difference between float and double

This tip submitted by ankit panwar on 2012-03-10 01:14:36. It has been viewed 54156 times.
Rating of 5.4 with 282 votes

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 "<<a;
}

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.

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