
chain a continue.

>

Is this the equivalence in Python?

>

 IF  x1 < limit:
        --- do a  ---
 elif x2 < limit:
        --- do b ---
 ----
 ----
 elif x10 < limit:
       --- do j ---

>
No.  That's not the same at all.
>
Here's one solution:
>
while True:
  if x1 limit: break
  do a
  if x2 limit: break
  do b
  if x3 limit: break
  do c
  ...
  if x10 limit: break
  do j
  break  
>
--
Grant Edwards                   grante             Yow! Eisenhower!!  Your
                                  at               mimeograph machine upsets
                               visi.com            my stomach!!

[Ethan Furman]

Re: Python IF THEN chain equivalence

jzakiya wrote:

I'm translating a program in Python that has this IF Then chain
>
>
IF x1 < limit: --- do a ---
IF x2 < limit: --- do b ---
IF x3 < limit: --- do c ---
.-----
------
IF x10 < limt: --- do j ---
THEN
THEN
-----
THEN
THEN
THEN
>
In other words, as long as 'xi' is less than 'limit' keep going
down the chain, and when 'xi' isn't less than 'limit' jump to end of
chain a continue.

if x1 < limit:
do a
if x2 < limit:
do b
if x3 < limit:
do c

[Grant Edwards]

Re: Python IF THEN chain equivalence

On 2008-11-14, Ethan Furman <ethan@stonelea f.uswrote:

jzakiya wrote:

>I'm translating a program in Python that has this IF Then chain
>>
>>
>IF x1 < limit: --- do a ---
> IF x2 < limit: --- do b ---
> IF x3 < limit: --- do c ---
> .-----
> ------
> IF x10 < limt: --- do j ---
> THEN
> THEN
> -----
> THEN
> THEN
>THEN
>>
>In other words, as long as 'xi' is less than 'limit' keep going
>down the chain, and when 'xi' isn't less than 'limit' jump to end of
>chain a continue.

>
if x1 < limit:
do a
if x2 < limit:
do b
if x3 < limit:
do c
.
.
.
etc

That doesn't do what the OP specified. If any of the
conditions fail, it should "jump" to the end and not execute
_any_ further "do" statements regardless of the values of
subsequent conditions.

On the plus side, it's easy to read and understand -- on the
minus side, it doesn't jump to the end once the tests start
failing.

If all you want is easy to read and understand, then this is
event simpler:


sys.exit(0)

--
Grant

[Ethan Furman]

Re: Python IF THEN chain equivalence

Grant Edwards wrote:

On 2008-11-14, Ethan Furman <ethan@stonelea f.uswrote:

>jzakiya wrote:

>>I'm translating a program in Python that has this IF Then chain
>>>
>>>
>>IF x1 < limit: --- do a ---
>> IF x2 < limit: --- do b ---
>> IF x3 < limit: --- do c ---
>> .-----
>> ------
>> IF x10 < limt: --- do j ---
>> THEN
>> THEN
>> -----
>> THEN
>> THEN
>>THEN
>>>
>>In other words, as long as 'xi' is less than 'limit' keep going
>>down the chain, and when 'xi' isn't less than 'limit' jump to end of
>>chain a continue.

>if x1 < limit:
> do a
>if x2 < limit:
> do b
>if x3 < limit:
> do c
> .
> .
> .
> etc

>
That doesn't do what the OP specified. If any of the
conditions fail, it should "jump" to the end and not execute
_any_ further "do" statements regardless of the values of
subsequent conditions.

Ack -- I was aware of the lack of jump, but missed that x7 might be less
than limit even if x6 is greater -- thanks.

>On the plus side, it's easy to read and understand -- on the
>minus side, it doesn't jump to the end once the tests start
>failing.

>
If all you want is easy to read and understand, then this is
event simpler:
>
>
sys.exit(0)
>

<sheepish grin>
~ethan~

[Steven D'Aprano]

Re: Python IF THEN chain equivalence

On Fri, 14 Nov 2008 01:24:32 +0100, M.-A. Lemburg wrote:

>Apparently you haven't seen
>any Forth, assembly, et al code. All you're doing is having the branch
>point for each conditional be the end of the chain, otherwise it falls
>through to the code after the conditional. This is done all the time
>in languages that let you actually manipulate the hardware.
>>
>Just as a suggestion a little humility would go a long way toward
>being open minded and receptive to different paradigms.

>
Without giving any hint as to what the quoted snippet of code is written
in, how do you expect people to make any sense of it ? Especially when
using an RPN stack oriented language in a Python forum.
>
There's a reason why we hide Python byte code running on the VM stack
machine from Python users ;-)

It's not like Forth is precisely an obscure little language. For a time,
it was possibly more popular than C. Or predated C? Whatever. I know I
learned about Forth long before I had even heard of C.

Other RPN languages include Postscript, not exactly unheard of either.
Open Firmware is Forth-like, and as you point out yourself, Python byte
code also is a stack-based language.

In conclusion, I'm not sure which is more disappointing: that the OP
couldn't be bothered to mention he was using a Forth-like syntax, or that
so many people failed to recognize it.

Anyway, for what it's worth, here's my translation into Python.

if x1 < limit:
a()
if x2 < limit:
b()
if x3 < limit:
c()
# blah blah blah...
if x10 < limt:
j()


Not very nice code. I think a better way is something like this:

keys = [x1, x2, x3, x4, x5, x6, x7, x8, x9, x10]
functions = [a, b, c, d, e, f, g, h, i, j]

for key, function in zip(keys, functions):
if key < limit:
function()
else:
break



--
Steven