一些不能精确计算的浮点数运算，列举

#include <iostream>
#include <cfloat>
#include <cmath>

using namespace std;

int main(int argc, char* argv[])
{
    float a = 102.1f;
    float b = 0.2f;
    float c = a + b;    // 102.3f
    if( c == 102.3f ) {
        cout << a << " + " << b << " = " << 102.3f << endl;
    } else {
        cout << a << " + " << b << " != " << 102.3f << endl;    // Do Else
    }

    float d = a - 100.0f;
    if( d == 2.1f ) {
        cout << a << " - 100 " << " = " << 2.1f << endl;
    } else {
        cout << a << " - 100 " << " != " << 2.1f << endl;    // Do Else
    }

    float e = 0.3f;
    if( (e * 3) == 0.9f ) {
        cout << e << " * 3 " << " = " << 0.9f << endl;
    } else {
        cout << e << " * 3 " << " != " << 0.9f << endl;    // Do Else
    }

    //
    // The Right judgement    expression
    // abs(a - xxx) <= 1.0e-5 // here  1.0e-5 is an epsilon, can't be other float number instead.
    //
    cout << "c = " << c << endl;
    float epsilon1 = 1.0e-5;
    float epsilon2 = 1.0e-6;
    float standard_EPSILON = FLT_EPSILON;    // #include <cfloat>
    cout << "FLT_EPSILON = " << standard_EPSILON << endl;

    cout << "Now use epsilon1 = " << epsilon1 << endl;
    if( abs(c - 102.3f) <= epsilon1 ) {
        cout << "In if   <= epsilon1  c == 102.3" << endl;    // In if epsilon1 is matched
    } else {
        cout << "In else > epsilon1 c != 102.3" << endl;
    }


    cout << "Now use epsilon2 = " << epsilon2 << endl;
    if( abs(c - 102.3f) <= epsilon2 ) {
        cout << "In if   <= epsilon2  c == 102.3" << endl;
    } else {
        cout << "In else > epsilon2 c != 102.3" << endl;    // In else epsilon2 is not matched
    }

    return 0;
}