# [Tutor] Need help w/ a for loop

W W srilyk at gmail.com
Thu Oct 23 12:56:43 CEST 2008

```On Wed, Oct 22, 2008 at 11:09 PM, Monte Milanuk <monte at milanuk.net> wrote:

> Hello all,
>
> New guy here, so go easy on me ;)
>

Welcome to python and the tutor list!

> I'm starting to work my way through Python Programming by Zelle, and have
> hit a bit of a wall on one of the programming exercises in Chapter 3 (#15 if
> anyone has the book handy).
>
> What the question ask is:  Write a program that approimates the value of pi
> by summing the terms of this series: 4/1 - 4/3 + 4/5 - 4/7 + 4/9 - 4/11+...
> The program should ask the user for 'n', the number of terms to sum, and
> then output the sum of the first 'n' terms of this series.
>
> Where I am running into problems is how to do the '-' & '+', depending on
> the value of 'n'.  i.e. if 'n' = 3, it's going to be a - & an +, if 'n' =5
> its going to be -, +, -, +, etc.  How to make that work in terms of an
> algorithm is making my head hurt (and its so early in the book yet... ;) )
>

Here's a suggestion: look at only the value of the denominator (as the top
is obviously constant at 4, for the sake of pattern searching you only need
to worry about the variable) -

1 - 3 + 5 - 7 + 9 - 11 + ...

Do you notice a pattern? (if it doesn't pop out, read on...)

1+2 = ?
1+4 = ?
1+6 = ?
1+8 = ?
1+10 = ?

If you want a fairly easy way to figure out whether it should be + or -,
take another look at the pattern.

If you need a hint, read past my name.

HTH,
Wayne

Hint: Think of the pattern in terms of relation to 2

WAIT!
Don't read beyond here if you want to discover the solution yourself!

2/2 = 1
4/2 = 2
6/2 = 3
8/2 = 4
etc.

> Here's what I have thus far:
>
> #   approximate_pi.py
> #   Approximates the value of 'pi' by summing the terms of a series.
> #
>
> import math
>
> def main():
>     print "This program will approximate the value of pi"
>     print "to a degree determined by the user. "
>     print
>
>     #  get the value of n from the user
>     n = input("How many terms do you want me to sum?  ")
>     print
>
>     #  create a loop from 1 to n+1, odd)
>     for i in range(1,n + 1,2):
>         #  each term is '4/i' as it steps thru the loop starting with 1
>         x = 4 / i
>         #  not sure where to go from here
>
>
>     print
>     #  output the sum - convert it to a float just in case
>     print "The sum of the numbers you entered is", (float(sum))
>
>     #  calculate the difference between our approximation and Python's pi
>     diff = sum - math.pi
>
>     #  output the difference
>     print
>     print "The difference between your 'pi' & Python's pi is", diff, "."
>
>
>
> main()
>
>
> Any assistance or nudges in the right direction would be most appreciated.
>
> Thanks,
>
> Monte
>
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor
>
>

--
To be considered stupid and to be told so is more painful than being called
gluttonous, mendacious, violent, lascivious, lazy, cowardly: every weakness,
every vice, has found its defenders, its rhetoric, its ennoblement and
exaltation, but stupidity hasn't. - Primo Levi
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