Advanced Array Usage
Array Initialization
Arrays can be initialized to the same quantity by an array operation:
A=0.0 !equivalent to A(:)=0.0
For small arrays, an array constructor can be written.
I=[1,0,0,0]
The constructor can use the implied do construct:
X=[(real(i),i=1,100)]
If an array of characters is declared, then each element must be initialized to a string of the specified length. The compiler will not pad with blanks.
Array Slices
Subarrays, also known as slices, may be extracted using the colon operator.
REAL, DIMENSION(100) :: A
REAL, DIMENSION(12) :: B
INTEGER, DIMENSION(20,10) :: N
INTEGER, DIMENSION(20) :: C
! Assign values to A and N
B=A(1:12)
C=N(:,i) !ith column of N
The upper bound of the range is always included. If the first bound is omitted, it starts from 1. If the second bound is absent, the slice is extracted to the end of the range. A single colon : represents the full range along a dimension.
Allocatable Arrays
So far we have examined static arrays, whose size is fixed at compile time. Arrays may be sized at runtime by making them allocatable . They are declared with an ALLOCATABLE` attribute and a colon for each dimension.
REAL, ALLOCATABLE, DIMENSION(:) :: A, B
REAL, ALLOCATABLE, DIMENSION(:,:) :: C
If any dimension is allocatable, all must be.
These arrays must be allocated before they are used, so their size must be known at runtime. More than one array may be allocated in a single ALLOCATE statement.
ALLOCATE(A(NMAX),B(MMAX),C(NMAX,MMAX))
Check whether an array is allocated with the intrinsic ALLOCATED(A)
if (allocated(A)) then
do something
else
allocate(A(some_size))
endif
or if we do not need to take any action if A is allocated:
if ( .not. allocated(A)) then
allocate(A(some_size))
Advanced Array Indexing
Arrays can be addressed with arrays of integers (but not logicals).
integer, dimension(1) :: maxtemp
real, dimension(365) :: temps
character(len=5),dimension(365) :: dates
maxtemp=maxloc(temps)
print *, "maximum temp was at ",dates(maxtemp)
Conditionals with Arrays
Logical arrays can be assigned with conditionals derived from other arrays to construct masks. The maxval intrinsic finds the (first) maximum value in an array.
logical, dimension(365) ::is_max
integer :: day
is_max=temps==maxval(temps)
print *, 'Maximum temperature(s) were at'
do day=1,size(is_max)
if (is_max(day)) then
write(*,advance='no'), dates(day)
endif
enddo
write(*,*)
Example
Pulling the array indexing capabilities all together we have a complete program:
program arrayinds
integer, dimension(1) :: maxtemp
integer, dimension(28) :: feb
real, dimension(365) :: x, temps
integer, dimension(365) :: nums
character(len=7),dimension(365) :: dates
character(len=3),dimension(12 ) :: months
integer, dimension(12 ) :: mons=[31,28,31,30,31,30,31,31,30,31,30,31]
character(len=3) :: month
character(len=2) :: day_of_month
integer :: i,m,mday,day
real, parameter :: pi=4.0*atan(1.0)
months=['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov', &
'Dec']
nums=[(real(i),i=1,365)]
x=nums/365.
! Dates as characters
day=0
do m=1,12
month=months(m)
do mday=1,mons(m)
day=day+1
write(day_of_month,'(i2)') mday
dates(day)=day_of_month//'-'//month
enddo
enddo
!Very artificial temperatures (in deg C)
temps=35.*sin(x*pi)
print *, "Temperatures for February"
feb=[(i,i=32,59)]
write(*,*) temps(feb)
maxtemp=maxloc(temps)
print *, "Maximum temp was at ",dates(maxtemp)
end program
This code contains some features, such as string concatenation, that we will study later.
Exercises
-
1 Download the program above. Add the code from the “Conditionals With Arrays” section appropriately. Compare your output to the maxloc (which returns an integer array of the indices of the maximum value).
-
2 Make all arrays that should be the same size as
tempsallocatable, leaving temps static. Allocate all to the size and shape of thetempsarray. For convenience you may introduce an integer that represents the size oftemps. This way we can accommodate data for a leap year by changing just the size oftemps.
Example Solution
program arrayinds
integer, dimension(1) :: maxtemp
integer, dimension(28) :: feb
real, dimension(365) :: temps
real, allocatable, dimension(:) :: x
integer, allocatable, dimension(:) :: nums
logical, allocatable, dimension(:) :: is_max
character(len=7),allocatable, dimension(:) :: dates
character(len=3),dimension(12 ) :: months
integer, dimension(12 ) :: mons=[31,28,31,30,31,30,31,31,30,31,30,31]
character(len=3) :: month
character(len=2) :: day_of_month
integer :: ndays
integer :: i,m,mday,day
real, parameter :: pi=4.0*atan(1.0)
months=['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov', &
'Dec']
ndays=size(temps)
allocate(nums(ndays),x(ndays),dates(ndays),is_max(ndays))
nums=[(real(i),i=1,size(temps))]
x=nums/365.
! Dates as characters
day=0
do m=1,12
month=months(m)
do mday=1,mons(m)
day=day+1
write(day_of_month,'(i2)') mday
dates(day)=day_of_month//'-'//month
enddo
enddo
!Very artificial temperatures (in deg C)
temps=35.*sin(x*pi)
print *, "Temperatures for February"
feb=[(i,i=32,59)]
write(*,*) temps(feb)
maxtemp=maxloc(temps)
print *, "Maximum temp was at ",dates(maxtemp)
is_max = temps==maxval(temps)
print *, "Maximum temperature(s) were at"
do day=1,size(temps)
if (is_max(day)) then
write(*,'(a)',advance='no') dates(day)
endif
enddo
write(*,*)
end program