Casting changes value strangely
Why does a1=72
instead of 73
in this (terrible) snippet of C++ code ?
#include <iostream>
#include <string>
using namespace std;
int main (int argc, char* argv[])
{
double a = 136.73;
unsigned int a1 = (100*(a-(int)a));
cout << (a-(int)a) << endl; // 0.73
cout << 100*(a-(int)a) << endl; // 73
cout << a1 <&开发者_运维百科lt; endl; // 72 !
}
You can execute it at http://codepad.org/HhGwTFhw
If you increase the output precision, you'll see that (a - (int) a)
prints 0.7299999999999898
.
Therefore, the truncation of this value (which you obtain when you cast it to an int
) is indeed 72
.
(Updated codepad here.)
This is a common precision issue. The literal 136.73
actually stands for the number
136.729999999999989768184605054557323455810546875
and the result of a-(int)a
is not 0.73
(even though that is what is displayed), but rather
0.729999999999989768184605054557323455810546875
When you multiply that by 100
, you get
72.9999999999989768184605054557323455810546875
And since converting from double to int cuts off everything after the decimal point, you get 72
.
0.73
cannot be represented exactly so it is rounded to a number close to it, which in this example is lower then 0.73
. when multiplying by 100, you get 72.[something]
, which is later trimmed to 72
.
more info
It might become clear if you read the following article: What Every Computer Scientist Should Know About Floating-Point Arithmetic.
The others have already shown that the result of (a - (int)a)
is not exactly 0.73, but a little lower. The article explains why that is so. It is not exactly an easy read, but really something everyone working with FP should read and try to understand, IMO.
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