validate string is valid maths

[Matthew_WARREN]

Hi pythoners.

I am generating strings of length n, randomly from the symbols

+-/*0123456789

What would be the 'sensible' way of transforming the string, for example
changing '3++++++8' into 3+8
or '3++--*-9' into '3+-9' such that eval(string) will always return a
number?

in cases where multiple symbols conflict in meaning (as '3++--*-9' the
earliest valid symbols in the sequence should be preserved

so for example,

'3++*/-9' = 3+-9
'45--/**/+7' = 45-+7
'55/-**+-6**' = 55/-6

....that is, unless there is a canonical method for doing this that does
something else instead..


this sounds like homework. It's not. I like making problems up and it's a
slow work day. So, as an aside, there is no real reason I want to do this,
nor other problem to solve, nor other background to what I'm trying to
achieve other than twiddling with python.

Matt.



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[Steven D'Aprano]

Hi pythoners.

I am generating strings of length n, randomly from the symbols

+-/*0123456789

What would be the 'sensible' way of transforming the string, for example
changing '3++++++8' into 3+8

That's easy: replace pairs of + into a single +, repeatedly until there's
nothing left to replace. And so forth. Here's an untested function. It's
probably not even close to optimal, but for playing around it is probably
fine.

def rationalise_signs(s):
while "++" in s or "+-" in s or "-+" in s or "--" in s:
s = s.replace("++", "+")
s = s.replace("--", "+")
s = s.replace("+-", "-")
s = s.replace("-+", "-")
return s

or '3++--*-9' into '3+-9' such that eval(string) will always return a
number?

in cases where multiple symbols conflict in meaning (as '3++--*-9' the
earliest valid symbols in the sequence should be preserved

You have four symbols, so there are just 4*4=16 sets of two symbols.
Probably the easiest way is to just list them and their replacements out
in a table:

table = {"++": "+", "+-": "-", "+*": "+", "+/": "+",
"-+": "-", "--": "+", "-*": "-", "-/": "-",
"*+": "*", "**": "*", "*/": "*",
"/+": "/", "/*": "/", "//": "/", }

Notice that both *- and /- don't get changed.



# Untested.
def rationalise_signs(s):
prev = ''
while s != prev:
prev = s
for key, value in table.items():
s = s.replace(key, value)
return s

 

[Christian Heimes]

def rationalise_signs(s):
while "++" in s or "+-" in s or "-+" in s or "--" in s:
s = s.replace("++", "+")
s = s.replace("--", "+")
s = s.replace("+-", "-")
s = s.replace("-+", "-")
return s

I assume it's faster to check the length of the string s after each
iteration and compare it to the last iteration. When the string hasn't
changed break out of the loop.