Python Forum

I am currently struggling on a way to compute a sum that changes its sign from addition to subtraction e.g. 1/1 - 1/3 + 1/5 - 1/7 + 1/9 - 1/11 + 1/13. I am adivised to use a forloop statement to compute this sum. Could somebody please help me out and explain the lines of code. Thanks!

You present this in a way that only needs a print statement in front of it. One line of code.
If a loop is required the data will be presented differently. Show us that and what you have tried,
than we can help.
Paul

sign = 1
total = 0
for x in range(1, 1002, 2):
    total += sign/x
    sign = -sign
print(total)

Not much to explain.

Example with operator, iterator.cycle and Fraction.
It's not the "better" way, just a different way to show up, what is Python able to do.
The better way is often lesser code and not more code.

import operator
from itertools import cycle

# Bonus: you can get the exact fraction instead of a float
from fractions import Fraction


def sum_fractions(iterations, fractions=False):
    # we start with total 1
    # if iterations == 0, then the int 1 is returned
    # if iterations > 0, then total will convert into a float
    # if fractions == True, then Fraction is used
    total = 1
    numerator = 1
    denominator = 3
    # isub and iadd replaces -= and +=
    # cycle cycle through this two operator functions
    op_cycle = cycle((operator.isub, operator.iadd))

    # we don't need the value from range
    # so assign it to the throw away name _
    for _ in range(iterations):
        # get the next operator function from op_cycle
        op_func = next(op_cycle)

        # Bonus, use Fraction to do the calculation
        # Keep in mind, that this is much slower, but
        # gives you exact results
        if fractions:
            current_value = Fraction(numerator, denominator)
        else:
            # without Fraction
            current_value = numerator / denominator

        # here the isub or iadd function is used
        # instead of -= or  += use here assignment
        total = op_func(total, current_value)

        # increment the denominator by 2
        denominator += 2

    return total

The benefit is, that you can do more than + and - operation.
You can extend the tuple of op_cycle to multiply, divide or do the modulo operation.