C++ "for" loop with a pow function in it gives incorrect results

Question

I'm studying coding basics, and had to make a code that calculates how many levels of a pyramid could be built with blocks available "x", if each level is squared (e.g. 1st=1, 2nd=4, 3rd=9 etc.) here's what I have so far, and for the life of me, I can't see where I'm wrong, but the code keeps returning a value of 2 more than it should (e.g. x=25 should result in 3 not 5)

#include <iostream>
#include <cmath>
using namespace std;

int main()
{
    int i, x, y=0;
    cout<< "How many blocks do you have?"<< endl;
    cin>> x;
        for (i=1; y<=x; i++) {
            y=y+pow(i, 2);
            
        } cout<< i <<endl;

return 0;
}

EDIT: Thanks for the answers. copied and tried them out, the for function seems to be the worse option here, so I ended up using while.

[Is floating point math broken?](https://stackoverflow.com/a/588014/4641116) The `pow` function works on `double` type, and uses logarithms to calculate the answer, which are likely slightly inexact. You've used a hammer to pound in a screw. — Eljay, May 30 '22 at 11:29
Does this answer your question? [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Brett Hale, May 30 '22 at 11:45
This task should be solved [using mathematics first](https://www.wolframalpha.com/input?i=sum+from+i+%3D+0+to+n+of+i%5E2) before starting coding. — Marek R, May 30 '22 at 11:45

Vlad from Moscow · Answer 1 · 2022-05-30T11:37:48.727

0

The for loop is incorrect

    for (i=1; y<=x; i++) {
        y=y+pow(i, 2);
        
    }

The result of this expression

y=y+pow(i, 2);

can be greater than x but the value of i will be increased.

You could write the for loop for example the following way

    int tmp;

    for ( i = 0; ( tmp = pow(i + 1, 2) ) + y <=x; i++) {
        y += tmp;
        
    }

edited May 30 '22 at 11:37

answered May 30 '22 at 11:32

Vlad from Moscow

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eww. Just do a while loop instead. – user253751 May 30 '22 at 11:34
TS just need to subtract from the `i` 2. There is no need to store temporary value. – trokymchuk May 30 '22 at 11:50
@trokymchuk If to subtract 2 you can get 0 instead of 1.:) – Vlad from Moscow May 30 '22 at 11:51
I believe, if you pass zero, you could build exactly zero levels of pyramid. ;) – trokymchuk May 30 '22 at 11:54

score 0 · Accepted Answer · edited May 30 '22 at 15:09

0

It is because your for loop works even if next layer is impossible to make and then it increments i once more . That's why your result is bigger by 2 than it should be . Try this:

int tmp;
while(true){
tmp = y+i*i;

if(tmp > x) //check if this layer is possible to create
    {
    i--; //its impossible , so answer is previous i 
    break;
    }
i+=1;
y = tmp;
} cout<< i <<endl;

edited May 30 '22 at 15:09

Pat. ANDRIA

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answered May 30 '22 at 11:48

whiteandwild

16
2

Good answer, formatting would help with clarity. – Wasif Hyder May 30 '22 at 15:28

score 0 · Answer 3 · answered May 30 '22 at 12:00

0

This works as well if you want to keep your code pretty much the same way it is already:

int i, x, y=1;  // y is now 1
cout<< "How many blocks do you have?"<< endl;
cin>> x;
for (i=2; y<=x; i++) {  // i now starts from 2
    y=y+pow(i, 2);

}
cout<<i-2<<endl; // now i-2

answered May 30 '22 at 12:00

PurpleHacker

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You should mentions that starting from `i=2`, other than a minor speed improvement, is there so a negative number of blocks will output `0` instead of `-1`. Arguably `-1` for a negative number of stones wouldn't be wrong. – Goswin von Brederlow May 30 '22 at 15:25

C++ "for" loop with a pow function in it gives incorrect results

3 Answers3