Math operations from string

Question

Let s say I have a standard Python string  such as one obtained from raw input     maybe  2   2  for simplicity s sake    I d like to convert this string to standard math operations in Python  such that  2   2  would return 4    Is there an easy way to do this  or would I have to split on the spaces and parse each number symbol manually  then do the math based on what I find    Do I want Regex

User · Answer

Regex won t help much  First of all  you will want to take into account the operators precedence  and second  you need to work with parentheses which is impossible with regex    Depending on what exactly kind of expression you need to parse  you may try either Python AST or  more likely  pyparsing  But  first of all  I d recommend to read something about syntax analysis in general and the Shunting yard algorithm in particular   And fight the temptation of using eval  that s not safe

User · Answer

The best way would be to do   print eval  2   2    If you wanted to you could use a variable   addition   eval  2   2    print addition  If you really wanted to  you could use a function   def add num1  num2        eval  num1   num2

User · Answer

Warning  this way is not a safe way  but is very easy to use  Use it wisely   Use the eval function   print eval  2   4     Output   6   You can even use variables or regular python code   a   5 print eval  a   4     Output   9   You also can get return values   d   eval  4   5   print d   Output   9   Or call functions   def add a  b       return a   b  def subtract a  b       return a - b  a   20 b   10     print eval  add a  b    print eval  subtract a  b      Output   30 10   In case you want to write a parser  maybe instead you can built a python code generator if that is easier and use eval to run the code  With eval you can execute any Python evalution   Why eval is unsafe   Since you can put literally anything in the eval  e g  if the input argument is   os system    rm -rf         It will remove all files on your system  at least on Linux Unix   So only use eval when you trust the input

User · Answer

You could use this function which is doing the same as the eval   function  but in a simple manner  using a function   def numeric equation       if     in equation          y   equation split              x   int y 0   int y 1       elif  -  in equation          y   equation split  -           x   int y 0  -int y 1       return x

User · Answer

The easiest way is to use eval as in     gt  gt  gt  eval  2        2    4   Pay attention to the fact I included spaces in the string  eval will execute a string as if it was a Python code  so if you want the input to be in a syntax other than Python  you should parse the string yourself and calculate  for example eval  2x7   would not give you 14 because Python uses   for multiplication operator rather than x

User · Answer

A simple way but dangerous way to do this would be to use eval    eval   executes the string passed to it as code  The dangerous thing about this is that if this string is gained from user input  they could maliciously execute code that could break the computer  I would get the input  check it with a regex  and then execute it if you determine if it s OK  If it s only going to be in the format  number operation number   then you could use a simple regex   import re s   raw input  What is your math problem     if re findall   d           d     s     print eval s  else    print  Try entering a math problem    Otherwise  you would have to come up with something a bit stricter than this  You could also do it conversely  using a regex to find if certain things are not in it  such as numbers and operations  Also you could check to see if the input contains certain commands

User · Answer

The asker commented      I figure that if I understand a problem well enough to write a program that can figure it out  I don t need to do the work manually    If he s writing a math expression solver as a learning exercise  using eval   isn t going to help   Plus it s terrible design   You might consider making a calculator using Reverse Polish Notation instead of standard math notation   It simplifies the parsing considerably   It would still be a good exercise

User · Answer

If you want to do it safely  you may want to use http   docs python org library ast html ast literal eval  from this answer  Python  quot safe quot  eval  string to bool int float None string   It might not do math  but you could parse the math operators and then operate on safely evaluated terms

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