if statement doesn't work VS2010

Question

(slopeDegrees=90 type long double)

cout<<slopeDegrees<<endl;

if(slopeDegrees==90)cout<<"0"<<endl;

this code doesn't work ... the console print only 90 (its need to be 90 \n 0) why ? i use VS2010

Maybe your value isn't exactly 90. Maybe it is 90.000000000001. It would still be printed as 90, but it wouldn't compare equal to 90. — Vaughn Cato, Aug 19 '12 at 14:35
http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm — Baiyan Huang, Aug 19 '12 at 14:36
possible duplicate of [strange output in comparison of float with float literal](http://stackoverflow.com/questions/1839422/strange-output-in-comparison-of-float-with-float-literal) — Bo Persson, Aug 19 '12 at 15:03

score 4 · Answer 1 · answered Aug 19 '12 at 14:35

4

You try to compare 90 with a float. Since a floating point is now always exactly 90.0000 but can be 89.9998 or 90.0001. When comparing to another int or float value directly the comparison not be true.

This should not be done that way, but like:

if (Math.Abs(slopeDegrees - 90.0) < 0.001)

The 0.001 is some accuracy you can define yourself.

answered Aug 19 '12 at 14:35

Michel Keijzers

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In standard C++ that would be `std::abs`. – Mike Seymour Aug 19 '12 at 14:37
Sorry I mixed C# and C++, you can use what Mike Seymore writes above or use the statement 'using namespace std;' and use abs directly. – Michel Keijzers Aug 19 '12 at 21:12

score 3 · Answer 2 · answered Aug 19 '12 at 14:34

Comparing equality with doubles and other floating point types is frought with peril. 90 in double is an approximation, not exactly the integer 90.

Its better to compare against being within a threshold, not against exact inequality. Something like:

if (slopeDegrees > 89.9 && slopeDegrees < 90.1)
{
    cout << "0" << endl;
}

If exact representations are needed of certain important values, you'll need to look into a fixed point way to represent these values.

MartyE · Answer 3 · 2012-08-19T15:04:39.887

1

Because you don't want to compare double for exact equality. Instead test within a range. Ex:

const double THRESHOLD = 0.005;
double slopeDegrees = 90;

cout<<slopeDegrees<<endl;

if((slopeDegrees <= (90 + THRESHOLD)) && (slopeDegrees >= (90 - THRESHOLD)))
{
    cout<<"0"<<endl;
}

edited Aug 19 '12 at 15:04

answered Aug 19 '12 at 14:36

MartyE

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if statement doesn't work VS2010

3 Answers3