Environments, Tables, And Troff Macros - initialize_arrays (7)
NAME
initialize_arrays(7f) - [FORTRAN:FAQ] Initializing small 2D numeric arrays with array constructors
DESCRIPTION
Intuitively, one might assume that if one wants to initialize a small array by rows that something like the following will work:
! DOES NOT WORK integer :: xx(3,5)= [ 1, 2, 3, 4, 5], & [10,20,30,40,50], & [11,22,33,44,55]or perhaps
! DOES NOT WORK integer :: xx(3,5)= [ [ 1, 2, 3, 4, 5], & [10,20,30,40,50], & [11,22,33,44,55] ]Someday something simpler might work, but currently the following syntax is required to specify the values in an intuitive row-column sequence using an array constructor:
integer,save :: xx(3,5)= reshape([&This is because an array constructor can be used to create and assign values only to rank-one arrays. To define arrays of more than one dimension with an array constructor, you must use the RESHAPE(3f) intrinsic function.1, 2, 3, 4, 5, & 10,20,30,40,50, & 11,22,33,44,55 &
],shape(xx),order=[2,1])
Note that the ORDER= option on RESHAPE(3f) is used to allow the values to be specified in row-column order instead of the default behavior, which fills columns first.
Also note that if the expressions are of type character, Fortran 95/90 requires each expression to have the same character length (there is a common compiler extension that extends all strings to the length of the longest value specified, but depending on it reduces portability).
## Printing small arrays in row-column format
When working with small arrays the issue that there is no default Fortran routine for printing an array in row-column order becomes apparent. So lets create a simple solution for integer arrays (PRINT_MATRIX_INT(3f)):
program demo_initialize_arrays ! initializing small arrays implicit none integer,save :: xx(3,5)= reshape([& & 1, 2, 3, 4, 5, & & 10,20,30,40,50, & & 11,22,33,44,-1055 & ],shape(xx),order=[2,1]) call print_matrix_int(’xx array:’,xx) contains subroutine print_matrix_int(title,arr) implicit none character(len=*),parameter::ident= "@(#)print_matrix_int(3f) - print small 2d integer arrays in row-column format" character(len=*),intent(in) :: title integer,intent(in) :: arr(:,:) integer :: i character(len=:),allocatable :: biggest write(*,*)trim(title) ! print title biggest=’ ’ ! make buffer to write integer into write(biggest,’(i0)’)ceiling(log10(real(maxval(abs(arr)))))+1 ! find how many characters to use for integers biggest=’(" > [",*(i’//trim(biggest)//’:,","))’ ! use this format to write a row do i=1,size(arr,dim=1) ! print one row of array at a time write(*,fmt=biggest,advance=’no’)arr(i,:) write(*,’(" ]")’) enddo end subroutine print_matrix_int end program demo_initialize_arraysResults:
xx array: > [ 1, 2, 3, 4, 5 ] > [ 10, 20, 30, 40, 50 ] > [ 11, 22, 33, 44, 55 ]We could do a more robust version that handles REAL and COMPLEX values as well as NaN values, but it has already been done. If you need to print a variety of small matrices see:
dispmodule(3f), "A Fortran 95 module for pretty-printing matrices". Kristjan Jonasson, Department of Computer Science, School of Science and Engineering, University of Iceland, Hjardarhaga 4, 107 Reykjavik, Iceland (jonasson@hi.is).#Initializing a 2D array using DATA statements
Note that DATA statements are very flexible, and allow for perhaps the most intelligible way of specifying small arrays row by row. For example:
! fill rows using DATA statements integer,save,dimension(3,5) :: gg data gg(1,:)/ 1, 2, 3, 4, 5 / data gg(2,:)/ 10, 20, 30, 40, 50 / data gg(3,:)/ 11, 22, 33, 44, 55 /There are other ways to use a DATA statement to fill in row-column order, including use of the SIZE(3f) function and an implied-DO:
! use implied-DO so data can be declared in row-column order integer, dimension(3,5) :: ff DATA (( ff(J,I), I=1,size(ff,dim=2)), J=1,size(ff,dim=1)) / & 01,02,03,04,05, & 10,20,30,40,50, & 11,22,33,44,55 /##Initializing a 2D array from a vector using EQUIVALENCE
Sometimes instead of using RESHAPE(3f) you will see someone initialize a vector and then equivalence it to a multi-dimensional array; especially if the code has a reason to access the data as both a vector and a matrix:
! multi-dimensional row1, row2, .... by equivalence integer,parameter :: d1=3,d2=5 integer :: ee(d1,d2) ! note that the DATA statements could be used to initialize the array instead integer :: e(d1*d2) =[1,10,11, 2,20,22, 3,30,33, 4,40,44, 5,50,55] equivalence (e(1),ee(1,1))##Notes
Remember that for simple initializations vector statements can be used
real :: arr(10,20)=0.0 ! array constructors can be used to define constants, not just vectors integer,parameter :: ii(10,10)=[(i,i=1,size(ii))] ! odd numbers using implied-DOand that if things are too complicated you can just set the values in the executable body of the code.
| Nemo Release 3.1 | initialize_arrays (7) | February 23, 2025 |
