iomanip, setprecision - inprecise?

[rugae]

Learning about all functions in <iomanip> and was testing out setprecision() and found that something weird is going on...? I did try to do a search on setprecision() limitatations or similar keywords but can't seem to find the right site... can someone please tell me what's going on?

double a = 0.1, b = 0.2, c = 0.3, d = 0.4, e = 0.5;
    double f = 0.6, g = 0.7, h = 0.8, i = 0.9;
    cout << setprecision(20) << a << endl;
    cout << setprecision(20) << b << endl;
    cout << setprecision(20) << c << endl;
    cout << setprecision(20) << d << endl;
    cout << setprecision(20) << e << endl;
    cout << setprecision(20) << f << endl;
    cout << setprecision(20) << g << endl;
    cout << setprecision(20) << h << endl;
    cout << setprecision(20) << i << endl << endl;
    
    float a2 = 0.1, b2 = 0.2, c2 = 0.3, d2 = 0.4, e2 = 0.5;
    float f2 = 0.6, g2 = 0.7, h2 = 0.8, i2 = 0.9;
    cout << setprecision(20) << a2 << endl;
    cout << setprecision(20) << b2 << endl;
    cout << setprecision(20) << c2 << endl;
    cout << setprecision(20) << d2 << endl;
    cout << setprecision(20) << e2 << endl;
    cout << setprecision(20) << f2 << endl;
    cout << setprecision(20) << g2 << endl;
    cout << setprecision(20) << h2 << endl;
    cout << setprecision(20) << i2 << endl;

outputs:

0.10000000000000001000
0.20000000000000001000
0.29999999999999999000
0.40000000000000002000
0.50000000000000000000
0.59999999999999998000
0.69999999999999996000
0.80000000000000004000
0.90000000000000002000

0.10000000149011612000
0.20000000298023224000
0.30000001192092896000
0.40000000596046448000
0.50000000000000000000
0.60000002384185791000
0.69999998807907104000
0.80000001192092896000
0.89999997615814209000

[GreenDay2001]

setprecision does nothting but increase decimal places at right.

For example setprecision(2) for 2 will give 2.00 and setprecision(5) will give 2.00000

Which compiler do you use.

[rugae]

I compiled in cygwin so g++. I didn't mean anything wrong with setprecision() but discovered it while playing with it but why ain't the numbers precise? Isn't that very bad for scientific calculations?

[Infarction]

Floating point numbers are not stored precisely in memory. They are usually stored according to the IEEE 754 standard. If you want exact precision, you need to use a 3rd party library or write your own. For most calculations, however, the small degree of error is acceptable.

[rugae]

I see, so for those scientic calculators in C++ they use other libraries for better precisions instead of the standard one.
Got it, thanks.

[barevhayerable]

#include<iostream>
#include<cstdlib>
#include<ctime>
using namespace std;


int main()
{
    const int n=4;
    int x[n][n] = {0}; int y[n][n] = {0}; int z[n][n] = {0};
    srand((unsigned)time(NULL));
    for(register int i = 0; i < n; i++){
        for(register int j = 0; j < n; j++){
            x[i][j] = rand()%2; y[i][j] = rand()%2;
            cout << x[i][j] << "  "; 
        }
        cout << endl;
    }
    cout << endl;
    for(register int i = 0; i < n; i++){
        for(register int j = 0; j < n; j++){
            cout << y[i][j] << "  "; 
        }
        cout << endl;
    }
    cout << endl;
    for(register int i = 0; i < n; i++){
        for(register int j = 0; j < n; j++){
            for(register int k = 0; k < n; k++){
                if((x[i][k] & y[k][j]) ==1){
                    z[i][j] = 1;
                    break;
                }
            }
        }
    }
	  for(register int i = 0; i < n; i++){
        for(register int j = 0;  j < n; j++){
            cout << z[i][j] << "  "; 
        }
        cout << endl;
    }
    return 0;
}