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;
This will show you the output as "bye 4.12"
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.