Develop Reference

How to use some operator that has been scraped from a string

I'm trying to add, subtract or multiply two integers based on the sign between the integers, as in 5 + 6. When I use + explicitly within the script, it works. However, I wish to parse that operator from the given string in order to use like int(numbers[0]) + operator + int(numbers[1]) or something similar that works.
I've tried with:
import re
str_number = "5 + 6"
numbers = re.findall('[0-9]+', str_number)
operator = re.findall('([^\s\d]+?)',str_number)[0]
result = int(numbers[0]) + operator + int(numbers[1])
print(result)
When I run the above, I get the error below:
result = int(numbers[0]) + operator + int(numbers[1])
TypeError: unsupported operand type(s) for +: 'int' and 'str'

You can use the inbuilt operator module and define a mappings dictionary that map the given operator to its corresponding operation.
Use:
import operator
mappings = {'+': operator.add, '-': operator.sub, '*': operator.mul, '/': operator.floordiv}
equation = "5 + 6"
n1, op, n2 = equation.split()
result = mappings[op](int(n1), int(n2))
print(result)
This prints:
11

If I am correct it is not possible because the operator variable is a str but the number number variable is an int and you can’t do anything with these without type conversion
But you could accomplish your idea by using an if else statement.

try it that works, transform you equation in a string and use the eval() function
result = numbers[0] + operator + numbers[1]
print(eval(result))

Python Performing Calculations with Strings

How do I perform calculations with strings? Like suppose I have:
a=('/')
print(10, a, 5)
How do I get the answer as '2'? Because 10/5 is 2.

There are multiple ways to do this.
The most common way, I guess, is using the operator module and a map (dictionary) to turn your strings into operators, as the next example.
import operator
run_ops = {
'+' : operator.add,
'-' : operator.sub,
'*' : operator.mul,
'/' : operator.truediv,
'%' : operator.mod,
'^' : operator.xor,
}.get
print(run_ops('/')(10, 5))
Or go with lambdas:
run_ops= {'+': lambda x, y: x + y,
'-': lambda x, y: x - y,
# ...
'/': lambda x, y: x / y
}
run_ops['/'] (10,5)
Cheers

A nice method has been posted by epap.
However if you want to stick to the order of the expression where the "/", comes between the two numbers then one of the ways you can do it, is as below.
You can wrap (Number_1, "/", Number_2) and send it to a class and then print it. Something like below.
class Divide(object):
def __new__(cls, a, symbol, b):
result = None
if symbol == "/":
result = a/b
return result
print(Divide(5,"/",2))
This yields the below output
2.5

You can use "eval" to evaluate expressions given as strings.
a = "/"
res = eval("10" + a + "5")
print(res)

How to identify a string as a binaryoperator

Perhaps this is an easy question, but I'm new to Python and haven't seen anyone else post anything similar to this.
We have a list:
lst1 = ["5", "+", "1"]
or
lst2 = ["10", "/", "2"]
How can one interpret this to: 5 + 1 or 10/2 ? I understand that you can write: int(lst2[0]) and then you would get 10, but how do you do the same with "+" or "/"?

Assuming you "know" that your list has three strings, with the first and last representing integers and the middle one representing one of a fixed set of operators, you just need a mapping from the operator to a function that implements it. For example:
import operator
ops = {"+": operator.add, "-": operator.sub, "*": operator.mul, "/": operator.div}
lst1 = ["5", "+", "1"]
f = ops[lst1[1]]
operand1 = int(lst1[0])
operand2 = int(lst1[2])
result = f(operand1, operand2)
However, things get complicated once you start allowing more general inputs, as you need some logic for deciding what parts of the list represent what. For that, you need:
A grammar, which identifies what role each element of your list plays
A parser, which uses the grammar to convert each string to a usuable value
An interpreter, which evaluates the thing produced by the parser to produce
a final value.
There may also be a tokenizer, which is what would convert a single string input like "5 + 1" into a list of tokens that the parser uses for its input.
Any further general discussion about how to define any of the 4 concepts defined above is far beyond the scope of Stack Overflow.

If I understand you right, here is a snippet you may start with:
import operator
operators = {'+': operator.add, '-': operator.sub, '/': operator.truediv, '*': operator.mul}
first, op, second = lst
first, op, second = int(first), operators[op], int(second)
op(first, second)
operator module contains operators as a functions, so you can map symbols to the operators. Note that I’ve used truediv for /, you may want to use variant which return int always

You can't transform the string "+" to the operator + because operators are not things that exist at runtime. Operators only exist in code. You can map the string "+" to a function that takes two arguments and returns their sum, though. Functional equivalents for most operators exist in the operator module:
lst1 = ["5", "+", "1"]
import operator
op_table = {
'+': operator.add,
'-': operator.sub,
'*': operator.mul,
'/': operator.truediv
}
lhs = int(lst1[0])
op = op_table[lst1[1]]
rhs = int(lst1[2])
print('result:', op(lhs, rhs)) # output: 6

You can use standard function eval()
https://www.journaldev.com/22504/python-eval-function
lst1 = ["5", "+", "1"]
result1 = eval("".join(lst1)) # 6
lst2 = ["10", "/", "2"]
result2 = eval("".join(lst2)) # 5.0

TypeError: 'bool' object is not callable - python

I'm trying to implement a simple maths game where the user is given random numbers and operators then they have to work out the answer, I found sources on the internet that suggested using the operator module, which is why i used it, if there is a more efficient/ easier way of achieving this I am very open for interpretation.
Essentially i am trying to remove this horrible <built-in function add> and swap it to be more user friendly by saying '+' or 'add', something along those lines but i keep getting the error 'TypeError: 'bool' object is not callable - python', i really dont know what this means, i am new to python and am very confused.
from operator import add, sub, mul, floordiv
operators = (add == str("add"), sub == str("subtract"), mul == str("multiply"), floordiv == str("divide"))
operator = random.choice(operators)
answer = operator(number1, number2)
question = print("What is ", number1, operator, number2, "?")

You want two seperate things: the string representation of the operator and the operator itself. You can use dictionary to store the function and the corresponding string representation:
from operator import add, sub, mul, floordiv
import random
number1 = 9
number2 = 3
operators = {'+':add, '-':sub, '*':mul, '/':floordiv}
operator_name = random.choice(operators.keys())
op = operators[operator_name]
answer = op(number1, number2)
question = "What is {}{}{}?".format(number1, operator_name, number2)
print question
print answer

What you get as a result of the first line is
operators = (False, False, False, False, False)
or something in those lines.
Then you are trying to call a boolean that gets you the exception you have.
add == str("add") will evaluate to False, since you're trying to compare a function to a string.
I'm assuming you are trying to implement a simple math game, hence instead of using operator, which are in fact math operation functions, you can just use a simple dictonary:
operators = { 'add': add, 'substract': sub, 'multiply': mul }
answer = operators[random.choice(operators.keys())](number1, number2)

Here's an implementation using tuples instead of dicts, I added in more ways to represent each operation.
import random
from operator import add, sub, mul, floordiv
number1 = 5
number2 = 10
operators = (
(add, ("add", "plus", "+")),
(sub, ("subtract", "minus", "-")),
(mul, ("multiply", "times", "*")),
(floordiv, ("divide", "over", "/")),
)
operator, (name, operation, symbol) = random.choice(operators)
print("What is ", number1, operation, number2, "?")
print(operator(number1, number2))
Output
13:50 ~ $ python3 StackOverflow/27128400.py
What is 5 times 10 ?
50
13:55 ~ $ python3 StackOverflow/27128400.py
What is 5 plus 10 ?
15
13:55 ~ $ python3 StackOverflow/27128400.py
What is 5 minus 10 ?
-5
13:55 ~ $ python3 StackOverflow/27128400.py
What is 5 over 10 ?
0

What you're trying to do here is certainly possible. The formation of the tuple above does not return anything useful. A dictionary is more appropriate.
You can construct the dictionary as:
operators = {'add':add, 'minus':sub, 'multiplied by':mul, 'divided by':floordiv}
This gives you a dictionary of related strings and functions. You can choose a random operator name (this will give the string value, not the operator itself.
operator = random.choice(list(operators.keys()))
Then compute the answer:
answer = operators.get(operator)(number1, number2)
This gets the operator function from the operators dictionary by 'searching' for its string partner.
Altogether now (with some numbers so it runs off the bat):
from operator import add, sub, mul, floordiv
import random
operators = {'add':add, 'minus':sub, 'multiplied by':mul, 'divided by':floordiv}
operator = random.choice(list(operators.keys()))
number1 = 2
number2 = 3
answer = operators.get(operator)(number1, number2)
print "What is " + str(number1) + " " + operator + " " + str(number2) + "?"
print answer
(I'm using 2.7 for this)

Evaluation of Postfix notation

How to calculate the value of post fix notation:
i tried with:
import operator
operator = {
'+': operator.add, '-': operator.sub,
'*': operator.mul, '/': operator.div, '%': operator.mod,
'**': operator.pow, '//': operator.floordiv,
}
l = []
exp = "11+2*"
for i in exp:
if i in operator:
a = operator[i]
x = l[0]
y = l[1]
l[-2:] = [a(*l[-2:])]
print l
else:
l.append(i)
print l
How to do this?

Make sure to have Python parse your characters into integers before adding them to the stack:
l.append(int(i))

Some hints:
You need a better parser, iterating just character by character will not be enough (unless you only want to have single digit numbers). To make it a little simpler, you could require that numbers and operators are delimited by whitespace each and use str.split:
elems = "11 2 3 + *".split()
You need to convert the numbers to integers before pushing them on the stack
The stack operation also looks a little weird. Having it as
l.append(a(l.pop(), l.pop())
makes it more explicit.