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# Thread: I need help with for loops

1. Junior Member
Join Date
Feb 2010
Posts
5

## I need help with for loops

Ok here is the assignment my instructor gave me:

A cancellation error occurs when you are manipulating a very large number with
a very small number. The large number may cancel out the smaller number. For example, the
result of 100000000.0 + 0.000000001 is equal to 100000000.0. To avoid cancellation errors and
obtain more accurate results, carefully select the order of computation. For example, in
computing the following series, you will obtain more accurate results by computing from right to
left rather than from left to right:

1+1/2+1/3+...+1/n.

Write a C++ program that compares the results of the summation of the preceding series,
computing from left to right and from right to left with n = 10, 100, 1000, and 10000.

Instructions:
Use a long double variable and accumulator for the summation.
Avoid mixed mode expressions.
May use a while or for loop (or both), but will need to nest the loops.
There is no input from the user.
Output the two summation values and the absolute value of their difference in a table
with the value of n.

n Left to right Right to left Abs(diff)
10
100
1000
10000

I wrote something that worked but it is so ugly and inefficient im scared to show it to my instructor.

I'm not asking anyone to write this for me, but i would appreciate some ideas about how you might approach this particular program. Thanks.

2. Junior Member
Join Date
Feb 2010
Posts
4

## Re: I need help with for loops

Originally Posted by jpetrilli
I wrote something that worked but it is so ugly and inefficient im scared to show it to my instructor.

I'm not asking anyone to write this for me, but i would appreciate some ideas about how you might approach this particular program. Thanks.
Hi. Most likely, we are not your instructor, so feel free to show it to us and we can help you improve your program.

3. Elite Member
Join Date
May 2009
Posts
2,413

## Re: I need help with for loops

Originally Posted by jpetrilli
1+1/2+1/3+...+1/n.
Here's a left to right summation,

Code:
```int N=.........;
double sum = 0.0;
for (int i=1; i<=N; ++i) {
sum = sum + 1.0/double(i);
}```

4. Junior Member
Join Date
Feb 2010
Posts
5

## Re: I need help with for loops

I got my program to do the summations, but I cannot figure out how to do the absolute value of the two differences of the sums. Any ideas?

5. Junior Member
Join Date
Feb 2010
Posts
6

## Re: I need help with for loops

Not entirely sure if this is correct, but can't you simply use the cmath library and use the abs() function?

6. Junior Member
Join Date
Feb 2010
Posts
5

## Re: I need help with for loops

Yes, I used that function I just had to declare 2 accumulators I guess that was my problem.

Does anyone know how to get the zeros on those answers to show up to 20 decimal places?

7. Elite Member Power Poster
Join Date
Oct 2007
Location
Seattle, WA
Posts
10,895

## Re: I need help with for loops

That's just a matter of output formatting. Read up on the setprecision() manipulator if using cout, or the detailed format string documentation if using printf().

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