Arrays Introduction

In scientific and engineering computing, it is very common to need to manipulate ordered sets of values, such as vectors and matrices. There is a common requirement in many applications to repeat the same sequence of operations on successive sets of data. In order to handle both of these requirements, F provides extensive facilities for grouping a set of items of the same type into an array which can be operated on  either as an object in its own right  or by reference to each of its individual elements.

The array concept A

n A set with n (=6) object, named A n In mathematical terms, we can call A, a vector n And we refer to its elements as A 1, A 2, A 3, …

The array concept n In F, we call such an ordered set of related variables an array n Individual items within an array array elements n A(1), A(2), A(3), …, A(n)

Array Concept In all the programs we have used one name to refer to one location in the computer’s memory. It is possible to refer a complete set of data obtained for example from repeating statements, etc. To do this is to define a group called “array” with locations in the memory so that its all elements are identified by an index or subscript of integer type. An array element is designated by the name of the array along with a parenthesized list of subscript expressions.

The array concept n Subscripts can be: x(10) y(i+4) z(3*i+max(i, j, k)) x(int(y(i)*z(j)+x(k))) x(1)=x(2)+x(4)+1 print*,x(5)

The array concept A A 20 A 31

Array declarations type, dimension(subscript bounds) :: list_of_array_names type, dimension(n:m) :: variable_name_list real, dimension(0:4) :: gravity, pressure integer, dimension(1:100) :: scores logical, dimension(-1, 3) :: table

Examples for Shape Arrays real, dimension ( 0 : 49 ) : : z ! the array “z” has 50 elements real, dimension ( 11 : 60 ) : : a, b, c ! a,b,c has 50 elements real, dimension ( -20 : -1 ) : : x ! the array “x” has 20 elements real, dimension ( -9 : 10 ) : : y ! the array “y” has 20 elements

Array declarations  Up to 7 dimensions  Number of permissible subscripts: rank  Number of elements in a particular dimension:extent  Total number of elements of an array: size  Shape of an array is determined by its rank and the extent of each dimension

Array constants and initial values Since an array consists of a number of array elements, it is necessary to provide values for each of these elements by means of an “ array constructor “. In its simplest form, an array constructor consists of a list of values enclosed between special delimiters :  the first of which consists of the 2 characters : ( /  the second of the 2 characters : / ) ( / value_1, value_2, ……………………., value_n /)

Examples integer, dimension ( 10 ) : : arr arr = (/ 3, 5, 7, 3, 27, 8, 12, 31, 4, 22 /) arr ( 1 ) = 3 arr ( 5 ) = 27 ! the value 27 is storen in the 5 th location of the array “arr” arr ( 7 ) = 12 arr ( 10 ) = 22

Array constructors n (/ value_1, value_2, … /) arr = (/ 123, 234, 567, 987 /) n Regular patterns are common: implied do (/ value_list, implied_do_control /) n arr = (/ (i, i = 1, 10) /) n arr = (/ -1, (0, i = 1, 48), 1 /)

Initialization n You can declare and initialize an array at the same time: integer, dimension(50) :: my_array = (/ (0, i = 1, 50) /)

Input and output n Array elements treated as scalar variables n Array name may appear: whole array n Subarrays can be referred too EXAMPLE : integer, dimension ( 5 ) : : value read *, value read *, value(3)

real, dimension(5) :: p, q integer, dimension(4) :: r print *, p, q(3), r read *, p(3), r(1), q print *, p, q (3), q (4), r print *, q (2) ! displays the value in the 2 nd location of the array “q” print *, p ! displays all the values storen in the array “p” Examples

Using arrays and array elements... The use of array variables within a loop, therefore, greatly increases the power and flexibility of a program. F enables an array to be treated as a single object in its own right, in much the same way as a scalar object. Assignment of an array constant to an array variable will be performed as is seen below : array_name = (/ list_of_values /) array_name = (/ value_1, value_2, ….., value_n /) An array element can be used anywhere that a scalar variable can be used a(2) = t(5) - t(3)*q(2) a(i) = t(j) - f(k)

Using arrays and array elements... n An array can be treated as a single object –Two arrays are conformable if they have the same shape –A scalar, including a constant, is conformable with any array –All intrinsic operations are defined between two conformable objects

Using arrays and array elements.... real, dimension(20) :: a, b, c. a = b*c do i = 1, 20 a(i) = b(i)*c(i) end do Arrays having the same number of elements may be applied to arrays and simple expressions. In this case, operation applied toan array are carried out elementwise.

Example integer, dimension ( 4 ) : : a, b integer, dimension ( 0 : 3 ) : : c integer, dimension ( 6 : 9 ) : : d a = (/ 1, 2, 3, 4 /) b = (/ 5, 6, 7, 8 /) c = (/ -1, 3, -5, 7 /) c(0) c(1) c(2) c(3) a = a + b ! will result a = (/ 6, 8, 10, 12 /) d = 2 * abs ( c ) + 1 ! will result d = (/ 3, 7, 11, 15 /) d(6) d(7) d(8) d(9)

Intrinsic procedures with arrays n Elemental intrinsic procedures with arrays array_1 = sin(array_2) arr_max = max(100.0, a, b, c, d, e) n Some special intrinsic functions: maxval(arr) maxloc(arr) minval(arr) minloc(arr) size(arr) sum(arr)

Sub-arrays n Array sections can be extracted from a parent array in a rectangular grid usinf subscript triplet notation or using vector subscript notation n Subscript triplet: subscript_1 : subscript_2 : stride Similar to the do – loops, a subscript triplet defines an ordered set of subscripts beginning with subscript_1, ending with subscript_2 and considering a seperation of stride between the consecutive subscripts. The value of stride must not be “zero”.

Sub-arrays n Subscript triplet: subscript_1 : subscript_2 : stride n Simpler forms: subscript_1 : subscript_2 subscript_1 : subscript_1 : : stride : subscript_2 : subscript_2 : stride : : stride :

Example real, dimension ( 10 ) : : arr arr ( 1 : 10 ) ! rank-one array containing all the elements of arr. arr ( : ) ! rank-one array containing all the elements of arr. arr ( 3 : 5 ) ! rank-one array containing the elements arr (3), arr(4), arr(5). arr ( : 9 ) ! rank-one array containing the elements arr (1), arr(2),…., arr(9). arr ( : : 4 ) ! rank-one array containing the elements arr (1), arr(5),arr(9).

Example integer, dimension ( 10 ) : : a integer, dimension ( 5 ) : : b, i integer : : j a = (/ 11, 22, 33, 44, 55, 66, 77, 88, 99, 110 /) i = (/ 6, 5, 3, 9, 1 /) b = a ( i ) ! will result b = ( 66, 55, 33, 99, 11 /) a ( 2 : 10 : 2 ) ! will result a = ( 22, 44, 66, 88, 110 /) a ( 1 : 10 : 2 ) = (/ j ** 2, ( j = 1, 5 ) /) ! will result a = ( 1, 4, 9, 16, 25 /) a(1) a(3) a(5) a(7) a(9)

Type of printing sub-arrays work = ( / 3, 7, 2 /) print *, work (1), work (2), work (3) ! or print *, work ( 1 : 3 ) ! or print *, work ( : : 1 ) ! or integer : : i do i = 1, 3, 1 print *, work (i) end do ! or another example print *, sum ( work (1: 3) ) ! or another example print *, sum ( work (1: 3 : 2) )

Type of INPUT of sub-arrays integer, dimension ( 10 ) : : a integer, dimension ( 3 ) : : b. b = a ( 4 : 6 ) ! b (1 ) = a (4) ! b (2 ) = a (5) ! b (3 ) = a (6). a ( 1 : 3 ) = 0! a(1) = a(2) = a(3) = 0 a ( 1 : : 2 ) = 1! a(1) = a(3) =…….= a(9) = 1 a ( 1 : : 2 ) = a ( 2 : : 2 ) + 1 ! a(1) = a(2) +1 ! a(3) = a(4) +1 ! a(5) = a(6) +1 ! etc.

PROBLEM : Write a function that takes 2 real arrays as its arguments returns an array in which each element is the maximum of the 2 corresponding elements in the input array function max_array ( array_1, array_2 ) result (maximum_array) ! this function returns the maximum of 2 arrays on an element-by- element basis ! dummy arguments real, dimension ( : ), intent (in) : : array_1, array_2 ! result variable real, dimension ( size ( array_1 ) ) : : maximum_array ! use the intrinsic function “max” to compare elements maximum_array = max ( array_1, array_2) ! or use a do-loop instead of the intrinsic function max to compare elements ! do i = 1, size (array_2) ! maximum_array ( i ) = max ( array_1 ( i ), array_2 ( i ) ) ! end do end function max_array

Do following problems for practice; n 7.8 n 7.10

If we wish to use an array as an argument to a procedure, it is needed to know the size or shape of the actual arguments used. On the other hand, as mentioned before a procedure does not need to know the details of the calling program unit the program unit called, in turn, also does not need to know anything about the procedure except the information about its arguments which form part of the procedure’s interface.

In fact, if an array is a procedure dummy argument, it must be declared in the procedure as an assumed- shape array. An assumed-shape array is a dummy argument array wose shape, as its mame implies, is not known but which assumes the shape as that of any actual argument that becomes associated with it.

The array specification for an “assumed-shape array” can take 2 forms as can be seen below, where the 2 nd form is equivalent to the 1 st with a “lower bound = 1” and on the other hand, the upper bound in both cases will only be established on entry to the procedure and will be whatever value is necessary to ensure that the extent of dummy array is the same as that of the actual array argument : * ( lower bound : )

Arrays and procedures n Explicit-shape array: no freedom! n Assumed-shape array –If an array is a procedure dummy argument, it must be declared in the procedure as an assumed-shape array!

Arrays and procedures Calling programProcedure Assumed-shape Explicit-shape

Arrays and procedures

Example main program unit: real, dimension ( b : 40 ) : : a subprogram: real, dimension ( d : ) : : x x ( d )  a ( b ) x ( d + 1 )  a ( b + 1 ) x ( d + 2 )  a ( b + 2 ) x ( d + 3 )  a ( b + 3 ).

Example main program unit: real, dimension ( 10 : 40 ) : : a, b. call array_example_l (a,b) subroutine array_example_1 ( dummy_array_1, dummy_array_2) real, intent (inout), dimension : : dummy_array_1, dummy_array_2. subprogram:

Example main program unit real : : p (-5 : 5 ), q ( 100 ). call array_example_2 ( p, q ). subprogram subroutine array_example_2 ( dummy_array_1, dummy_array_2) real, intent (inout), dimension : : dummy_array_1, dummy_array_2)

Example integer, dimension ( 4 ) : : section = (/ 5, 1, 2, 3 /) real, dimension ( 9 ) : : a call array_example_3 ( a ( 4 : 8 : 2 ), a ( section ) ) dummy_array_1 = (/ a (4), a (6), a (8) /) dummy_array_2 = (/ a (5), a (1), a (2), a (3) /) dummy_argument_2 (4) dummy_argument_2 (3) dummy_argument_2 (2) dummy_argument_2 (1)

Example PROBLEM : Write a subroutine that will sort a set of names into alphabetic order. İnitial order After 1 st exc After 2 nd exc After 3 rd exc After 4 th exc After 5 th exc After 6 th exc After 7 th exc

Array-valued functions As is well known, arrays can be passed to procedures as arguments and a subroutine can return information by means of an array. It is convenient for a function to return its result in the form of an array of values rather than as a single scalar value  array-valued function, as can be seen below : function name ( ……………………..) result (arr) real, dimension ( dim ) : : arr. end function name

Array-valued functions n A function can return an array n Such a function is called an array-valued function function name(…..) result(arr) real, dimension(dim) :: arr... end function name Explicit-shape

Example Consider a trivial subroutine that simply adds 2 arrays together, returning the result through a third dummy argument array. subroutine trivial_fun ( x, y ) result (sumxy) real, dimension ( : ), intent (in) : : x, y real, dimension ( size (x) ) : : sumxy end subroutine trivial_fun

Example Write a function that takes 2 real arrays as its arguments, returns an array in which each element is the maximum of the 2 corresponding elements in the input array

function max_array ( array_1, array_2 ) result (maximum_array) ! this function returns the maximum of 2 arrays on an element-by-element basis dummy arguments real, dimension ( : ), intent (in) : : array_1, array_2 ! result variable real, dimension ( size ( array_1 ) ) : : maximum_array ! use the intrinsic function “max” to compare elements maximum_array = max ( array_1, array_2) ! or use a do-loop instead of the intrinsic function max to compare elements do i = 1, size (array_2) maximum_array ( i ) = max ( array_1 ( i ), array_2 ( i ) ) end do end function max_array