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Why doesn't it print the full decimal number?

#include<iostream>

using namespace std;

int main (void)
{
    int test,r,i=0;
    float f;
    cin>>test;
    i++;
    while ( test!=0 )
    {
        cin>>r;
        f = 4*r*r + 0.25;
        cout<<"Case "<<i<<":"<<" "<<f<<"\n";
        i++;
        test--;
    }
    return 0;
}

I made this simple program that evaluates the value of an expression 4*r*r+0.25

. It r

will always be an integer from 1 <= r <= 1000000 here. I have to print the value of the above expression to two decimal places. However, when I enter r

anything -> 50 values , it only shows the result as output.2

, that is, for example, when entering a value as 50

, it shows the result as 10000.2

. Why is this happening? I tried using double and not float but it didn't help.

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3 answers


You need to set the precision, for example:

f = 4*r*r + 0.25;    
cout<<"Case "<<i<<":"<<" "<< setprecision(2) << fixed << f<<"\n";

Output:



10000.25

Be sure to include iomanip

.

+3


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By default, floating point values โ€‹โ€‹are printed with six significant digits. You can use a manipulator setprecision

to specify more precision:

#include <iomanip>

std::cout << std::setprecision(10);  // specify up to 10 significant figures


There are two more problems you will run into for large values:

  • the computation 4*r*r

    uses integer arithmetic and will overflow if the integer type is not large enough. You can use int64_t

    either floating point type to support input values โ€‹โ€‹up to 1,000,000.
  • float

    can only contain up to seven decimal digits and so you still lose precision for values r

    over a few thousand. double

    should be sufficient for input values โ€‹โ€‹up to 1,000,000.
+5


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Let's take a look at some simplified code:

#include <iostream>
#include <iomanip>

int main (void) {
    std::cout << 10000.25 << "\n";
    std::cout << std::setprecision(20);
    std::cout << 10000.25 << "\n";
}

Output:

10000.2
10000.25

If you'd like to see this in action, I've included an example of this at ideone.com .

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