difference between float and double

In c++ any value like 4.12 is treated as a double by default.

Comparing floats and doubles can be a tricky business because of the difference in precision leading to minute errors. For example:

float a = 4.12;

if(a==4.12)
{
   cout<<"hello";
}
else
{
    cout<<"bye "<
} 


This will show you the output as "bye 4.12" 



Why?



Because by default 4.12 is a double (such as in the if statement or in the assignment 
to our variable), but storing it in a float it loses some precision, and so comparing 
then comparing a double with a float lead to microscopic changes in the precision of 
the number--remember that floats and doubles are not precise. 



Two lessons here: one is that floating point numbers shouldn't be compared directly 
most of the time, and the other is that the default size and type of a hard-coded 
floating point number is double.



For more on floating point numbers, read understanding floating point numbers--accuracy and precision.