FORTRAN 90 FAQ: Arrays

Our FORTRAN 90 compiler does not recognize my array declarations. What is wrong with our compiler?

Ok, your program perhaps looks like the following:

PROGRAM  Array
   IMPLICIT  NONE
   INTEGER :: n
   INTEGER, DIMENSION(1:n) :: x
   INTEGER :: i

   READ(*,*)  n
   READ(*,*)  (x(i), i=1, n)
END PROGRAM  Array

I guess this is what you are trying to do. Because you don't know the size of array x(), you read in the number of data items into INTEGER variable n and use n to declare an array of exactly n elements. However, you would definitely get the following message:

   INTEGER, DIMENSION(1:n) :: x
                        ^
cf90-521 f90comp: ERROR ARRAY, File = array1.f90, Line = 4, Column = 25
  Local variable "N" must be a dummy argument or in common to be used in a bounds specification expression.

f90: SunSoft F90 Version 1.0.1.0  (21229283) Fri Nov 14, 1997  18:12:12
f90: COMPILE TIME 0.060000 SECONDS
f90: MAXIMUM FIELD LENGTH 2479728 DECIMAL WORDS
f90: 9 SOURCE LINES
f90: 1 ERRORS, 0 WARNINGS, 0 OTHER MESSAGES, 0 ANSI
f90: CODE: 0 WORDS, DATA: 0 WORDS

While the lower bound and upper bound of an array extent can be constants, PARAMETERs and variables, only formal argument array can use all three types. More precisely, in a main program or a module, the lower bound and upper bound of an array extent must be constants or PARAMETERs. In the above, variable n is used as an upper bound of array x()'s extent in the main program. This is incorrect.

To solve this problem, you should declare the array as follows:

PROGRAM  Array
   IMPLICIT  NONE
   INTEGER, PARAMETER :: SIZE = 10
   INTEGER, DIMENSION(1:SIZE) :: x
   INTEGER :: n, i

   READ(*,*)  n
   READ(*,*)  (x(i), i=1, n)
END PROGRAM  Array

Of course, you should make sure that you won't read in more than 10 elements into array x().