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Safe addition: Difference between revisions
→{{header|C}}: Add solution with Microsoft _controlfp().
(→{{header|E}}: Add C using fpsetround().) |
(→{{header|C}}: Add solution with Microsoft _controlfp().) |
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* Round up (toward +infinity).
If a C program can change the rounding mode, then [a + b rounded down, a + b rounded up] solves the task. There is no portable way to change the rounding mode. Microsoft has _controlfp(). Some Unix clones, like [[OpenBSD]], have fpsetround().
{{improve|C|Add
=== _controlfp() ===
{{works with|MinGW}}
<lang c>#include <float.h> /* _controlfp() */
#include <stdio.h> /* printf() */
/*
* Calculates an interval for a + b.
* interval[0] <= a + b
* a + b <= interval[1]
*/
void
safe_add(volatile double interval[2], volatile double a, volatile double b)
{
unsigned int orig;
orig = _controlfp(0, 0);
_controlfp(_RC_DOWN, _MCW_RC); /* round to -infinity */
interval[0] = a + b;
_controlfp(_RC_UP, _MCW_RC); /* round to +infinity */
interval[1] = a + b;
_controlfp(orig, _MCW_RC);
}
int
main()
{
const double nums[][2] = {
{1, 2},
{0.1, 0.2},
{1e100, 1e-100},
{1e308, 1e308},
};
double ival[2];
int i;
for (i = 0; i < sizeof(nums) / sizeof(nums[0]); i++) {
/*
* Calculate nums[i][0] + nums[i][1].
*/
safe_add(ival, nums[i][0], nums[i][1]);
/*
* Print the result. %.17g gives the best output.
* %.16g or plain %g gives not enough digits.
*/
printf("%.17g + %.17g =\n", nums[i][0], nums[i][1]);
printf(" [%.17g, %.17g]\n", ival[0], ival[1]);
printf(" size %.17g\n\n", ival[1] - ival[0]);
}
return 0;
}</lang>
=== fpsetround() ===
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