Question

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How should I do floating point comparison?

this is a snippet of code and I can’t figure out what’s wrong with it.

The if statement is evaluating the numbers incorrectly and going to “else”.

double m,k;

m = (y3-y1)/(x3-x1);
k = m*(-x1)+y1;

cout <<"\n\n"<< m <<"  "<< k << "\n";
cout <<"\n\n"<< ((-a)/b) <<"  "<< c/b << "\n";

if(m==((-a)/b) && k==c/b)
 {
  cout << "\nTaisne sakrīt ar kādu no trīsstūra malām!" << endl;
  goto beigas;
 }
  else
   cout << "\n\n WRONG \n\n";

The user inputs values x1,y1,x3,y3,a,b,c – all are double.

It seems to work fine only with “m”. And it is strange that when I tried the following, the if statement was true with the same values:

if(m==((-a)/b) && k<c/b)

Here is a screenshot to get the whole problem:

The code starts after user has input all the values.

Thanks, any help would be greatly appreciated!

Answer 1

when you want to compare two floating points use a threshold like that one:

double m = a/b;

if (abs(m - (a/b)) < 0.0001)
{
    // execute code where m == a/b here
}

so basically to compare float/double subtract them and make sure the result is less than certain threshold.