setprecision + fixed - 打印值没有不必要的零

Question

The following code:

double x = 3.14;
double y = 3.14159265359;
cout<<fixed<<setprecision(6)<<x<<", "<<y<<endl;

prints: 3.140000, 3.141593

I want to print values without unnecessary zeros: 3.14, 3.141593 How to do that without using the string and stringstream classes ?

Answer 1

When neither fixed nor scientific format is chosen, the meaning of setprecision is the number of all digits to output (not just after the dot).

Therefore, this should work for you

double x = 3.14;
double y = 3.14159265359;
cout<<setprecision(7)<<x<<", "<<y<<endl;

Output:

3.14, 3.141593

Answer 2

You may use cmath to count the digit of integer parts as shown:

but if number small than 0.1 will be wrong

#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main(){
    double a = -123.456789;
    double b = 4567.45;
    if (a > 1 || a < -1){
        cout << setprecision(5 + int(log10(fabs(a)) + 1)) << a << endl; //anser is -123.45679
    }
    else {
        cout << setprecision(5) << a << endl;    // if a = 0.0123456 answer will be 0.012347
    }
    if (b > 1 || b < -1){
        cout << setprecision(5 + int(log10(fabs(b)) + 1)) << b << endl; //anser is 4567.45
    }
    else {
        cout << setprecision(5) << b << endl;
    }
}