CODEWITHSUNDEEP
python
JonCode
JonCode

Reputation: 241

Python lambda expression, use String in list as operant

Lets say I have a simple list as lis = [5, *, 2]

Is it possible to do something along the line of:

print(reduce(lambda x,y,z: float(x) y float(z), lis))
10

I'd like to use the string given in the list to determine the operand.

Upvotes: 1

Views: 140

Answers (3)

jwilner
jwilner

Reputation: 6606

Operators can't be passed around in Python -- reduce(+, [1,2,3]) would give you a SyntaxError. Similarly, your operator in the middle of the list would as well.

If you're looking to use strings to look up operators, you can construct a dictionary using the operator module.

SYMBOL_TO_OPERATOR = {"*": operator.mul,
                      "+": operator.add, ...}

Those are just normal methods though -- you can't use them with 'infix' notation as you can the symbols.

Additionally, this doesn't make much sense as a reduce operation (nor would that be very idiomatic python). You could easily do something like:

statements = [[1, "*", 5], [2, "+", 9]]
results = [SYMBOL_TO_OPERATOR[symbol](a, b) for a, symbol, b in statements]

Upvotes: 2

WilHall
WilHall

Reputation: 11994

Let me know if I am misunderstanding, but I believe something like below would accomplish what you are asking:

import operator

operators = {
    '*': operator.mul,
    '/': operator.div
}

dynamic_operator = lambda x,y,z: operators[y](x, z)

some_list = [5, '*', 5]
print dynamic_operator(*some_list)

You would need to expand on the operators dictionary to encompass all the operations you want to handle dynamically. For more operator functions, see: https://docs.python.org/2/library/operator.html

Upvotes: 2

DJG
DJG

Reputation: 6563

First off, you can't place the * operator in a list, as it is not a value in Python. Instead you can use the mul function from the operator package:

from operator import mul

Then, assuming that you have the following list:

lst = [5, mul, 2]

you can do the following:

a, op, b = lst
print(op(a,b))

which should print:

10

.

Upvotes: 3

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