How do I compute pi in c++? No programming background

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2.00/5 (3 votes)

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How do I compute pi in c++ withing 10^-5 of actual pi using u=2 and v= 1/sqrt(2) and replacing u with u/v and v with sqrt((1+v)/2) each iteration. I have no programming background! This is all I got:

C++

#include <iostream>
#include <math.h>

using namespace std;

int main()
{
  float u,v;

  u=2;
  v=1/sqrt(2);

PLEASE HELP!!!

It is due tomorrow

Posted 28-Jan-14 6:30am

MalDrHoop

Updated 28-Jan-14 6:52am

Richard MacCutchan

v5

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Richard MacCutchan 28-Jan-14 12:53pm

What is due, and since you have no programming background why are you being asked for this?

[no name] 28-Jan-14 13:13pm

Here's another hint: Taylor Series.

MalDrHoop 28-Jan-14 14:15pm

I know its a Taylor series. I just don't know what to do. I don't know programming

4 solutions

Solution 3

its quite simple
double dPI = 2*(asin(1));

Posted 28-Jan-14 23:08pm

tv suresh

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Matt T Heffron · Accepted Answer · 2014-01-28T06:46:00

Solution 1

Your code shown is incomplete.
But that is beside the point...
Your homework is given to help you to learn.
If we give you a solution you will not have the "think through" solving the problem.

I'll give you a HINT:
type double will give more precision than float

EDIT:
Since you imply this is not for a c++ programming course, but that you were expected to have some programming background, here is the iteration part of the program.

C++

#include <iostream>
#include <math.h>

using namespace std;
 
int main()
{
  float u,v, prevu;
 
  u=2;
  v=1/sqrt(2);
  prevu = 0;
  while (fabs(prevu - u) > 10e-6f)
  {
    prevu = u;
    u = u/v;
    v = sqrt((1 + v)/2);
  }
  // Here u will contain the approximation to pi
  // Here is where the result should be output
}

My c++ I/O is very rusty, sorry.

Posted 28-Jan-14 6:46am

Matt T Heffron

Updated 29-Jan-14 6:48am

v3

Comments

MalDrHoop 28-Jan-14 14:15pm

I was told to use float. I have NO programming background. I know the code is incomplete but I have no idea what I am doing therefore your hint does not help me. I was not taught ANY c++ and do not have hardly any time to teach myself.

Matt T Heffron 28-Jan-14 14:20pm

OK, then you are already pretty close to the limit of precision that is available with float.
If you use the "Improve question" and show all of what you are doing we may be able to help you "fine tune" it.
(And like Richard MacCutchan noted: If you have no programming background, why are you taking a course which seems to assume c++ as a pre-requisite?)

MalDrHoop 28-Jan-14 14:31pm

I've shown all I've done. Except for some reason it removed the <iostream> and <math.h> after the #include (s) respectively when I submitted. I've given all the instructor supplied. They placed me in the course because it is the only time I can take it and it is a requirement.

Matt T Heffron 28-Jan-14 15:05pm

Can you show the code that you have that follows what is above?
All we can see is the initial assignments to u and v.
What is your iteration code?

MalDrHoop 28-Jan-14 15:15pm

My question preceding the program is all that was given to me in the class.

Matt T Heffron 28-Jan-14 16:44pm

I updated my solution...does it help?

MalDrHoop 28-Jan-14 14:39pm

Ok it removed it in my comment as well. The iostream and math.h

CPallini 28-Jan-14 17:46pm

Don't know why the univoters are fighting with you, have my 5.

Matt T Heffron 28-Jan-14 18:00pm

Could be someone didn't like being told we won't do their homework...

MalDrHoop 28-Jan-14 19:27pm

May I ask what the fabs and prevu means?

Matt T Heffron 28-Jan-14 19:39pm

fabs is the floating point absolute value function
prevu is the previous value of u
the loop is running until u has changed by less or equal 10e-6

MalDrHoop 28-Jan-14 19:45pm

Thanks for putting up with me. And I wasn't asking you to do my homework. I just have no clue where to start since I was never exposed to programming before being thrown in. I guess I won't ask anymore questions on here so that I'm not such a bother

Matt T Heffron 28-Jan-14 19:49pm

Not a bother, considering how many really bad questions get asked here...
And, seriously, discuss your situation with instructor/professor or TA.
What type of course is it? Numerical methods for non-CS majors, etc. ?

MalDrHoop 28-Jan-14 19:58pm

I am a chemistry/math major. Programming is a pre req but I wasn't declared until more recent and my Chemistry advisor didn't think to put me in programming last semester. But I have to have this class while it's offered. I'm a junior and this is my only chance.

MalDrHoop 28-Jan-14 20:03pm

Its Numerical Analysis and the instructor already knows

Matt T Heffron 28-Jan-14 20:14pm

Can you "formally" Accept my Solution with the "Accept" above?

At least you don't have to do it in Fortran! ;-)

You might want to ask if you can use Matlab (or the free equivalent: Octave at octave.org)
Things there "look" a bit more like math and it has vectors, matrices and complex numbers built-in.
It is also "interpreted" so you can just type a line of code to it and see what happens.
The CS-types will say it has a REPL (for "Read-Eval-Print-Loop")

MalDrHoop 28-Jan-14 20:39pm

When I get a chance to look at the program on my computer and see how it goes. Now that my question has been given one star and I've been trash talked :-\

MalDrHoop 28-Jan-14 22:47pm

I did the program and finished up some things and ran it and got 2pi + 1 lol. I just subtracted one from my addition and divided by two. thank you. I will remove my one star if possible. I am sorry for the controversy. I did not know it was anything bad. Just thought it was a rating as to how well it answered my question and it honestly didnt at all at the time. And now everybody on here dislikes me :/ and now im receiving one stars and will likely never receive help on here after this it seems since everyone has a bad opinion of me now.

MalDrHoop 28-Jan-14 22:51pm

Well, I am sorry, but I can not change the star. I am brand new to this site and must not have been clear on the process of the site. I did accept your solution however.

Matt T Heffron 28-Jan-14 19:43pm

And, before you ask, the f at the end of 10e-6f indicates that it is type float.
The compiler treats 10e-6 as type double unless the suffix of f (or F) is present.

[no name] 28-Jan-14 22:04pm

Of course - and having no programming background qualified to judge (destined for management?). My 5 also.

MalDrHoop 28-Jan-14 22:44pm

I gave my judgement based off of how well it answered my question. (It was before the edit btw). Why does me not having any programming experience due to no fault of my own give my question a one star? From my deduction I gave the star based on how well it answered my question at the time, and at the time it did not.

[no name] 28-Jan-14 23:00pm

Well your honesty is laudable. We can all benefit from being less trigger happy in voting. I did not rate your question but I would suggest the reason it was down voted was that you posted homework the day before it was due. Many of those questions are simply voted off the forum so you did well in the end.
BTW: Had you simply used google a few days earlier you would easily have got it in on time.

MalDrHoop 28-Jan-14 23:22pm

Actually, I had used google and tried to figure this out on my own and everything I saw did not approach it the way my instructor instructed us to. I waited till now because I just now knew about the site, and did not know about the assignment very long to be honest. I have tried to work on it. You have to keep in mind I am a beginner in all respects to the world of programming. I can not help that I am. I just am.

Matt T Heffron 29-Jan-14 12:47pm

As pwasser said, posting at the last minute is something of a red flag on CodeProject.
Also, your statement about no programming experience didn't indicate that it was "no fault of my own".
We get way too many questions here asking for code from people in programming/CS courses who seem to think that being able to find someone else to write the code FOR them qualifies them as a programmer. The sense of entitlement of some of these is incredible!
I am NOT accusing you of this! Just noting that many of the helpful folks here can get fed-up and a bit "trigger-happy" on down-voting.

MalDrHoop 29-Jan-14 12:57pm

I promise you I am not like that! I hate the predicament I am in. I just want to learn and I fail, in the most epic sense, at learning just by reading. I need lectures and reinforcements, people explaining it to me. That is how it makes sense. I also am not looking for someone to do the work for me. Seeing code with explanations behind the different parts has been what has explained it to me the best thus far. I just met with an instructor from elsewhere who explained and helped me work through the solution to the question that I just posted and it made sense to me. I want to learn the material and do my work not be spoon-fed through the course. What's the point in going to class then right? I have gotten ambitious this semester and overloaded myself, once again, with my course load and work load. Sorry if that stress came off badly in my tone in any manner.

Matt T Heffron 29-Jan-14 13:02pm

As I indicated, I believe you are in a difficult situation.

MalDrHoop · Accepted Answer · 2014-01-28T17:35:00

Solution 2

Here is what I finally came up with. Definitely not the best way by no means but best I can do with my very limited knowledge.

C++

#include < iostream>
#include < math.h> //so that sqrt function may be used
 
using namespace std;
 
int main()
{
float u,v, prevu;
 
u=2;
v=1/sqrt(2);
prevu = 0;
 
while (fabs(prevu-u) > 10e-6f) //fabs: floating point absolute value function. f: means it's a type of float
{
prevu = u;
u=u/v;
v=sqrt((1+v)/2);
}
cout << " pi ~ " << u << endl;
}

Posted 28-Jan-14 17:35pm

MalDrHoop

Updated 29-Jan-14 7:01am

v5

Comments

Matt T Heffron 29-Jan-14 12:39pm

At the end of the loop u should contain the approximation to π (it does for me).
The calculation in the cout line should be unnecessary.

MalDrHoop 29-Jan-14 12:45pm

I have a better solution now. I will submit it in a moment.

MalDrHoop · Accepted Answer · 2014-01-29T06:58:00

Solution 5

Apparently I forgot to mention if didn't matter which code the program was in at the beginning when I entered my question (new to site remember) but I prefer c or c++ over the others cause of the small amount of stuff I've looked at and understood it was in c++ and c++ and c is what I will be taking next semester and what some people have tried to explain to me.

C#

#include <math.h>               // to use square root function
#include <stdio.h>              // to use print

int main()
{
    float u = 2.0;
    float v = 1/ sqrt((float)2.0);

    printf("Pi approximation\n\n");  //prints output

    for (int i = 0; i < 12; i++)
    {
        u = u/v;
        v = sqrt((1 + v) / 2);
        printf("%f\n", u);      //prints u for each loop onto screen
    }

    getchar();
    return 0;
}

Posted 29-Jan-14 6:58am

MalDrHoop

Updated 29-Jan-14 7:42am

v2

Comments

Matt T Heffron 29-Jan-14 13:11pm

Now, I'm going to put on my "former TA" hat, if you don't mind and ask a couple of questions for you to think about (no answer here necessary)
Why 12 iterations? Why not 5, 10, 100?
Is that the optimal number of iterations to get the desired precision?
Numerical Analysis includes not doing unnecessary computation as well.

MalDrHoop 29-Jan-14 13:27pm

Your deduction is correct, 12 iterations was all that was necessary to get the desired outcome.

Matt T Heffron 29-Jan-14 14:05pm

Are you sure it is 12 and not 11?
I modified mine to output the fabs(prevu - u) and that value was ~9.5e-7 after the 11th iteration.

MalDrHoop 29-Jan-14 13:26pm

This works perfectly and someone gave me one star? Wow I guess I should just quit on here.

Matt T Heffron 29-Jan-14 13:59pm

I think the one star is because NORMALLY you shouldn't post SOLUTIONS to your own questions.
Especially, multiple Solutions.
(It looks like abuse of the Reputation Points system, because Solutions are worth more.)
In the future, the recommended way is to use the "Improve question" (above, at the bottom right of your original question) and add them to the end of the question with comments like "This is my latest attempt", or "Thanks, this is what I ended up with".

MalDrHoop 30-Jan-14 7:37am

Well, the button that said "I've solved this myself" took me straight to the solution box. So apparently everybody on the site just doesn't like to use it the way it is set up, or someone needs to make some changes somewhere, I just did exactly what it told me to do. And this is how he was wanting it done so that you could see each iteration as it approached closer to pi. And it isn't the same as the other way so why would I pair it with that? Oh well. People jump too quickly. I'm not into this for the reputation points. But I dislike people judging me unjustly. I get enough of that from other sources.

Matt T Heffron 30-Jan-14 12:56pm

Like all things, the conventions here take a bit of getting used to.
"I've solved this myself" means "I came up with a solution independent of the suggested solutions" or "Never mind, I figured it out before anyone suggested a solution"