The principle of printf rounding in GCC

• VC + + is using the Round half away from zero

• GCC is using the Round half to even which is also known as banker ' s rounding.

• Nearest integer function in computer science, the Nearest integer function of real number x denoted variously by Round (x), is a function which returns the nearest an integer to X. To avoid ambiguity if operating on half-integers, a rounding rule must be chosen. On most computer implementations, the selected rule was to round half-integers to the nearest even Integer-for example,

Test program on gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.2)

#include <iostream> #include <vector> #include <stdio.h> #include <algorithm>using namespace Std;void printvector (float& elem) {    printf ("Value%f is change to%0.1f \ n", Elem, Elem);} void Testroundrules () {    cout << "using gcc" << Endl;    Float fvalues[] = {3.05f, 3.15f, 3.151f, 3.155f, 3.25f, 3.251f, 3.255f,-0.45f, -0.15f};    Using the vector's constructor, copy the first element of the array from the beginning to the end of the content to the vector    vector<float> vdatas (fvalues, fvalues+sizeof (fvalues)/ sizeof (Fvalues[0]));    For_each (Vdatas.begin (), Vdatas.end (), printvector);}

　　

Output

Using GCC

Value 3.050000 is change to 3.0
Value 3.150000 is change to 3.2
Value 3.151000 is change to 3.2
Value 3.155000 is change to 3.2
Value 3.250000 is change to 3.2
Value 3.251000 is change to 3.3
Value 3.255000 is change to 3.3
value-0.450000 is Change to-0.4
value-0.150000 is Change to-0.2

Note that 3.251 was rounding to 3.3 not 3.2, because 3.3 was nearer to 3.251.

The principle of printf rounding in GCC