How to display a fixed number of digits in C++ without rounding

Question

I have this code (very basic):

#include <iostream>
#include <iomanip>

using namespace std;
int main()
{
float   a = 0.0,
        b = 0.0,
        c = 0.0;

cout<<"Input a: ";
cin>>a;
cout<<"input b: ";
cin>>b;
cout<<endl;
c = a / b;

cout<<"Result: "<<fixed<<setprecision(2)<<c<<endl;
return 0;
}

When I enter two numbers (say, a = 513 and b = 791) I get 0.65. Calculator shows that the correct answer is 0.648. I understand that my code rounds up the last decimal number but this is not what I want.

How can I get it to where it just stays as 0.64 and not 0.65?

You should check if `cin >> a` returns `true`. Otherwise the input has failed. — Roland Illig, Nov 03 '13 at 22:53

Sergey Kalinichenko · Accepted Answer · 2013-11-03T23:08:36.917

10

If you would like to truncate the value to two decimal places, you can multiply it by 100, truncate to integer, and then divide by 100, like this:

c = a / b;
c = floor(100 * c) / 100;
cout<<"Result: "<<fixed<<setprecision(2)<<c<<endl;

Demo on ideone.

edited Nov 03 '13 at 23:08

answered Nov 03 '13 at 22:54

Sergey Kalinichenko

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which library has trunc cause I have cmath in vs 2012 and its not taking it – user2529011 Nov 03 '13 at 23:03
@user2529011 It's `` (see the demo, I added an `#include` for it). – Sergey Kalinichenko Nov 03 '13 at 23:03
I did and it still doesnt take it – user2529011 Nov 03 '13 at 23:05
error C3861: 'trunc': identifier not found I'm using VS2012 BTW – user2529011 Nov 03 '13 at 23:07
1

@user2529011 Ah, I see, `std::trunc` was added in C++11, and VS 2012 is not C++11. You need to replace it with `floor` (edited, and changed the demo). – Sergey Kalinichenko Nov 03 '13 at 23:07

score 3 · Answer 2 · answered Nov 03 '13 at 22:54

3

You can use trunc to truncate to a certain number of digits:

c = a / b;

// truncate past two decimals:
c = trunc(c * 100) / 100;

cout<<"Result: "<<fixed<<setprecision(2)<<c<<endl;

of for a generic function:

int trunc(double val, int digits)
{
    double pow10 = pow(10,digits);
    return trunc(val * pow10) / pow10;
}

then use

cout << "Result: " << fixed << setprecision(2) << trunc(c,2) << endl;

answered Nov 03 '13 at 22:54

D Stanley

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which library has trunc cause I have cmath in vs 2012 and its not taking it – user2529011 Nov 03 '13 at 23:03
Or you can cast `double` to `int` without using `trunc` method. – ruslaniv Feb 22 '18 at 11:35

How to display a fixed number of digits in C++ without rounding

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