I have to calculate some simple mathematical expression, but when I do it in one row, the result always will be zero. But the correct result is obviously not zero. And its interesting but when I separate the parts of expression, I get the correct answer. Later I will divide with this result, so it should not be 0.
The expression is like this:
(X-X1)/(X2-X1)
In this case the delta: 0
double delta = (x - x1) / (x2 - x1);
But this way the delta will be correct:
double top = x - x1;
double bottom = x2 - x1;
double delta = top/bottom;
Do you have any idea, how could this happen?
This is a typical case of "integer division used to make a float value". If x, x1 and x2 are all integers, then the compiler should, according to the rules of C and C++ use integer calculations to calculate x-x1 and x2-x1, and then divide the differences of those as integer division. If x2-x1 is greater than x-x1, then the result is zero [assuming positive values like 7/14. -14 / -7 is obviously 2].
The easy fix is to convert the type of one expression to a floating point type:
double delta = static_cast<double>(x - x1) / (x2 - x1);
In the first case, integer / integer, result is still integer, then convert integer to double.
In the second case, double / double, then you get correct result.
In the first case, you are performing int / int which will result in a int value. So, if your denominator is larger than the numerator, the result will be truncated to 0.
In the second case, you are evaluating a double/double expression which will evaluate to a double value, hence preserving the fractional and integer parts.
You have to cast the integer division to double. type casting
来源：https://stackoverflow.com/questions/15550708/c-wrong-result-of-mathematical-expression