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So I am making a quick calculator script in Python, and I need to take apart a short string. The program first displays a short welcome message, then a prompt asks What they want to do calculate, and shows them the correct format. The functionality is there to do the calculation but unfortunately, I can not get the string dissecting bit working.

Here's my code

print ("--------------------------------------")
print (" ")

print ("Advanced Quick Calculator")
print ("By Max M, licenced under GPLv3")

print (" ")
print ("--------------------------------------")

statement = raw_input ("Please enter your mathematical statement [3 3 plus minus times            divide]: ")

strnum1 = statement[:1]
print ("strnum1 : " + strnum1)
#num1 = int (strnum1)

strnum2 = statement[:4]
print ("strnum2 : " + strnum2)
#num2 = int (strnum2)

operation = statement[5:11]
print ("operation : " + operation)
#if operation == "+":
#     ans = num1 + num2

#if operation == "-":
#   ans = num1 - num2

#if operation == "*":
#   ans = num1 * num2

#if operation == "/":
#   ans = num1 / num2

#print ("The answer is : "), ans
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3
  • 1
    Perhaps you should consider actually parsing the string instead of using fixed offsets. Commented Jul 14, 2013 at 3:23
  • Please clarify what the correct format is supposed to be. Is it 3 + 3, 3 - 3, 3 / 3, etc. ? Commented Jul 14, 2013 at 3:23
  • I would start by splitting the string by spaces Commented Jul 14, 2013 at 3:29

2 Answers 2

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This looks like a job for regular expressions:

>>> import re
>>> match = re.search(r'(\d+)\s*([+*/-])\s*(\d+)', '42 + 7') 
>>> match.group(1)  # <-- num1
'42'
>>> match.group(2)  # <-- operation
'+'
>>> match.group(3)  # <-- num2
'7'

Slicing the input like you're currently doing is probably not a good idea as it greatly restricts the allowed formats. For instance, what if the user accidentally precedes his input with a couple of spaces? Regular expressions can handle such cases well.

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2 Comments

You got the division operator backward, and you don't need to escape metacharacters in a character class.
Thanks I guess I will just, use a different use a different input methods
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I'm not going to do your homework for you, but I will point you to the answer to your question (hint: it looks like you're trying to split on spaces instead of comma's like in the link, so adjust the code accordingly).

How to read formatted input in python?

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