Why is Less Than or Equal To (<=) Returning False For Equal Number (C++)

Asked May 24 '20 at 20:05

Active May 24 '20 at 20:05

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I have created a simple function that calculates the BMI of a person given the weight and height (metric). Everything works fine except when the BMI is 30. It's returning Obese instead of Overweight even though I have put less than or equal to (<=). I have found a fix for it, but I am not understanding why this fixes it. The fix I found is casting the bmi variable to an integer. Thank you!

#include <iostream>

std::string bmi(double w, double h){
    double bmi = w / (h * h);
    std::cout << bmi << '\n';
    if(bmi <= 18.5){
        return "Underweight";
    }else if(bmi <= 25){
        return "Normal";
    }else if(bmi <= 30){ // BUG, fix: cast bmi to (int), but WHY ?
        return "Overweight";
    }else{ // bmi > 30.0
        return "Obese";
    }
}

int main(){
    std::cout << bmi(86.7, 1.7) << '\n';
    return 0;
}

asked May 24 '20 at 20:05

Vrej

Because the double is probably represented as `30.0000000x` and when you cast to int its rounded down to `30` – Tony Tannous May 24 '20 at 20:07
@cigien Didn't know about that, thank you! – Vrej May 24 '20 at 20:07
@TonyTannous Yep I understood that, but then how would one compare to a double number without casting it to an integer? – Vrej May 24 '20 at 20:08
add an epsilon to the comparison. – Tony Tannous May 24 '20 at 20:09
1

You'd add a fudge factor. `bmi <= 30.0 + 0.01` – Eljay May 24 '20 at 20:09
Thank you very much everyone, I understand the issue now! – Vrej May 24 '20 at 20:16
I think nearly every programmer learns this lesson. It takes some experience with the idiosyncrasies of floating point to be aware of its pitfalls. – Eljay May 24 '20 at 20:23

Why is Less Than or Equal To (<=) Returning False For Equal Number (C++)

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