Help please!

[Matt Cromer]

Hi guys, I am trying to do this problem:
Find the Fifth root of the sum of the squares of the first 100 ODD numbers only. (Beginner).

This is my code for it:

//  Fifth root of the sum of the squares of the first 100 ODD numbers
#include <iostream>
#include <cmath>

using namespace std;



int main()
{
    
int a=1;
int x=0;
double numbers [x];
double answer;
int repeat = 100;
    
 cout << "This program finds the fifth root of the sum of the squares of x numbers." << endl;
 cout << "How many odd numbers?  ";
 cin >> repeat;
 cout << endl;
 // for statement repeats the mapping process for how many odd numbers you want, in this case 100.
 // x=array size   and   a=initial number and counts up from there
 for (int n=0; n<repeat; n++)
    {
          numbers[x] = sqrt(a);
          x+=1;
          a+=2;       
    }  
 // adds all of the numbers in the array, based on the x variable 
 // that was used to create the array in the previous statement
 for (int n=repeat; n>0; n--)
    {
          answer += numbers[x];
          x-=1;      
    }
 // display the result
 cout << "The result is: " << answer << endl;
    
    system ("pause");
    return (0);
}

I haven't made it do the fifth root yet, but it is already giving me the wrong answer.
For some reason if I try 5, it is giving me 9.something... when I add the first 5 squares (sqrt1, 3, 5, 7, 9) on my calculator, I get 10.61.
ALSO, if I do a larger number, the entire code crashes.

What have I done wrong?

[Dinesh subedi]

here is your corrected code

#include <iostream>
#include <cmath>
#include <cstdlib>

using namespace std;



int main()
{

int a=1;
int count=0;
//double numbers [x];
double answer;
int repeat;

 cout << "This program finds the fifth root of the sum of the squares of x numbers." << endl;
 cout << "How many odd numbers?  ";
 cin >> repeat;
 cout << endl;
 double numbers[repeat];
 // for statement repeats the mapping process for how many odd numbers you want, in this case 100.
 // x=array size   and   a=initial number and counts up from there
 for (int n=0; n<repeat; n++)
    {
          numbers[n] = sqrt(a);
          a+=2;
          count++;
    }
 // adds all of the numbers in the array, based on the x variable
 // that was used to create the array in the previous statement
 for (int n=count; n>0; n--)
    {
          answer += numbers[n-1];
          //x-=1;
    }
 // display the result
 cout << "The result is: " << answer << endl;

    system ("pause");
    return (0);
}

I have included <cstdlib> for system("pause") and other error is also corrected.

[Hippogriff]

The problem is the way you are reading your 'numbers' array.

If you are doing 100 like your example, it reads the numbers into numbers[0] - numbers[99] and then increments x by 1 each time. In your next for loop it will then start reading at numbers[100] which doesn't have a value and will give undefined behavior (C++ doesn't check to see if you went out of bounds on an array).

There are several ways you could fix this by rearranging code, but a simple way is to change line 34 to
answer += numbers[x - 1];

Another problem is you are declaring an "x" amount of values in your numbers array when x hasn't been defined yet which will cause strange behavior. So you will either have to change that to an actual number or switching to vectors might not be a bad idea either (which would also make the whole thing safer).

(And beaten to it~)

[histrungalot]

You are saying the first 5 squares but you are using square root (sqrt).
1,3,5,7,9 -> squared 1,9,25,49,81 = 165 -> fifth root = 2.776494316734079

[Matt Cromer]

Thanks Dinesh subedi for the code, and thanks James for explaining it.
I really appreciate it.

I can't believe I read that question wrong too!
Of course it is squares! Gosh.

I know absolutely nothing about vectors, I will have to read up on that.

Thanks again.

[Matt Cromer]

I also noticed you included cstdlib? What is the difference there?

[Dinesh subedi]

In code blocks to run system("pause"); we have to include cstdlib so I have included it.I use codeblocks as a IDE.