python 3 prefix to infix without too many parethesis

[DL Neil]

On 10/12/19 8:40 AM, Terry Reedy wrote:
> On 12/9/2019 6:21 AM, jezkator at gmail.com wrote:
>> Hi, I have got a problem.
> 
> Is this homework?

Same question - that way we know that our task is to help you learn to 
code in Python, cf a problem with Python itself...

Similarly, you may like to  know that there is a separate Python-Tutor 
Discussion List.


>> I wrote a code for prefix to infix. It works, but I need improve it
>> so on input there will be only necessary parentheses.

The example code processes, like a calculator (um, yes, well...). What 
do you mean by "prefix to infix"?


> Define 'necessary'; give multiple input/output examples.
> Put them in a test function.

+1


>> Here is the code:

Question: with abbreviated variableNMs, eg "p", "a", even "op"; how 
easily will you comprehend the code in six month's time? How easy do you 
think it is for someone else to read the code now? NB the print 
functions are no help because the labels are equally-abbreviated, plus 
my aged-eyes start to strain when following lists of %s-es and regexps - 
mea culpa!


>> import re
>> a = input()
>>
>> class Calculator:
>>
>>      def __init__ (self):
>>          self.stack = []
>>
>>      def push (self, p):
>>          if p in ['+', '-', '*', '/', "**"]:
>>              op1 = self.stack.pop ()
>>              op2 = self.stack.pop ()
>>              self.stack.append ('(%s%s%s)' % (op1, p, op2))
>>
>>              print("op1 =", op1)
>>              print("op2 =", op2)
>>              print("p=", p)
>>              print(self.stack)
>>
>>          else:
>>              self.stack.append (p)
>>
>>
>>      def convert (self, l):
>>          l.reverse ()
>>          for e in l:
>>              self.push (e)
>>          return self.stack.pop ()
>>
>> c = Calculator ()
>>
>> print (c.convert (re.findall(r'\d+|\*\*|[-+*/]', a)))
>>
>>
>> input is like /-*+*++**85 27 39 87 65 65 37 63 91

ALWAYS, ALWAYS, ALWAYS, document a RegExp with a comment!!!

After 'translating' the abbreviations according to my recollection of 
this form of a frequently-used ComSc course problem, I was then 
surprised by the format of the input - using an infix calculator we 
enter a value, then an operator, and then another value, etc; and using 
a postfix calculator one usually inputs a couple of values and then an 
operator, another value, and then an operator, and so on - rather than 
receiving it all at once. Is this 'all at once' a feature of an infix 
calculator or am I missing/forgetting something?

For which I am obviously not the only one with a confused expression on 
my face...


> Since '*' and '**' are both allowed operators (see 'p in' above), spaces 
> are needed between operators to disambiguate.  In any case, what output 
> does your code produce now, and what do you want it to produce.

I hate RegExps because it is sooooo easy to write a wrong 'un. However, 
because the "\*\*" selection precedes the "[-+*/]", it appears (rheumy 
eyes allowing) to enact arithmetic priority correctly. However, as 
mentioned before, most of us will have created a serial and algorithmic 
approach to this problem, back-when; so it does seem a little strange.


OK, let's address your actual question: the parentheses!

How do you plan to make them work? As elsewhere: please provide example 
input and expected output, so that we can be sure of the code's 
objective. Multiple examples can only help, especially if there are 
exceptions and "edge-cases". Also, if your progress in Python is ready 
for it, we can talk PyTest (or equivalent), plus such a plan enables you 
to practice Test-Driven Development...


These problems stretch one's mental acuities. It would seem that I am 
not the only one looking forward to hearing back from you...
-- 
Regards =dn