C++ Division – Why 3/5 Results in Zero Despite Being Stored as Double

c++divisionfloating-pointinteger-division

I was working on something else, but everything came out as zero, so I made this minimalistic example, and the output is still 0.

#include <iostream>

int main(int argc, char** argv)
{
  double f=3/5;
  std::cout << f;
  return 0;
}

What am I missing?

Best Answer

You are missing the fact that 3 and 5 are integers, so you are getting integer division. To make the compiler perform floating point division, make one of them a real number:

 double f = 3.0 / 5;

Related Solutions

C++ – Why Do Division Results Appear to Be Off

It's because you're doing integer division. Integer division in C/C++ rounds down. So your following code:

    rPercent = ((rTotal/total)*100);
    dPercent = ((dTotal/total)*100);
    oPercent = ((oTotal/total)*100);

is all rounding down to 0.

To fix this, you should cast to a floating-point type:

    rPercent = (int)((double)rTotal/total*100);
    dPercent = (int)((double)dTotal/total*100);
    oPercent = (int)((double)oTotal/total*100);

EDIT:

The code above could give some weird results due to rounding behavior. Perhaps something like this might be more appropriate since it rounds to the nearest %:

    rPercent = (int)((double)rTotal/total*100 + 0.5);
    dPercent = (int)((double)dTotal/total*100 + 0.5);
    oPercent = (int)((double)oTotal/total*100 + 0.5);

C++ – Why Does 1 / 0.2 Result in 4 Instead of 5?

You have the age old problem of rounding floats which stems from the fact that floating point numbers are base 2 and therefore cannot represent all base 10 numbers perfectly (eg. 0.3). Read this tutorial and this SO question for more information.

As for your example try adding this to your program:

cout << std::setprecision(16) << (float)(a-b)/c << endl;

You will get this print out:

4.999999925494195

And what is (int)4.9999...? It is 4.

Live example.

Best Answer

Related Solutions

C++ – Why Do Division Results Appear to Be Off

C++ – Why Does 1 / 0.2 Result in 4 Instead of 5?

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