Gotchas

Variable Initialization Using Initialization Expression

The following code:

integer :: a = 5

is equivalent to:

integer, save :: a = 5

and not to:

integer :: a
a = 5

See for example this question.

Floating Point Numbers

Assuming the definitions:

integer, parameter :: dp=kind(0.d0)           ! double precision
integer, parameter :: sp=kind(0.0 )           ! single precision

Then the following code:

real(dp) :: a
a = 1.0

is equivalent to:

real(dp) :: a
a = 1.0_sp

and not to:

real(dp) :: a
a = 1.0_dp

As such, always use the _dp suffix as explained in Floating Point Numbers. However, the following code:

real(dp) :: a
a = 1

is equivalent to:

real(dp) :: a
a = 1.0_dp

And so it is safe to assign integers to floating point numbers without loosing any accuracy (but one must be careful about integer division, e.g. 1/2 is equal to 0 and not 0.5).