How to make a string to calculation-command?

Question

Is there a way to convert "1 plus 3 minus 4" to "1 + 3 - 4" through replacing, to calculate it as such?

• Obversity: You'd probably have to hard code it to check for words like "plus" and "minus" and replace them with the appropriate symbol, then use eval() -- I think Python has this.
• Caramiriel: If you're using eval(), please validate (as usual, but this case in particular) the input. I would actually prefer some kind if library that parses the string as an expression, so that injection isn't possible.
• l4mpi: Don't use eval() for this unless you don't care about somebody deleting your hard drive with your cute calculator program. It's also dead simple to write a little stack based state machine to calculate mathematical expressions like this, which is considerably safer and could probably even be faster. If you still prefer using eval, with limited input like this you could actually be safe by using a character-based whitelist; e.g. only allowing 0123456789+- in the strings you try to evaluate.

Answer 1

Yes, you can use str.replace to replace the words with the symbols, then eval to evaluate the calculation:

def evaluate(s):
 replacements = {'plus': '+', 'minus': '-'} # define symbols to replace
 for word in replacements:
 s = s.replace(word, replacements[word]) # replace them
 return eval(s) # evaluate calculation
>>> s = "1 plus 3 minus 4"
>>> evaluate(s)
0

• Ashwini Chaudhary: ast.literal_eval can only be used with list, tuples, string etc, not with expressions.

Answer 2

One way is to use eval, but it should be used with proper validations.

abc = "1 plus 3 minus 4"
operatorMap = {"plus":"+","minus":"-"}
evalString = ""
for value in abc.split():
 try:
 val = str(int(value))
 except:
 try:
 val = operatorMap[value]
 except:
 print "Error!!"
 break
 evalString += val
print eval(evalString)

Answer 3

If the input is coming from a known source, then use eval on the string after replacing plus, minus with appropriate operators.

>>> s = "1 plus 3 minus 4"
>>> d = {'plus':'+', 'minus':'-'}
>>> eval(' '.join(d.get(x, x) for x in s.split()))
0