Error in program

[matanuragi]

Dear sir ,
I want to make a program to calculate
sum=1^3-2^3+3^3-4^3........+n^3
I have written the following code
#include<iostream>
#include<conio>

int main()
{
int sum=0,i,b,n;
std::cout<<"\n enter a value \n";
std::cin>>n;
for(i=1;i<=n;i++)
{
b=(-1)^(i+1);
sum=sum+b*i*i*i;
}
std::cout<<"\n the sum is \n"<<sum;
getch();
return 0;
}
But I am not getting correct output.
I would be glad you can solve my problem
With regards,
Matanuragi

[Duthomhas]

Your math is incorrect.

For the sequence

1³ - 2³ + 3³ - 4³ ...

you have correctly indicated that the difference in each term is in the value of the coefficient and its sign.

What you need to do is write a single equation that represents a single term, then turn that equation into code. This is what belongs in the loop (as I think you know).

Here's the equation for you:

_n
Σ (-1)^k+1(k³)
^k=1

Now make sure your code matches that math and let us know if you have any troubles.

Good luck!

[edit] Fixed the power of -1 to k+1. Sorry about that!

[matanuragi]

Dear sir,
As per my knowlege in maths the equation of kth term given by you i.e.
n
Σ (-1)^k(k³)
k=1
gives the following series
-1³ + 2³ - 3³ + 4³ ...

According to me to calculate the series
1³ - 2³ + 3³ - 4³ ...
the kth term is
n
Σ (-1)^k+1 k[sup]3
k=1
If I am still wrong ,Kindly forgive me for my foolishness and
explain explicitly the reason for me being wrong.
With regards,
Matanuragi

[Duthomhas]

Sorry about that... My brain failed me on that one. :-S

But the problem is that ^ does not raise a number to a power -- it is the XOR operator.

You can use the pow() function in <cmath> or decide it from i directly.

Sorry again.

[matanuragi]

It is alright enfact I am thankful that you devoted your time in solving my problem.But I am still getting error that pow ()function is not in math header file in the following program

#include<iostream>
#include<conio>
#include<cmath>

int main()
{
int sum=0,i,n,b;
std::cout<<"\n enter a value \n";
std::cin>>n;
for(i=1;i<=n;i++)
{
b=pow(-1,i+1);
sum=sum+b*i*i*i;
}
std::cout<<"\n the sum is \n"<<sum;
getch();
return 0;
}
Please rectify the error
With regards,
Matanuragi

[z05DSL3A]

You could try #include<math.h>

http://www.cplusplus.com/reference/clibrary/cmath/pow.html

[matanuragi]

I tried #include<math>
and I got a correct output.
Thanks for your help.

With regards,
Matanuragi