C++ double rounds up when unwanted

Question

I am writing a program in C++ for the distance formula. The answer to x1=0 y1=0 x2=1 y2=1 should be around 1.14, however the answer printed out is 2.00. Every single variable is stored as double I don't know what is going wrong here. Here is my code, and thank you for any help!!

//  main.cpp
//  Chap6_42
//
//  Created on 10/21/14.
//

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
double distance(double,double,double,double); //distance prototype


int main()
{
    double d = 0;

    double x1 = 0; //coordinate x1
    double x2 = 0; //coord x2
    double y1 = 0; //coord y1
    double y2 = 0; //coord y2




    cout << "Enter four cords (x1,y1,x2,y2) to find the distance between them " << endl;
    cout << "x1 = ";
    cin >> x1;
    cout << "y1 = ";
    cin >> y1;
    cout << "x2 = ";
    cin >> x2;
    cout << "y2 = ";
    cin >> y2;

    d = distance (x2,x1, y2,y1); //calls to distance function, performs computations

    cout << "The distance is " << fixed << setprecision(2) << showpoint << d << endl;



    return 0;
}
double distance(double x2,double x1,double y2,double y1) //distance function header
{

    return sqrt(pow(x2-x1,2.0)) + sqrt(pow(y2-y1,2.0)); //distance function computations


}
                                                //function definition

apart from the downvote, why everyone posted an answer with "pow(x,2.0)". Basically there are 3 wrong answers because "pow(-3.0,2.0)" returns "-9.0" wich is wrong in this case because Zach wanted Pythagora. — CoffeDeveloper, Oct 22 '14 at 20:58
possible duplicate of [distance calculation error in c++](http://stackoverflow.com/questions/4217733/distance-calculation-error-in-c) — CoffeDeveloper, Oct 22 '14 at 21:01
@DarioOO This question is not a duplicate of that post. That question is about polluting the global namespace with `using namespace std;` and causing function name resolution problems. — Cory Kramer, Oct 22 '14 at 21:06

Cory Kramer · Answer 1 · 2014-10-22T21:02:13.767

2

Why do you think the answer is around 1.14? Given your example, it should return 2.0

sqrt((pow(2.0 - 1.0, 2.0))) + sqrt((pow(2.0 - 1.0, 2.0)))
sqrt((pow(1.0, 2.0))) + sqrt((pow(1.0, 2.0)))
sqrt(1.0) + sqrt(1.0)
1.0 + 1.0
2.0

Tada!

If you are calculating distance, which the function name implies, you need to adjust your formula.

return sqrt(pow(abs(x2 - x1), 2.0) + pow(abs(y2 - y1), 2.0));

edited Oct 22 '14 at 21:02

answered Oct 22 '14 at 20:40

Cory Kramer

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score 2 · Accepted Answer · answered Oct 22 '14 at 20:40

2

Your calculation is wrong. You're calling sqrt twice when you should only call it once on the entire sum.

return sqrt(pow(x2-x1,2.0) + pow(y2-y1,2.0));

answered Oct 22 '14 at 20:40

Rob Kennedy

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score 2 · Answer 3 · answered Oct 22 '14 at 20:41

2

Your formula is wrong.

You wrote:

sqrt(pow(x2-x1,2.0)) + sqrt(pow(y2-y1,2.0));

It should be:

sqrt(pow(x2-x1,2.0) + pow(y2-y1,2.0));

Anyway, do not use pow there but multiply by hand, that's (probably) faster and more accurate.

sqrt( (x2-x1)*(x2-x1) + (y2-y1)*(y2-y1) );

Also, often you can use squared distances instead of the distance for a small performance-boost.

answered Oct 22 '14 at 20:41

Deduplicator

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Happens to every one. Sometimes getting out, doing something else and then re-reading will clear such a cogitive block. – Deduplicator Oct 22 '14 at 20:52
actually pow(-3.0,2.0) returns -9.0 un my machine, missing an "abs" in the second version. or the integer parameter fix that. (or using multiplication by hand) – CoffeDeveloper Oct 22 '14 at 20:56
2

@DarioOO: That's ... interesting math by the math library. – Deduplicator Oct 22 '14 at 21:01
Wow really? Why is that the case? That is more than just unexpected, frankly that is just mathematically incorrect on the part of the `pow` function. – Cory Kramer Oct 22 '14 at 21:04
Indeed esoteric using "pow" instead of "var*var" :D I saw you added "abs" at last minute :D – CoffeDeveloper Oct 22 '14 at 21:07

score 1 · Answer 4 · answered Oct 22 '14 at 20:40

1

You and pythagorus disagree about how to calculate the distance

answered Oct 22 '14 at 20:40

Martin Beckett

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score -1 · Answer 5 · answered Oct 22 '14 at 20:55

-1

Given you intend to use "pow" for some esoteric reason, your code should be

return sqrt(abs(pow(x2-x1,2.0)) + abs(pow(y2-y1,2.0)));

or

return sqrt(pow(x2-x1,int(2)) + pow(y2-y1,int(2)));

If you are going to programming you'll probably need a lot of math. you need to know "on paper" the algorithms you are going to use (even simple ones like the one by Pitagora). Also a rounding error with doubles is not going to change a result so much (unless you are using some bad conditioned algorithm, wich is not the case for Pitagora.

answered Oct 22 '14 at 20:55

CoffeDeveloper

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Apparently my answer is downvoted for no reason, it is not worst than other answers. downvoter, be polite and explain your reasons. If the downvote is for "abs" usage, then downvoter read more carefully what abs and pow are really doing and you'll see my code is working, while code of currently upvoted answers is not working. – CoffeDeveloper Dec 23 '14 at 14:58

C++ double rounds up when unwanted

5 Answers5