在C++中可以人为控制浮点数的精度,也就是说可以用流操纵算子setprecision或成员函数percision控制小数点后面的位数。设置了精度以后,该精度对之后所有的输出操作都有效,直到下一次设置精度为止。无参数的成员函数percision返回当前设置的精度。图11.17中的程序用成员函数precision和流操纵算子setprecision打印出了2的平方根表,输出结果的精度从0连续变化到9。
// Fig. 11.17: fig11_17.cpp // Controlling precision of floating-point values #include < iostream.h> #include < iomanip.h> #include < math.h> int main() double root2 = sqrt( 2.0 ); int places; cout << setiosflags(ios::fixed) << "Square root of 2 with precisions 0-9.\n" << "Precision set by the" << "precision member function:" << endl; for ( places = 0; places <= 9; places++ ) { cout.precision( places ); cout << root2 << '\n'; } cout << "\nPrecision set by the" << "setprecision manipulator:\n"; for ( places = 0; places <= 9; places++ ) cout << setprecision( places ) << root2 << '\n'; return 0; }
输出结果:
Square root of 2 with pzecisions 0-9.
Precision set by the precision member function:
1
1.4
1.41
1.414
1.4142
1.41421
1.414214
1.4142136
1.41421356
1.414213562
Precision set by the setprecision manipulator:
1
1.4
1.4l
1.414
1.4142
1.41421 ·
1.414214
1.4142136
1.41421356
1.414213562
图11.17 控制浮点数的精度
FROM: http://blog.163.com/kevinlee_2010/blog/static/16982082020141287163589/