CODEWITHSUNDEEP
pythonpython-3.x
yol torres newbie
yol torres newbie

Reputation: 45

How to split string that is a mathematical expression into separate parts (including operators)

I want to separate the digits from characters and letters and add them to a list.

n = "1+22-3*4/5"
eq=list(n)
c=0
for i in eq:
  if "*" in eq:
    while "*" in eq:
      c=eq.index("*")
      eq[c-1]=float(eq[c-1])*float(eq[c+1])
      del eq[c]
      del eq[c]
      print(eq)
  if "/" in eq:
    while "/" in eq:
      c=eq.index("/")
      eq[c-1]=float(eq[c-1])/float(eq[c+1])
      del eq[c]
      del eq[c]
      print(eq)
  if "+" in eq:
    while "+" in eq:
      c=eq.index("+")
      eq[c-1]=float(eq[c-1])+float(eq[c+1])
      del eq[c]
      del eq[c]
      print(eq)
  if "-" in eq:
    while "-" in eq:
      c=eq.index("-")
      eq[c-1]=float(eq[c-1])-float(eq[c+1])
      del eq[c]
      del eq[c]
  print(eq)
print(n,"=",eq)

It can only append every digit in the list. The current output is ['1','+','2','2','-','3','*','4','/','5']

Upvotes: 1

Views: 270

Answers (4)

aghast
aghast

Reputation: 15310

I suggest you process the characters of the string in order (for ch in str) and either (a) add them to your list; or (b) accumulate them into a number:

str = "1+22-3*4/5"
tokens = []
number = None
operators = "+-*/"
digits = "0123456789"

for ch in str:
    if ch in operators:
        if number is not None:
            tokens.append(number)
        tokens.append(ch)
        continue
    elif ch in digits:
        if number is None:
            number = ch
        else:
            number += ch
        continue
    else:
        # what else could it be?
        pass

# After loop, check for number at end 
if number is not None:
   tokens.append(number)

Upvotes: 0

Charles Landau
Charles Landau

Reputation: 4265

Some great solutions here using stdlib, here's a pure python try:

i = "11+11*11"

def parser(i):
  out = []
  gram = []
  for x in i:
    if x.isdigit():
      gram.append(x)
    else:
      out.append("".join(gram))
      out.append(x)
      gram = []
  if gram:
    out.append("".join(gram))
  return out

parser(i) # ['11', '+', '11', '*', '11']

Upvotes: 1

blhsing
blhsing

Reputation: 106533

You can use itertools.groupby with str.isdigit as the key function:

from itertools import groupby
[''.join(g) for _, g in groupby(n, key=str.isdigit)]

This returns:

['1', '+', '22', '-', '3', '*', '4', '/', '5']

Upvotes: 4

Jack Moody
Jack Moody

Reputation: 1771

You could use a regular expression:

import re
s = "1+22-3*4/5"
re.split('(\W)', s)

Output

['1', '+', '22', '-', '3', '*', '4', '/', '5']

Upvotes: 2

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