问题
I'am trying to expand exp(x) function to Taylor series. Here is code:
double CalcExp(){
double eps = 0.0000000000000000001;
double elem = 1.0;
double sum = 0.0;
int i = 1;
sum = 0.0;
do {
sum += elem;
elem *= x / i;
i++;
} while (elem >= eps);
return sum;
}
The problem is when I enter big X or negative X my program crashes. And when I enter X like "0.00000000001" the result is -1.
Need advice. Thank's for help.
回答1:
For big X values (around 700 and above), you'll hit the range limit for doubles (10^308) and cause an infinite loop. You can't do much about it, you should either limit X input range or use some big number library to have extended range.
Another workaround is to add this to your loop:
if (sum > 1E305) {
// we'll most likely run into an infinite loop
break;
}
Note you should handle this case outside the loop afterwards to avoid printing a very large incorrect result.
I can't reproduce the problem for 0.00000000001
, this just returns 1 for me. Negative values run fine, too, although the result is wrong which seems to be an error/limitation in the algorithm. EDIT: To correct this, we can use the fact that e^-x
is the same as 1 / e^x
.
Code:
#include <stdio.h>
double CalcExp(double x){
double eps = 0.0000000000000000001;
double elem = 1.0;
double sum = 0.0;
bool negative = false;
int i = 1;
sum = 0.0;
if (x < 0) {
negative = true;
x = -x;
}
do {
sum += elem;
elem *= x / i;
i++;
if (sum > 1E305) break;
} while (elem >= eps);
if (sum > 1E305) {
// TODO: Handle large input case here
}
if (negative) {
return 1.0 / sum;
} else {
return sum;
}
}
int main() {
printf("%e\n", CalcExp(0.00000000001)); // Output: 1.000000e+000
printf("%e\n", CalcExp(-4)); // Output: 1.831564e-002
printf("%e\n", CalcExp(-45)); // Output: 2.862519e-020
printf("%e\n", CalcExp(1)); // Output: 2.718282e+000
printf("%e\n", CalcExp(750)); // Output: 1.375604e+305
printf("%e\n", CalcExp(7500000)); // Output: 1.058503e+305
printf("%e\n", CalcExp(-450000)); // Output: 9.241336e-308
return 0;
}
回答2:
Need advice.
Try stepping through your program in a debugger to see where it's going wrong. If you don't have a debugger, insert print statements within the loop to monitor the values of variables that change.
来源:https://stackoverflow.com/questions/5362573/exp-to-taylor-series