Arrays and strings#
More often than not, we need to store and operate on long lists of numbers as opposed to just the single scalar variables that we have been using so far; in computer programming such lists are called arrays.
Arrays are multidimensional variables that contain more than one value where each value is accessed using one or more indices.
Arrays in Fortran are one-based by default; this means that the first element along any dimension is at index 1.
We can declare arrays of any type. There are two common notations for declaring array variables:
dimension attribute or by appending the array dimensions in parentheses to the variable name.
Example: static array declaration
program arrays implicit none ! 1D integer array integer, dimension(10) :: array1 ! An equivalent array declaration integer :: array2(10) ! 2D real array real, dimension(10, 10) :: array3 ! Custom lower and upper index bounds real :: array4(0:9) real :: array5(-5:5) end program arrays
A powerful feature of the Fortran language is its built-in support for array operations; we can perform operations on all or part of an array using array slicing notation:
Example: array slicing
program array_slice implicit none integer :: i integer :: array1(10) ! 1D integer array of 10 elements integer :: array2(10, 10) ! 2D integer array of 100 elements array1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] ! Array constructor array1 = [(i, i = 1, 10)] ! Implied do loop constructor array1(:) = 0 ! Set all elements to zero array1(1:5) = 1 ! Set first five elements to one array1(6:) = 1 ! Set all elements after five to one print *, array1(1:10:2) ! Print out elements at odd indices print *, array2(:,1) ! Print out the first column in a 2D array print *, array1(10:1:-1) ! Print an array in reverse end program array_slice
Fortran arrays are stored in column-major order; the first index varies fastest.
Allocatable (dynamic) arrays#
So far we have specified the size of our array in our program code—this type of array is known as a static array since its size is fixed when we compile our program.
Quite often, we do not know how big our array needs to be until we run our program, for example, if we are reading data from a file of unknown size.
For this problem, we need
These are allocated while the program is running once we know how big the array needs to be.
Example: allocatable arrays
program allocatable implicit none integer, allocatable :: array1(:) integer, allocatable :: array2(:,:) allocate(array1(10)) allocate(array2(10,10)) ! ... deallocate(array1) deallocate(array2) end program allocatable
Allocatable local arrays are deallocated automatically when they go out of scope.
Example: static character string
program string implicit none character(len=4) :: first_name character(len=5) :: last_name character(10) :: full_name first_name = 'John' last_name = 'Smith' ! String concatenation full_name = first_name//' '//last_name print *, full_name end program string
Example: allocatable character string
program allocatable_string implicit none character(:), allocatable :: first_name character(:), allocatable :: last_name ! Explicit allocation statement allocate(character(4) :: first_name) first_name = 'John' ! Allocation on assignment last_name = 'Smith' print *, first_name//' '//last_name end program allocatable_string
Array of strings#
An array of strings can be expressed in Fortran as an array of
All elements in a
character array have equal length.
However, strings of varying lengths can be provided as input to the array constructor, as shown in the example below.
They will be truncated or right-padded with spaces if they are longer or shorter, respectively, than the declared length of the
Finally, we use the intrinsic function
trim to remove any excess spaces when printing the values to the standard output.
Example: string array
program string_array implicit none character(len=10), dimension(2) :: keys, vals keys = [character(len=10) :: "user", "dbname"] vals = [character(len=10) :: "ben", "motivation"] call show(keys, vals) contains subroutine show(akeys, avals) character(len=*), intent(in) :: akeys(:), avals(:) integer :: i do i = 1, size(akeys) print *, trim(akeys(i)), ": ", trim(avals(i)) end do end subroutine show end program string_array