C++ using E in a mathematical equation

Question

I'm trying to solve aX² + bX + c = 0 but I can't seem to make it work with using the math header (which I'm not supposed to use).

printf("%E",(-b+(b*b-4*a*c)E0.5)/2a);

I must say I'm genuinely surprised and bemused to see the ideas people come up with -- of course this doesn't work, but one might very well have expected it to work. +1 for sharing this idea! — Kerrek SB, Sep 09 '11 at 16:12

score 7 · Answer 1 · edited May 23 '17 at 12:29

7

Use std::sqrt from header <cmath>. Also, you must write (2 * a), not 2a.

Another thing: don't use the textbook formula for solving quadratic equations. Use the method described there.

If you can't use the math header, then you have to implement the square root eg. as described there:

double my_abs(double x)
{
    return x > 0 ? x : -x;
}

double my_sqrt(double x)
{
    static const double eps = 1e-12;
    double u = x, uold;

    do { uold = u; u = (u * u + x) / (2 * u); }
    while (my_abs(u - uold) < eps * x);

    return u;
}

edited May 23 '17 at 12:29

Community

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answered Sep 09 '11 at 16:09

Alexandre C.

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Funny thing, though - he *can't* use ``. – Ken Wayne VanderLinde Sep 09 '11 at 16:13
*not supposed to* is different from *can't* ... smells like homework but he can roll his own loops and get an approximate of square root if he must. – AJG85 Sep 09 '11 at 16:17
You can't include anything aside from stdio.h – user779444 Sep 09 '11 at 16:26
2

@user - A C++ course assignment in which the only allowed header is ``? Not ``, not even ``, but ``? My first suggestion is that you should ask for your money back. – Robᵩ Sep 09 '11 at 16:46

score 2 · Answer 2 · answered Sep 09 '11 at 16:12

2

That is not at all how E works.

E is used in floating point literals, to express a number scientific notation (more or less)

// x will be the same as 0.00104
double x = 1.04e-3

If you want to take a square root, then you should be using a sqrt function:

sqrt(-b+(b*b-4*a*c))/2 / a

Of course, since you can't use #include <cmath>, you'd have to roll your own!

answered Sep 09 '11 at 16:12

Ken Wayne VanderLinde

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Very nice idea, but how do I make such a function ? We're right back where we started – user779444 Sep 09 '11 at 16:21
Wouldn't writing the function be your job? Lucky for you, Alexandre gave you the answer, but the idea is that you find a way to write one yourself, or at least search for one on your own. Btw, a quick google search brought me here [Wikipedia: Methods of computing square roots](http://en.wikipedia.org/wiki/Methods_of_computing_square_roots), so it's not like the stuff is difficult to find. – Ken Wayne VanderLinde Sep 09 '11 at 17:41

score 0 · Answer 3 · edited May 23 '17 at 12:04

0

You can't use E as pow in C/C++ (see for example mathematical power operator not working as expected). And the E in the printf will print the number as Scientific notation, you know? (like 3.9265E+2).

edited May 23 '17 at 12:04

Community

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answered Sep 09 '11 at 16:09

xanatos

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score 0 · Answer 4 · answered Sep 09 '11 at 16:12

0

E only works when you're typing out a constant floating point, like 2.2E6. To compute exponentials, you need to use std::pow() from <cmath>. In this case, you could use std::sqrt().

answered Sep 09 '11 at 16:12

Maister

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score 0 · Answer 5 · answered Sep 09 '11 at 16:14

I suppose with E you mean the power, but there is no such power operator in C++. Use either the pow function or the, in your case more appropriate, sqrt function. But these are both in <cmath>. If you cannot use <cmath> (homework assignment?), you might have to implement your own square root function.

score 0 · Answer 6 · answered Sep 09 '11 at 16:15

I think you are confusing scientific notation (3.2E6 = 3.2 x 10^6) with exponentiation (sqrt(5) = 5^(1/2)), where I am using ^ for "raise to the power of". Unfortunately, c++, like C, doesn't have a built-in power operator. So you would normally use either sqrt(x) or pow(x,0.5) from the math library.

However, if you want to solve this without the math header, you'll have to find a different way to calculate square roots. You could write a subroutine to use the Babylonian or Heron method, for example...

C++ using E in a mathematical equation

6 Answers6