Subtraction of floating point numbers in python and c doesn't behave the same, when I try "0.3 - 0.1", c tells me 0.2 and python tells me 0.199999...98.
I wrote the following c code:
// a.c
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
double a, b;
a = strtod(argv[1], NULL);
b = strtod(argv[2], NULL);
printf("%lf, %lf, %lf\n", a, b, a - b);
return 0;
}
and i calculate 0.3 - 0.1
> gcc a.c -O3
> ./a.out 0.3 0.1
0.300000 0.100000 0.200000
looks good so far.
but when i calculate in python:
# idle
>>> 0.3 - 0.1
0.19999999999999998
why? Why do the two behave differently. Is there any way to improve Python? Or the authors of Python have tried their best?
CodePudding user response：
why? Why do the two behave differently
Because printing a floating-point number in C using printf with %lf format specifier prints the number with precision of 6 digits after the decimal point, while in python the number is printed using "precision high as needed to represent the actual value". See https://docs.python.org/3.7/library/string.html#formatspec and https://en.cppreference.com/w/cpp/io/c/fprintf and Printf width specifier to maintain precision of floating-point value .
Is there any way to improve Python?
If by the word "improve" you mean to get the same output from python as in C code, you would format it properly. To get the same output as your C code should have outputted, you can:
>>> print("{:f}, {:f}, {:f}".format(0.3, 0.1, 0.3 - 0.1))
0.300000, 0.100000, 0.200000
the authors of Python have tried their best?
I will strongly assume that they did. They are giving an amazing project for free, which is amazing. Consider supporting them, see https://www.python.org/about/help/ .
CodePudding user response：
There is no difference. Print more digits in C and you will see that printf has simply rounded the values.
printf("%.20f, %.20f, %.20f\n", a, b, a - b);