- By default,
Fortran
uses
implicit typing
- In implicit typing
the type of the variable
is
implied :
- Variables with names starting
I, J, K, L,
M and N
are integer variables
- Variables with names starting with a letter other than the 6 letters above, are real variables
Also, you do NOT need to define a variable before using it...
- Variables with names starting
I, J, K, L,
M and N
are integer variables
- Example:
program Hello i = 3.14; // You don't need to define i to use it... print *,"i = ", i; end program Hello
- Example Program:
(Demo above code)
- Prog file: click here
- Note:
If you should hate anything in a programming language, this feature would be it...
With this "feature", you can mistype a variable name and instead of the compiler alerting of your mistake, your program will compile but will not work....
- Fortran (just like C/C++) provides
type conversion
for the cenvenience of the programmer
Example:
program Hello r = 3.14; i = r; print *,"r = ", r, " i = ", i; end program Hello
- Example Program:
(Demo above code)
- Prog file: click here
- F90
can
turn off
implicit typying using the
compiler directive :
implicit none
Example:
program Hello implicit none i = 3.14; ! Compile error print *,"i = ", i; end program Hello
- Example Program:
(Demo above code)
- Prog file: click here
- ADVISE: ALWAYS use EXPLICIT typing
- Defining variables in F90:
Syntax: TYPE VariableName [, VariableName ...] or: TYPE [, ATTRIBUTE] (KIND) :: VariableName [, VariableName ...]
- Variables names:
- Begins with a letter
- Followed by a letter, a digit or underscore
- Case insensitive
- Upto 32 characters in length
- Types of variables: (functional property)
integer integer valued variable real floating point valued variable complex Complex number, is just a PAIR of float variables character Character (not discussed in this course) logical Boolean valued variables ((not discussed in this course)
- Attributes of variables: (quality of variable)
parameter constant value public "Class" usage private "Class" usage external defined outside a function allocatable dynamic allocation, like "new" in C++ intent (in | out | inout ) specify how a parameter variable is used dimension(...) used to define array variables
- Kind of variables: (precision of variable)
4 4 bytes 8 8 bytes
- Examples:
integer i1, i2 ! F77 style is acceptable real r1, r2
integer :: var1, my_var1 ! Single precision integer integer(1) :: var2, my_var2 ! Very short integer (1 byte) integer(2) :: var3, my_var3 ! Medium length integer integer(4) is same as integer integer(8) :: var4, my_var4 ! Double precision integer
real :: var5, my_var5 ! Single precision real double precision :: var6, my_var6 ! Double precision real real(4) is same as single precision real(8) is same as double precision
complex :: var7, my_var7 ! Complex integer complex(8) :: var8, my_var8 ! Double precision complexSorry, no arrays yet... they will be covered in a separate webpage...
-
Example Program:
(integer typed variables) ---
click here
-
Example Program:
(real typed variables) ---
click here
- A complex type variable is a
pair of real
variables
- Fortran has
built-in
complex number arithmetic
Example:
complex :: x1, y1, z1 x1 = (0, 1); y1 = (0, 1) ! x1 = i, y1 = i z1 = x1 * y1 ! i*i = -1
- Example Program:
(Demo above code)
- Prog file: click here
- Type hierarchy:
REAL(8) -> REAL(4) -> INTEGER(8) -> INTEGER(4) -> INTEGER(2) -> INTEGER(1)
- It is always safe to assign a value to
a variable of equal or higher capable type
-
E.g., assigning INTEGER(2) to INTEGER(8) is safe
E.g., assigning INTEGER(8) to REAL(4) is safe
- When you assign a value to
a variable of a less capable type,
there is the potential for information to be lost
or can cause the program to abort due to
overflow .
- Fortran can convert
between
real
and
complex
types
Example:
complex(8) :: x; real(8) :: r; ! ================================================ ! Convert complex to real ! ================================================ x = (123, 456) r = x ! ================================================ ! Convert real to complex ! ================================================ r = 12345 x = r
- Example Program:
(Demo above code)
- Prog file: click here
- A constant is a named variable whose value cannot be
modified (changed)
- Constants are defined in F90 using the
attribute parameter
Example:
integer, parameter :: MAX = 20 real, parameter :: Pi = 3.14159
Notice that constants in Fortran has a type
- Example Program:
(Demo above code)
- Prog file: click here