Convert a string with arithmetic operation to an interger in python

Question

I have a question about how to convert strings with arithmetic operations like + or * from a string to an integer.

Here is my input and the error output:

    a = int('4*5')
    Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    ValueError: invalid literal for int() with base 10: '4*5'

I've expected this operation saves the number 20 as an interger in variable a.

Thanks for helping

score 3 · Answer 1 · answered Jul 31 '18 at 18:09

It is evaluating the string as a whole as an integer without applying the mathematical operations present within the string, so obviously this fails, as * is not a valid digit.

There are certain ways to take code like this and evaluate it that can also be dangerous, such as eval. However, there is a library called pandas which has a much safer eval function:

From the pandas documentation:

The following arithmetic operations are supported: +, -, *, /, **, %, // (python engine only) along with the following boolean operations: | (or), & (and), and ~ (not)

>>> import pandas as pd
>>> pd.eval('4*5')
20

This is a much safer alternative to using plain eval, which can run potentially malicious code on your machine.

score 1 · Accepted Answer · answered Jul 31 '18 at 18:05

1

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'>

answered Jul 31 '18 at 18:05

Green Cloak Guy

23,793
4
33
53

It Works :-) Thanks for helping and your advice – lghh Jul 31 '18 at 18:10

GetHacked · Answer 3 · 2018-07-31T18:10:00.587

1

If you must take the expression as a string, use Python's eval function.

a = eval('4*5')
a
> 20

Otherwise, just remove the quotes to make it an expression instead of a string:

a = 4*5
a
> 20

edited Jul 31 '18 at 18:10

answered Jul 31 '18 at 18:06

GetHacked

546
4
21

Then `int` is redundant ... Why is this upvoted ? – scharette Jul 31 '18 at 18:08
Question literally says "convert strings with arithmetic operations" – user3483203 Jul 31 '18 at 18:08
@scharette You're right. I usually like to edit as little as possible from the original question, but in this case it's unhelpful. Thanks! – GetHacked Jul 31 '18 at 18:08
@user3483203 I know it does, but I could also suggest`int(int(4*5))` following this logic... – scharette Jul 31 '18 at 18:10
@scharette, you can just do `int(eval('4 * 5'))` if you really want to make sure it returns an integer. – Jack Moody Jul 31 '18 at 18:11
@JackMoody My comment is not valid anymore since he edited. My point was that saying to remove quotes and let the `int` cast was redundant. – scharette Jul 31 '18 at 18:12
In this special case I had to use the expression – lghh Jul 31 '18 at 18:13
@scharette it's still an excellent point; I find myself doing that a bit too much. – GetHacked Jul 31 '18 at 18:15
@scharette sorry if it was unclear, I wasn't replying to you, I was commenting on the answer – user3483203 Jul 31 '18 at 18:56
@user3483203 Oh, I see. That was my bad. – scharette Jul 31 '18 at 20:31

score 0 · Answer 4 · answered Jul 31 '18 at 18:08

The dangerous way is to use the eval function. eval is quite powerful, and often results in unexpected or unwanted behaviour from small errors in the input.

To do this properly, you need to process the string. Extract each of the numerical strings and the operation. Convert the strings to ints, then test the operation and apply the needed code. For instance:

expr = '4*5'
num1 = int(expr[0])
num2 = int(expr[2])
op = expr[1]

if op == '*':
    result = num1 * num2

In general, Python language syntax (keywords, math symbols, punctuation, etc.) are not easily transferred between the program itself and the data you're using.

Convert a string with arithmetic operation to an interger in python

4 Answers4