Fortran Modern alternatives to historical features Implicit variable types
Example
When Fortran was originally developed memory was at a premium. Variables and procedure names could have a maximum of 6 characters, and variables were often implicitly typed. This means that the first letter of the variable name determines its type.
- variables beginning with i, j, ..., n are
integer - everything else (a, b, ..., h, and o, p, ..., z) are
real
Programs like the following are acceptable Fortran:
program badbadnotgood
j = 4
key = 5 ! only the first letter determines the type
x = 3.142
print*, "j = ", j, "key = ", key, "x = ", x
end program badbadnotgood
You may even define your own implicit rules with the implicit statement:
! all variables are real by default
implicit real (a-z)
or
! variables starting with x, y, z are complex
! variables starting with c, s are character with length of 4 bytes
! and all other letters have their default implicit type
implicit complex (x,y,z), character*4 (c,s)
Implicit typing is no longer considered best practice. It is very easy to make a mistake using implicit typing, as typos can go unnoticed, e.g.
program oops
real :: somelongandcomplicatedname
...
call expensive_subroutine(somelongandcomplEcatedname)
end program oops
This program will happily run and do the wrong thing.
To turn off implicit typing, the implicit none statement can be used.
program much_better
implicit none
integer :: j = 4
real :: x = 3.142
print*, "j = ", j, "x = ", x
end program much_better
If we had used implicit none in the program oops above, the compiler would have noticed immediately, and produced an error.
