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+---
+language: Fortran
+contributors:
+ - ["Robert Steed", "https://github.com/robochat"]
+filename: learnfortran.f90
+---
+
+Fortran is one of the oldest computer languages. It was developed in the 1950s
+by IBM for numeric calculations (Fortran is an abbreviation of "Formula
+Translation"). Despite its age, it is still used for high-performance computing
+such as weather prediction. However, the language has changed considerably over
+the years, although mostly maintaining backwards compatibility; well known
+versions are FORTRAN 77, Fortran 90, Fortran 95, Fortran 2003, Fortran 2008,
+Fortran 2015, and Fortran 2018.
+
+This overview will discuss the features of Fortran 95 since it is the most
+widely implemented of the more recent specifications and the later versions are
+largely similar (by comparison FORTRAN 77 is a very different language).
+
+```fortran
+
+! This is a comment.
+
+
+program example !declare a program called example.
+
+ ! Code can only exist inside programs, functions, subroutines or modules.
+ ! Using indentation is not required but it is recommended.
+
+
+ ! Declaring Variables
+ ! ===================
+
+ ! All declarations must come before statements and expressions.
+
+ implicit none !prevents dynamic declaration of variables (recommended!)
+ ! Implicit none must be redeclared in every function/program/module...
+
+ ! IMPORTANT - Fortran is case insensitive.
+ real z
+ REAL Z2
+
+ real :: v,x ! WARNING: default initial values are compiler dependent!
+ real :: a = 3, b=2E12, c = 0.01
+ integer :: i, j, k=1, m
+ real, parameter :: PI = 3.1415926535897931 !declare a constant.
+ logical :: y = .TRUE. , n = .FALSE. !boolean type.
+ complex :: w = (0,1) !sqrt(-1)
+ character (len=3) :: month !string of 3 characters.
+
+ real :: array(6) !declare an array of 6 reals.
+ real, dimension(4) :: arrayb !another way to declare an array.
+ integer :: arrayc(-10:10) !an array with a custom index.
+ real :: array2d(3,2) !multidimensional array.
+
+ ! The '::' separators are not always necessary but are recommended.
+
+ ! many other variable attributes also exist:
+ real, pointer :: p !declare a pointer.
+
+ integer, parameter :: LP = selected_real_kind(20)
+ real (kind = LP) :: d !long precision variable.
+
+ ! WARNING: initialising variables during declaration causes problems
+ ! in functions since this automatically implies the 'save' attribute
+ ! whereby values are saved between function calls. In general, separate
+ ! declaration and initialisation code except for constants!
+
+
+ ! Strings
+ ! =======
+
+ character :: a_char = 'i'
+ character (len = 6) :: a_str = "qwerty"
+ character (len = 30) :: str_b
+ character (len = *), parameter :: a_long_str = "This is a long string."
+ !can have automatic counting of length using (len=*) but only for constants.
+
+ str_b = a_str // " keyboard" !concatenate strings using // operator.
+
+
+ ! Assignment & Arithmetic
+ ! =======================
+
+ Z = 1 !assign to variable z declared above (case insensitive).
+ j = 10 + 2 - 3
+ a = 11.54 / (2.3 * 3.1)
+ b = 2**3 !exponentiation
+
+
+ ! Control Flow Statements & Operators
+ ! ===================================
+
+ ! Single-line if statement
+ if (z == a) b = 4 !condition always need surrounding parentheses.
+
+ if (z /= a) then !z not equal to a
+ ! Other symbolic comparisons are < > <= >= == /=
+ b = 4
+ else if (z .GT. a) then !z greater than a
+ ! Text equivalents to symbol operators are .LT. .GT. .LE. .GE. .EQ. .NE.
+ b = 6
+ else if (z < a) then !'then' must be on this line.
+ b = 5 !execution block must be on a new line.
+ else
+ b = 10
+ end if !end statement needs the 'if' (or can use 'endif').
+
+
+ if (.NOT. (x < c .AND. v >= a .OR. z == z)) then !boolean operators.
+ inner: if (.TRUE.) then !can name if-construct.
+ b = 1
+ endif inner !then must name endif statement.
+ endif
+
+
+ i = 20
+ select case (i)
+ case (0) !case i == 0
+ j=0
+ case (1:10) !cases i is 1 to 10 inclusive.
+ j=1
+ case (11:) !all cases where i>=11
+ j=2
+ case default
+ j=3
+ end select
+
+
+ month = 'jan'
+ ! Condition can be integer, logical or character type.
+ ! Select constructions can also be named.
+ monthly: select case (month)
+ case ("jan")
+ j = 0
+ case default
+ j = -1
+ end select monthly
+
+
+ do i=2,10,2 !loops from 2 to 10 (inclusive) in increments of 2.
+ innerloop: do j=1,3 !loops can be named too.
+ exit !quits the loop.
+ end do innerloop
+ cycle !jump to next loop iteration.
+ enddo
+
+
+ ! Goto statement exists but it is heavily discouraged though.
+ goto 10
+ stop 1 !stops code immediately (returning specified condition code).
+10 j = 201 !this line is labeled as line 10
+
+
+ ! Arrays
+ ! ======
+ array = (/1,2,3,4,5,6/)
+ array = [1,2,3,4,5,6] !using Fortran 2003 notation.
+ arrayb = [10.2,3e3,0.41,4e-5]
+ array2d = reshape([1.0,2.0,3.0,4.0,5.0,6.0], [3,2])
+
+ ! Fortran array indexing starts from 1.
+ ! (by default but can be defined differently for specific arrays).
+ v = array(1) !take first element of array.
+ v = array2d(2,2)
+
+ print *, array(3:5) !print all elements from 3rd to 5th (inclusive).
+ print *, array2d(1,:) !print first column of 2d array.
+
+ array = array*3 + 2 !can apply mathematical expressions to arrays.
+ array = array*array !array operations occur element-wise.
+ !array = array*array2d !these arrays would not be compatible.
+
+ ! There are many built-in functions that operate on arrays.
+ c = dot_product(array,array) !this is the dot product.
+ ! Use matmul() for matrix maths.
+ c = sum(array)
+ c = maxval(array)
+ print *, minloc(array)
+ c = size(array)
+ print *, shape(array)
+ m = count(array > 0)
+
+ ! Loop over an array (could have used Product() function normally).
+ v = 1
+ do i = 1, size(array)
+ v = v*array(i)
+ end do
+
+ ! Conditionally execute element-wise assignments.
+ array = [1,2,3,4,5,6]
+ where (array > 3)
+ array = array + 1
+ elsewhere (array == 2)
+ array = 1
+ elsewhere
+ array = 0
+ end where
+
+ ! Implied-DO loops are a compact way to create arrays.
+ array = [ (i, i = 1,6) ] !creates an array of [1,2,3,4,5,6]
+ array = [ (i, i = 1,12,2) ] !creates an array of [1,3,5,7,9,11]
+ array = [ (i**2, i = 1,6) ] !creates an array of [1,4,9,16,25,36]
+ array = [ (4,5, i = 1,3) ] !creates an array of [4,5,4,5,4,5]
+
+
+ ! Input/Output
+ ! ============
+
+ print *, b !print the variable 'b' to the command line
+
+ ! We can format our printed output.
+ print "(I6)", 320 !prints ' 320'
+ print "(I6.4)", 3 !prints ' 0003'
+ print "(F6.3)", 4.32 !prints ' 4.320'
+
+ ! The letter indicates the expected type and the number afterwards gives
+ ! the number of characters to use for printing the value.
+ ! Letters can be I (integer), F (real), E (engineering format),
+ ! L (logical), A (characters) ...
+ print "(I3)", 3200 !print '***' since the number doesn't fit.
+
+ ! we can have multiple format specifications.
+ print "(I5,F6.2,E6.2)", 120, 43.41, 43.41
+ print "(3I5)", 10, 20, 30 !3 repeats of integers (field width = 5).
+ print "(2(I5,F6.2))", 120, 43.42, 340, 65.3 !repeated grouping of formats.
+
+ ! We can also read input from the terminal.
+ read *, v
+ read "(2F6.2)", v, x !read two numbers
+
+ ! To read a file.
+ open(unit=11, file="records.txt", status="old")
+ ! The file is referred to by a 'unit number', an integer that you pick in
+ ! the range 9:99. Status can be one of {'old','replace','new'}.
+ read(unit=11, fmt="(3F10.2)") a, b, c
+ close(11)
+
+ ! To write a file.
+ open(unit=12, file="records.txt", status="replace")
+ write(12, "(F10.2,F10.2,F10.2)") c, b, a
+ close(12)
+
+ ! There are more features available than discussed here and alternative
+ ! variants due to backwards compatibility with older Fortran versions.
+
+
+ ! Built-in Functions
+ ! ==================
+
+ ! Fortran has around 200 functions/subroutines intrinsic to the language.
+ ! Examples -
+ call cpu_time(v) !sets 'v' to a time in seconds.
+ k = ior(i,j) !bitwise OR of 2 integers.
+ v = log10(x) !log base 10.
+ i = floor(b) !returns the closest integer less than or equal to x.
+ v = aimag(w) !imaginary part of a complex number.
+
+
+ ! Functions & Subroutines
+ ! =======================
+
+ ! A subroutine runs some code on some input values and can cause
+ ! side-effects or modify the input values.
+
+ call routine(a,c,v) !subroutine call.
+
+ ! A function takes a list of input parameters and returns a single value.
+ ! However the input parameters may still be modified and side effects
+ ! executed.
+
+ m = func(3,2,k) !function call.
+
+ ! Function calls can also be evoked within expressions.
+ Print *, func2(3,2,k)
+
+ ! A pure function is a function that doesn't modify its input parameters
+ ! or cause any side-effects.
+ m = func3(3,2,k)
+
+
+contains ! Zone for defining sub-programs internal to the program.
+
+ ! Fortran has a couple of slightly different ways to define functions.
+
+ integer function func(a,b,c) !a function returning an integer value.
+ implicit none !best to use implicit none in function definitions too.
+ integer :: a,b,c !type of input parameters defined inside the function.
+ if (a >= 2) then
+ func = a + b + c !the return variable defaults to the function name.
+ return !can return the current value from the function at any time.
+ endif
+ func = a + c
+ ! Don't need a return statement at the end of a function.
+ end function func
+
+
+ function func2(a,b,c) result(f) !return variable declared to be 'f'.
+ implicit none
+ integer, intent(in) :: a,b !can declare and enforce that variables
+ !are not modified by the function.
+ integer, intent(inout) :: c
+ integer :: f !function return type declared inside the function.
+ integer :: cnt = 0 !GOTCHA - initialisation implies variable is
+ !saved between function calls.
+ f = a + b - c
+ c = 4 !altering the value of an input variable.
+ cnt = cnt + 1 !count number of function calls.
+ end function func2
+
+
+ pure function func3(a,b,c) !a pure function can have no side-effects.
+ implicit none
+ integer, intent(in) :: a,b,c
+ integer :: func3
+ func3 = a*b*c
+ end function func3
+
+
+ subroutine routine(d,e,f)
+ implicit none
+ real, intent(inout) :: f
+ real, intent(in) :: d,e
+ f = 2*d + 3*e + f
+ end subroutine routine
+
+
+end program example ! End of Program Definition -----------------------
+
+
+! Functions and Subroutines declared externally to the program listing need
+! to be declared to the program using an Interface declaration (even if they
+! are in the same source file!) (see below). It is easier to define them within
+! the 'contains' section of a module or program.
+
+elemental real function func4(a) result(res)
+! An elemental function is a Pure function that takes a scalar input variable
+! but can also be used on an array where it will be separately applied to all
+! of the elements of an array and return a new array.
+ real, intent(in) :: a
+ res = a**2 + 1.0
+end function func4
+
+
+! Modules
+! =======
+
+! A module is a useful way to collect related declarations, functions and
+! subroutines together for reusability.
+
+module fruit
+ real :: apple
+ real :: pear
+ real :: orange
+end module fruit
+
+
+module fruity
+ ! Declarations must be in the order: modules, interfaces, variables.
+ ! (can declare modules and interfaces in programs too).
+
+ use fruit, only: apple, pear ! use apple and pear from fruit module.
+ implicit none !comes after module imports.
+
+ private !make things private to the module (default is public).
+ ! Declare some variables/functions explicitly public.
+ public :: apple,mycar,create_mycar
+ ! Declare some variables/functions private to the module (redundant here).
+ private :: func4
+
+ ! Interfaces
+ ! ==========
+ ! Explicitly declare an external function/procedure within the module
+ ! (better in general to put functions/procedures in the 'contains' section).
+ interface
+ elemental real function func4(a) result(res)
+ real, intent(in) :: a
+ end function func4
+ end interface
+
+ ! Overloaded functions can be defined using named interfaces.
+ interface myabs
+ ! Can use 'module procedure' keyword to include functions already
+ ! defined within the module.
+ module procedure real_abs, complex_abs
+ end interface
+
+ ! Derived Data Types
+ ! ==================
+ ! Can create custom structured data collections.
+ type car
+ character (len=100) :: model
+ real :: weight !(kg)
+ real :: dimensions(3) !i.e. length-width-height (metres).
+ character :: colour
+ end type car
+
+ type(car) :: mycar !declare a variable of your custom type.
+ ! See create_mycar() routine for usage.
+
+ ! Note: There are no executable statements in modules.
+
+contains
+
+ subroutine create_mycar(mycar)
+ ! Demonstrates usage of a derived data type.
+ implicit none
+ type(car),intent(out) :: mycar
+
+ ! Access type elements using '%' operator.
+ mycar%model = "Ford Prefect"
+ mycar%colour = 'r'
+ mycar%weight = 1400
+ mycar%dimensions(1) = 5.0 !default indexing starts from 1!
+ mycar%dimensions(2) = 3.0
+ mycar%dimensions(3) = 1.5
+
+ end subroutine
+
+ real function real_abs(x)
+ real :: x
+ if (x<0) then
+ real_abs = -x
+ else
+ real_abs = x
+ end if
+ end function real_abs
+
+ real function complex_abs(z)
+ complex :: z
+ ! long lines can be continued using the continuation character '&'
+ complex_abs = sqrt(real(z)**2 + &
+ aimag(z)**2)
+ end function complex_abs
+
+
+end module fruity
+
+```
+
+### More Resources
+
+For more information on Fortran:
+
++ [wikipedia](https://en.wikipedia.org/wiki/Fortran)
++ [Fortran-lang Organization](https://fortran-lang.org/)
++ [Fortran_95_language_features](https://en.wikipedia.org/wiki/Fortran_95_language_features)
++ [fortranwiki.org](http://fortranwiki.org)
++ [www.fortran90.org/](http://www.fortran90.org)
++ [list of Fortran 95 tutorials](http://www.dmoz.org/Computers/Programming/Languages/Fortran/FAQs%2C_Help%2C_and_Tutorials/Fortran_90_and_95/)
++ [Fortran wikibook](https://en.wikibooks.org/wiki/Fortran)
++ [Fortran resources](http://www.fortranplus.co.uk/resources/fortran_resources.pdf)
++ [Mistakes in Fortran 90 Programs That Might Surprise You](http://www.cs.rpi.edu/~szymansk/OOF90/bugs.html)