cout.setf(ios::fixed)

[masiht]

cout.setf(ios::fixed)
please tell me what is this and why do we use this ?

[Zhuge]

This makes cout display floating point values in fixed point notation.

http://en.wikipedia.org/wiki/Fixed-point_arithmetic

[masiht]

how do we know that we have to use this ?

[Zhuge]

Well, that would depend on what type of display you want. Usually a floating point display looks fine but a fixed point display can be useful if you want all the decimal points to line up with a mono spaced font.