Re: [Tutor] Need help w/ a for loop

[bob gailer]

Monte Milanuk wrote: Hello all, New guy here, so go easy on me ;) We save the whips and chains for the more hardened questers. Newcomers get the feathers. 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... ;) ) There are many ways to handle this. Others have given some hints. The simplest IMHO is to set the range stride to 4 instead of 2 and then use x += 4.0/i - 4.0/(i + 2). You could also use a multiplier (let's call it m) that alternates between 1 and -1. Roughly: x = 0 m = 1 for in in range...   x += 4.0/i*m   m =  -m For more generality and anticipating more complex algorithms: import operator ops = (operator.add, operator.sub) x = 0 m = 0 for in in range...   x = ops[m](x, 4.0/i)   m =  1-m And just for the heck of it you could write 2 for loops, each with a stride of 4. The first would just add all the fractions to be added and the second would add all the fractions to be subtracted, then combine them. Throwing in the sum function and generator expressions: pi = sum(4.0/i for i in range(1, n*2, 4)) - sum(4.0/i for i in range(3, n*2, 4)) Applying sum and generator expressions to my original solution you get: pi = sum(4.0/i - 4.0/(i + 2) for i in range(1, 4*n, 4)) Ah I can go on can't I? A lot more than you asked for! -- Bob Gailer Chapel Hill NC 919-636-4239 When we take the time to be aware of our feelings and needs we have more satisfying interatctions with others. Nonviolent Communication provides tools for this awareness. As a coach and trainer I can assist you in learning this process. What is YOUR biggest relationship challenge?

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