Arrays
References: Section 7 in the FORTRAN95 Manual and the Fortran Lecture4
A variable:
a named position of memory.
An array: a collection
of variables of the same type that are referenced by a common name. A specific
element of an array can be accessed by using a subscript, which is a number
indicating the position of the element in the array.
An array must be declared at
the beginning of a program before being used. The declaration defines the type
and the size of the array. For
example:
real x(100)
! x is a real type array and it has 100
elements, x(1), x(2)…..x(100)
integer I(0:10)
! I is an integer type array and it has 11 elements, I(0),
I(1)…..I(10)
dimension x(100), I(0:10) ! x: real type,
I: integer type
When dimension is used to declare an array without a
type-declaration statement, array type is determined implicitly; i.e. all are
real type, except the first letter of the name is I, J, K, L, M or N.
To get access to every subscript of an array, the easiest way is by using
a DO loop, when the number of elements in the array is known. For example, if we
know there are 10 numbers need to be put in x(1) to x(10):
DO I = 1, 10
READ(*,*)
x(i)
END DO
if we need to print those 10 numbers on
screen:
DO I = 1, 10
WRITE(*,*)
x(i)
END DO
if we need to get the average of those 10
numbers, 
:
sum =
0
DO I = 1, 10
sum = sum
+ x(i)
END DO
average =
sum / 10
More information on array declaration and initialization can be found in Section 7 in the FORTRAN95 Manual.
Exercises
1) (optional) Copy the Prog16 (from Fortran Lecture4) into U: drive, save it as prog16.f95 and execute it.
PROGRAM PROG16
DIMENSION
X(100)
!-----------------------------
! read in a long list of numbers and calculate the average
!------------------------
WRITE (*,*)
‘ Please type in a list of real numbers. To end input
type a letter’
N=0
SUM=0.0
AVERAGE=0.0
I=1 ! ARRAY SUBSCRIPT
101 READ
(*,1000, ERR=100) X(I) ! trap
format errors ourselves
N=N+1
SUM=SUM+X(I)
I=I+1
IF (I.GT.100)
GO TO 102
GOTO 101 ! loop until format error
!-----------------------------
! ran out of array memory
!-----------------------------
102 WRITE
(*,*) ‘ONLY ROOM FOR 100 NUMBERS’
!-----------------------------
! user ended input
!with non
numeric character
!------------------------------
100 IF ( N.GT.0 )AVERAGE=SUM/N
WRITE (*,*)
‘Sum of ‘, N ,’ numbers is ‘,SUM, ‘average is ‘,AVERAGE
!-------------------------------
! we can go back and recheck the data
!-------------------------------
NABOVE=0
IF (N.LE.0) GO
TO 106
DO 105
II=1,N
IF(X(II).GT.AVERAGE) NABOVE=NABOVE+1
105 CONTINUE
106 Write
(*,*) ‘number above average-=’,NABOVE
!
-------------------------------
!sort into
numerical order
!-------------------------------