Question

Given the exampleString = "[9+[7*3+[1+2]]-5]"
How does one extract and store elements enclosed by [] brackets, and then evaluate them in order?

1+2 --+
      |
  7*3+3 --+
          |
        9+24-5

Does one have to create somekind of nested list? Sorry for this somewhat broad question and bad English.

I see, this question is really too broad… Is there a way to create a nested list from that string? Or maybe i should simply do regex search for every element and evaluate each? The nested list option (if it exists) would be a IMO “cleaner” approach than looping over same string and evaluating until theres no [] brackets.

Answer 1

Here is a regex solution:

import re

def evaluatesimple(s):
  return eval(s)

def evaluate(s):
  while 1:
    simplesums=re.findall("\[([^\]\[]*)\]",s)
    if (len(simplesums) == 0):
      break
    replacements=[('[%s]' % item,str(evaluatesimple(item))) for item in simplesums]
    for r in replacements:
      s = s.replace(*r)
  return s

print evaluate("[9+[7*3+[1+2]]-5]")

But if you want to go the whole hog and build a tree to evaluate later, you can use the same technique but store the expressions and sub expressions in a dict:

def tokengen():
  for c in 'abcdefghijklmnopqrstuvwyxz':
    yield c

def makeexpressiontree(s):
  d=dict()
  tokens = tokengen()
  while 1:
    simplesums=re.findall("\[([^\]\[]*)\]",s)
    if (len(simplesums) == 0):
      break
    for item in simplesums:
      t = tokens.next()
      d[t] = item
      s = s.replace("[%s]"% item,t)
  return d

def evaltree(d):
  """A simple dumb way to show in principle how to evaluate the tree"""
  result=0
  ev={}
  for i,t in zip(range(len(d)),tokengen()):
    ev[t] = eval(d[t],ev)
    result = ev[t]
  return result

s="[9+[7*3+[1+2]]-5]"
print evaluate(s)
tree=makeexpressiontree(s)
print tree
print evaltree(tree)

(Edited to extend my answer)