Math Operations from String

Math operations from string

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.

How to do math operations with string?

You could write something like:

def parse_exp(s):
    return eval(s.replace('x','*'))

and expand for whatever other exotic symbols you want to use.

To limit the risks of eval you can also eliminate bad characters:

import string

good = string.digits + '()/*+-x'

def parse_exp(s):
    s2 = ''.join([i for i in s if i in good])
    return eval(s2.replace('x','*'))

Edit: additional bonus is that the in-built eval function will take care of things like parenthesis and general calculation rules :)

Edit 2: As another user pointed out, evalcan be dangerous. As such, only use it if your code will ever only run locally

Math operations from string using Javascript

It's a little hacky, but you can try something like this:

var eqs = [
'1+2*3*4+1+1+3',
'1+2*3*400+32/2+3',
'-5+2',
'3*-2',
];

for(var eq in eqs) { console.log(mdas(eqs[eq])); }

function mdas(equation) {

console.log(equation);

var failsafe = 100;
var num = '(((?<=[*+-])-|^-)?[0-9.]+)';
var reg = new RegExp(num + '([*/])' + num);

while(m = reg.exec(equation)) {
    var n = (m[3] == "*") ? m[1]*m[4] : m[1]/m[4];
    equation = equation.replace(m[0], n);
    if(failsafe--<0) { return 'failsafe'; } 
}

var reg = new RegExp(num + '([+-])' + num);
while(m = reg.exec(equation)) {
    var n = (m[3] == "+") ? 1*m[1] + 1*m[4] : m[1]-m[4];
    equation = equation.replace(m[0], n);
    if(failsafe--<0) { return 'failsafe'; }
}

return equation;

}

Convert a string with arithmetic operation to an interger in python

You're looking for eval:

>>> a = eval('4 * 5')
>>> a
20
>>> type(a)
<type 'int'>

Caveat: BE CAREFUL WITH THIS. eval() will evaluate mathematical expressions like this one, but will also execute any valid python code it's given. For example,

>>> eval( 'type(a)' )
<type 'int'>

Performing math operation when operator is stored in a string

I don't recommend this but is funny. in java6

String op = '+';
int first= 10;
int second = 20;
ScriptEngineManager scm = new ScriptEngineManager();
ScriptEngine jsEngine = scm.getEngineByName("JavaScript");
Integer result = (Integer) jsEngine.eval(first+op+second);

go with the switch, but remember to convert the string operator to char as switch don't works with strings yet.

switch(op.charAt(0)){
    case '+':
        return first + second;
        break;
   // and so on..

How to perform math operation on numbers in string of python

This might help. Use the isalpha method to remove all alpha chars and the use eval

Ex:

s = "1abc0+5*1hv0"
s = "".join([i for i in s if not i.isalpha()])
print eval(s)

Output:

60

How to use math operators with strings?

int RTPower = Int32.Parse(powerTextBox.Text);

or for decimal values

decimal RTPower = Decimal.Parse(powerTextBox.Text);

You need to convert the value from a string to an int.

Also, I assume you are new to c# - my advice would be to avoid using var and explicitly declare your variables. It will make things clear and easier for you to learn and understand.

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