Question

I tried to add two digits with different weights. Here is my code:

void onTimeStepOp::updatePointsType1_2(boost::tuples::tuple<float,int,int,int> &_prev,
                                     boost::tuples::tuple<float,int,int,int> &_result,
                                       boost::tuples::tuple<float,float> weights)
{
    _result.get<0>() = _result.get<0>() * weights.get<0>() + _prev.get<0>() * weights.get<1>();
    std::cout<<"deb:"<<(float)_result.get<2>() * weights.get<0>()<<" "<<(float)_prev.get<2>() * weights.get<1>()<<std::endl;
    _result.get<2>() = (int)((float)(_result.get<2>()) * weights.get<0>() + (float)(_prev.get<2>()) * weights.get<1>());
    std::cout<<"deb2:"<<(float)_result.get<3>() * weights.get<0>() <<" "<< (float)_prev.get<3>() * weights.get<1>()<<std::endl;
    _result.get<3>() = (int)((float)(_result.get<3>()) * weights.get<0>() + (float)(_prev.get<3>()) * weights.get<1>());
}

weights.get<0> = 0.3,weights.get<1> = 0.7.

The output I get looks like this:

resultBefore=36.8055 4 69 91 previousPPos=41.192 4 69 91
deb:20.7 48.3
deb2:27.3 63.7
resultAfter=39.8761 4 **68** 91

The third number should be 69(69 * 0.3 + 69 * 0.7). However, it is 68 instead. What’s the problem with the type conversion expression?

Answer 1

Conversion to int truncates, so the slightest rounding error could cause you to be one off. Rather than converting directly to int, you might want to use the function round.

I might add that weights.get<0> is certainly not 0.3, and weights.get<1> is certainly not 0.7, since neither 0.3 nor 0.7 are representable in machine floating point (at least not on any machine you’re likely to be using).