Static Cast Error Truncation

Hey all when I use the following code, and enter 300.34 it returns 30033, but that is the only value it returns one less for..... I don't understand why?

Code:

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#include <iostream>


using namespace std;


int main() {
       
        double d, temp;
        int i;
        cout << "Enter a number: ";
        cin >> d;
        i=static_cast<int>(d*100);
        cout << "New Number: " << i;

        return 0;

}

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Thanks!

Because the computer cannot represent 300.34 exactly. When it stores this in a float, it actually stores it as 300.3399999999999

Hence the value "30033" when you multiply by 100, and cast to an int.

Viggy

i dont know why it happens :confused:

but i can suggest something to overcome it.

Code:

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i=static_cast<int>(d*1000);
i/=10;

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it will work but i really also wanna to know why its happening.

Cheers

Quote:

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Originally Posted by MrViggy

Because the computer cannot represent 300.34 exactly. When it stores this in a float, it actually stores it as 300.3399999999999

Hence the value "30033" when you multiply by 100, and cast to an int.

Viggy

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I don't understand, wouldn't it do the exact same thing with 200.34 or any other number? But 300.34 is the only number that it happens with.

Quote:

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Originally Posted by DivZr0

I don't understand, wouldn't it do the exact same thing with 200.34 or any other number? But 300.34 is the only number that it happens with.

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MrViggy explained that 300.34 cannot be stored exactly with the binary representation of floats, so it stores something like 300.3399999999999
And, a conversion from float to int (a static_cast) is done by rounding to the greater integer not greater than 30033.99999999999, that is 30033
To avoid this error, you must round the floating number before casting it to int:

Code:

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double fround(double val) // i don't know if there is any standard function to round a float number to the nearest integer.
{
double f=floor(val);
if (val-f>(f+1)-val) return f+1;
else return f;
}
// now use static_cast<int>(fround(d)*100);

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Floating point numbers have floating point, that is, they are represented by a mantissa (a binary number) and an exponent (an other binary number), and a sign bit.
The value of the number is sign*mantissa*2^exponent
The best representation for a number whose value cannot be exactly represented, uses the whole mantissa bits, and the exponent can be negative.
200.34 and 300.34 have very different representations with a different exponent and a different mantissa.

Check the FAQ:
http://www.codeguru.com/forum/showthread.php?t=323835