is there an easy way to parse a nested list ?

[Stef Mientki]

hello,

I need to parse a nested list, given as a string, like this

line = " A [ B [ C+2 ] + 3 ] "

( I probably can do it with find, but I guess that's not the most
elegant way, besides recusrion is not my strongest point)

which should be parsed so I get an list from inner to outer side (don't
know if these are the correct words),
but I should like to have a list or tuple for tis case:

parsed = [ C+2, B[C+2]+3, A [ B [ C+2 ] + 3 ] ]
or (last term is not needed, as well as the constants aren't)
parsed = [ C, B[C+2] ]

this all, to improve the improved error / exception message even more ;-)

thanks,
Stef

 

[Aaron Brady]

hello,

I need to parse a nested list, given as a string, like this

line = " A  [  B  [ C+2 ] + 3 ] "

( I probably can do it with find, but I guess that's not the most
elegant way, besides recusrion is not my strongest point)

which should be parsed so I get an list from inner to outer side (don't
know if these are the correct words),
but I should like to have a list or tuple for tis case:

parsed = [ C+2,  B[C+2]+3,   A  [  B  [ C+2 ] + 3 ] ]
   or (last term is not needed, as well as the constants aren't)
parsed = [ C,  B[C+2] ]

this all, to improve the improved error / exception message even more ;-)

thanks,
Stef

Hi Stef,

This looks like what you want:

def parse_list( s, start= 0 ):
x= []
i= start
while i< len( s ):
c= s[ i ]
if c== '[':
y, i= parse_list( s, i+ 1 )
x= x+ y
if c== ']':
return x+ [ s[start: i ] ], i
i+= 1
return x+ [ s ]

line = " A [ B [ C+2 ] + 3 ] "
print( parse_list( line ) )
#[' C+2 ', ' B [ C+2 ] + 3 ', ' A [ B [ C+2 ] + 3 ] ']

line = " A [ B [ C+2 ] [ D+2 ] + 3 ] "
print( parse_list( line ) )
#[' C+2 ', ' D+2 ', ' B [ C+2 ] [ D+2 ] + 3 ', ' A [ B [ C+2 ] [ D
+2 ] + 3 ] ']

....So long as you don't have any brackets inside strings or what not.
It just admits two special characters, '[' and ']'.

 

[Stef Mientki]

thanks Aaron, Paul and Vlastimil,
sorry I phrased my question wrong,
I was looking for parsing an expression of an element of a nested list
the code below seems to do what I'm looking for.

cheers,
Stef

hello,

I need to parse a nested list, given as a string, like this

line = " A [ B [ C+2 ] + 3 ] "

( I probably can do it with find, but I guess that's not the most
elegant way, besides recusrion is not my strongest point)

which should be parsed so I get an list from inner to outer side (don't
know if these are the correct words),
but I should like to have a list or tuple for tis case:

parsed = [ C+2, B[C+2]+3, A [ B [ C+2 ] + 3 ] ]
or (last term is not needed, as well as the constants aren't)
parsed = [ C, B[C+2] ]

this all, to improve the improved error / exception message even more ;-)

thanks,
Stef

Hi Stef,

This looks like what you want:

def parse_list( s, start= 0 ):
x= []
i= start
while i< len( s ):
c= s[ i ]
if c== '[':
y, i= parse_list( s, i+ 1 )
x= x+ y
if c== ']':
return x+ [ s[start: i ] ], i
i+= 1
return x+ [ s ]

line = " A [ B [ C+2 ] + 3 ] "
print( parse_list( line ) )
#[' C+2 ', ' B [ C+2 ] + 3 ', ' A [ B [ C+2 ] + 3 ] ']

line = " A [ B [ C+2 ] [ D+2 ] + 3 ] "
print( parse_list( line ) )
#[' C+2 ', ' D+2 ', ' B [ C+2 ] [ D+2 ] + 3 ', ' A [ B [ C+2 ] [ D
+2 ] + 3 ] ']

...So long as you don't have any brackets inside strings or what not.
It just admits two special characters, '[' and ']'.