forloop to compute sum by alternating from addition to subtraction

[JulianZ]

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!

[DPaul]

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

[deanhystad]

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

Not much to explain.

[DeaD_EyE]

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.