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Norwid Behrnd 2022-07-08 19:47:18 +02:00
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@ -5,8 +5,8 @@ contributors:
filename: learnfortran.f90 filename: learnfortran.f90
--- ---
Fortran is one of the oldest computer languages. It was developed in the 1950s 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 by IBM for numeric calculations (Fortran is an abbreviation of "Formula
Translation"). Despite its age, it is still used for high-performance computing Translation"). Despite its age, it is still used for high-performance computing
such as weather prediction. However, the language has changed considerably over such as weather prediction. However, the language has changed considerably over
the years, although mostly maintaining backwards compatibility; well known the years, although mostly maintaining backwards compatibility; well known
@ -30,29 +30,29 @@ program example !declare a program called example.
! Declaring Variables ! Declaring Variables
! =================== ! ===================
! All declarations must come before statements and expressions. ! All declarations must come before statements and expressions.
implicit none !prevents dynamic declaration of variables (recommended!) implicit none !prevents dynamic declaration of variables (recommended!)
! Implicit none must be redeclared in every function/program/module... ! Implicit none must be redeclared in every function/program/module...
! IMPORTANT - Fortran is case insensitive. ! IMPORTANT - Fortran is case insensitive.
real z real z
REAL Z2 REAL Z2
real :: v,x ! WARNING: default initial values are compiler dependent! real :: v,x ! WARNING: default initial values are compiler dependent!
real :: a = 3, b=2E12, c = 0.01 real :: a = 3, b=2E12, c = 0.01
integer :: i, j, k=1, m integer :: i, j, k=1, m
real, parameter :: PI = 3.1415926535897931 !declare a constant. real, parameter :: PI = 3.1415926535897931 !declare a constant.
logical :: y = .TRUE. , n = .FALSE. !boolean type. logical :: y = .TRUE. , n = .FALSE. !boolean type.
complex :: w = (0,1) !sqrt(-1) complex :: w = (0,1) !sqrt(-1)
character (len=3) :: month !string of 3 characters. character (len=3) :: month !string of 3 characters.
real :: array(6) !declare an array of 6 reals. real :: array(6) !declare an array of 6 reals.
real, dimension(4) :: arrayb !another way to declare an array. real, dimension(4) :: arrayb !another way to declare an array.
integer :: arrayc(-10:10) !an array with a custom index. integer :: arrayc(-10:10) !an array with a custom index.
real :: array2d(3,2) !multidimensional array. real :: array2d(3,2) !multidimensional array.
! The '::' separators are not always necessary but are recommended. ! The '::' separators are not always necessary but are recommended.
! many other variable attributes also exist: ! many other variable attributes also exist:
@ -65,8 +65,8 @@ program example !declare a program called example.
! in functions since this automatically implies the 'save' attribute ! in functions since this automatically implies the 'save' attribute
! whereby values are saved between function calls. In general, separate ! whereby values are saved between function calls. In general, separate
! declaration and initialisation code except for constants! ! declaration and initialisation code except for constants!
! Strings ! Strings
! ======= ! =======
@ -75,7 +75,7 @@ program example !declare a program called example.
character (len = 30) :: str_b character (len = 30) :: str_b
character (len = *), parameter :: a_long_str = "This is a long string." character (len = *), parameter :: a_long_str = "This is a long string."
!can have automatic counting of length using (len=*) but only for constants. !can have automatic counting of length using (len=*) but only for constants.
str_b = a_str // " keyboard" !concatenate strings using // operator. str_b = a_str // " keyboard" !concatenate strings using // operator.
@ -98,7 +98,7 @@ program example !declare a program called example.
! Other symbolic comparisons are < > <= >= == /= ! Other symbolic comparisons are < > <= >= == /=
b = 4 b = 4
else if (z .GT. a) then !z greater than a else if (z .GT. a) then !z greater than a
! Text equivalents to symbol operators are .LT. .GT. .LE. .GE. .EQ. .NE. ! Text equivalents to symbol operators are .LT. .GT. .LE. .GE. .EQ. .NE.
b = 6 b = 6
else if (z < a) then !'then' must be on this line. else if (z < a) then !'then' must be on this line.
b = 5 !execution block must be on a new line. b = 5 !execution block must be on a new line.
@ -145,32 +145,32 @@ program example !declare a program called example.
cycle !jump to next loop iteration. cycle !jump to next loop iteration.
enddo enddo
! Goto statement exists but it is heavily discouraged though. ! Goto statement exists but it is heavily discouraged though.
goto 10 goto 10
stop 1 !stops code immediately (returning specified condition code). stop 1 !stops code immediately (returning specified condition code).
10 j = 201 !this line is labeled as line 10 10 j = 201 !this line is labeled as line 10
! Arrays ! Arrays
! ====== ! ======
array = (/1,2,3,4,5,6/) array = (/1,2,3,4,5,6/)
array = [1,2,3,4,5,6] !using Fortran 2003 notation. array = [1,2,3,4,5,6] !using Fortran 2003 notation.
arrayb = [10.2,3e3,0.41,4e-5] arrayb = [10.2,3e3,0.41,4e-5]
array2d = reshape([1.0,2.0,3.0,4.0,5.0,6.0], [3,2]) array2d = reshape([1.0,2.0,3.0,4.0,5.0,6.0], [3,2])
! Fortran array indexing starts from 1. ! Fortran array indexing starts from 1.
! (by default but can be defined differently for specific arrays). ! (by default but can be defined differently for specific arrays).
v = array(1) !take first element of array. v = array(1) !take first element of array.
v = array2d(2,2) v = array2d(2,2)
print *, array(3:5) !print all elements from 3rd to 5th (inclusive). print *, array(3:5) !print all elements from 3rd to 5th (inclusive).
print *, array2d(1,:) !print first column of 2d array. print *, array2d(1,:) !print first column of 2d array.
array = array*3 + 2 !can apply mathematical expressions to arrays. array = array*3 + 2 !can apply mathematical expressions to arrays.
array = array*array !array operations occur element-wise. array = array*array !array operations occur element-wise.
!array = array*array2d !these arrays would not be compatible. !array = array*array2d !these arrays would not be compatible.
! There are many built-in functions that operate on arrays. ! There are many built-in functions that operate on arrays.
c = dot_product(array,array) !this is the dot product. c = dot_product(array,array) !this is the dot product.
! Use matmul() for matrix maths. ! Use matmul() for matrix maths.
@ -180,13 +180,13 @@ program example !declare a program called example.
c = size(array) c = size(array)
print *, shape(array) print *, shape(array)
m = count(array > 0) m = count(array > 0)
! Loop over an array (could have used Product() function normally). ! Loop over an array (could have used Product() function normally).
v = 1 v = 1
do i = 1, size(array) do i = 1, size(array)
v = v*array(i) v = v*array(i)
end do end do
! Conditionally execute element-wise assignments. ! Conditionally execute element-wise assignments.
array = [1,2,3,4,5,6] array = [1,2,3,4,5,6]
where (array > 3) where (array > 3)
@ -196,30 +196,30 @@ program example !declare a program called example.
elsewhere elsewhere
array = 0 array = 0
end where end where
! Implied-DO loops are a compact way to create arrays. ! 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 = 1,6) ] !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, 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 = [ (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] array = [ (4,5, i = 1,3) ] !creates an array of [4,5,4,5,4,5]
! Input/Output ! Input/Output
! ============ ! ============
print *, b !print the variable 'b' to the command line print *, b !print the variable 'b' to the command line
! We can format our printed output. ! We can format our printed output.
print "(I6)", 320 !prints ' 320' print "(I6)", 320 !prints ' 320'
print "(I6.4)", 3 !prints ' 0003' print "(I6.4)", 3 !prints ' 0003'
print "(F6.3)", 4.32 !prints ' 4.320' print "(F6.3)", 4.32 !prints ' 4.320'
! The letter indicates the expected type and the number afterwards gives ! The letter indicates the expected type and the number afterwards gives
! the number of characters to use for printing the value. ! the number of characters to use for printing the value.
! Letters can be I (integer), F (real), E (engineering format), ! Letters can be I (integer), F (real), E (engineering format),
! L (logical), A (characters) ... ! L (logical), A (characters) ...
print "(I3)", 3200 !print '***' since the number doesn't fit. print "(I3)", 3200 !print '***' since the number doesn't fit.
! we can have multiple format specifications. ! we can have multiple format specifications.
print "(I5,F6.2,E6.2)", 120, 43.41, 43.41 print "(I5,F6.2,E6.2)", 120, 43.41, 43.41
print "(3I5)", 10, 20, 30 !3 repeats of integers (field width = 5). print "(3I5)", 10, 20, 30 !3 repeats of integers (field width = 5).
@ -230,7 +230,7 @@ program example !declare a program called example.
read "(2F6.2)", v, x !read two numbers read "(2F6.2)", v, x !read two numbers
! To read a file. ! To read a file.
open(unit=11, file="records.txt", status="old") open(unit=11, file="records.txt", status="old")
! The file is referred to by a 'unit number', an integer that you pick in ! 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'}. ! the range 9:99. Status can be one of {'old','replace','new'}.
read(unit=11, fmt="(3F10.2)") a, b, c read(unit=11, fmt="(3F10.2)") a, b, c
@ -241,39 +241,39 @@ program example !declare a program called example.
write(12, "(F10.2,F10.2,F10.2)") c, b, a write(12, "(F10.2,F10.2,F10.2)") c, b, a
close(12) close(12)
! There are more features available than discussed here and alternative ! There are more features available than discussed here and alternative
! variants due to backwards compatibility with older Fortran versions. ! variants due to backwards compatibility with older Fortran versions.
! Built-in Functions ! Built-in Functions
! ================== ! ==================
! Fortran has around 200 functions/subroutines intrinsic to the language. ! Fortran has around 200 functions/subroutines intrinsic to the language.
! Examples - ! Examples -
call cpu_time(v) !sets 'v' to a time in seconds. call cpu_time(v) !sets 'v' to a time in seconds.
k = ior(i,j) !bitwise OR of 2 integers. k = ior(i,j) !bitwise OR of 2 integers.
v = log10(x) !log base 10. v = log10(x) !log base 10.
i = floor(b) !returns the closest integer less than or equal to x. i = floor(b) !returns the closest integer less than or equal to x.
v = aimag(w) !imaginary part of a complex number. v = aimag(w) !imaginary part of a complex number.
! Functions & Subroutines ! Functions & Subroutines
! ======================= ! =======================
! A subroutine runs some code on some input values and can cause ! A subroutine runs some code on some input values and can cause
! side-effects or modify the input values. ! side-effects or modify the input values.
call routine(a,c,v) !subroutine call. call routine(a,c,v) !subroutine call.
! A function takes a list of input parameters and returns a single value. ! A function takes a list of input parameters and returns a single value.
! However the input parameters may still be modified and side effects ! However the input parameters may still be modified and side effects
! executed. ! executed.
m = func(3,2,k) !function call. m = func(3,2,k) !function call.
! Function calls can also be evoked within expressions. ! Function calls can also be evoked within expressions.
Print *, func2(3,2,k) Print *, func2(3,2,k)
! A pure function is a function that doesn't modify its input parameters ! A pure function is a function that doesn't modify its input parameters
! or cause any side-effects. ! or cause any side-effects.
m = func3(3,2,k) m = func3(3,2,k)
@ -297,7 +297,7 @@ contains ! Zone for defining sub-programs internal to the program.
function func2(a,b,c) result(f) !return variable declared to be 'f'. function func2(a,b,c) result(f) !return variable declared to be 'f'.
implicit none implicit none
integer, intent(in) :: a,b !can declare and enforce that variables integer, intent(in) :: a,b !can declare and enforce that variables
!are not modified by the function. !are not modified by the function.
integer, intent(inout) :: c integer, intent(inout) :: c
integer :: f !function return type declared inside the function. integer :: f !function return type declared inside the function.
@ -328,14 +328,14 @@ contains ! Zone for defining sub-programs internal to the program.
end program example ! End of Program Definition ----------------------- end program example ! End of Program Definition -----------------------
! Functions and Subroutines declared externally to the program listing need ! Functions and Subroutines declared externally to the program listing need
! to be declared to the program using an Interface declaration (even if they ! 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 ! are in the same source file!) (see below). It is easier to define them within
! the 'contains' section of a module or program. ! the 'contains' section of a module or program.
elemental real function func4(a) result(res) elemental real function func4(a) result(res)
! An elemental function is a Pure function that takes a scalar input variable ! 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 ! 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. ! of the elements of an array and return a new array.
real, intent(in) :: a real, intent(in) :: a
res = a**2 + 1.0 res = a**2 + 1.0
@ -345,7 +345,7 @@ end function func4
! Modules ! Modules
! ======= ! =======
! A module is a useful way to collect related declarations, functions and ! A module is a useful way to collect related declarations, functions and
! subroutines together for reusability. ! subroutines together for reusability.
module fruit module fruit
@ -358,7 +358,7 @@ end module fruit
module fruity module fruity
! Declarations must be in the order: modules, interfaces, variables. ! Declarations must be in the order: modules, interfaces, variables.
! (can declare modules and interfaces in programs too). ! (can declare modules and interfaces in programs too).
use fruit, only: apple, pear ! use apple and pear from fruit module. use fruit, only: apple, pear ! use apple and pear from fruit module.
implicit none !comes after module imports. implicit none !comes after module imports.
@ -367,7 +367,7 @@ module fruity
public :: apple,mycar,create_mycar public :: apple,mycar,create_mycar
! Declare some variables/functions private to the module (redundant here). ! Declare some variables/functions private to the module (redundant here).
private :: func4 private :: func4
! Interfaces ! Interfaces
! ========== ! ==========
! Explicitly declare an external function/procedure within the module ! Explicitly declare an external function/procedure within the module
@ -377,14 +377,14 @@ module fruity
real, intent(in) :: a real, intent(in) :: a
end function func4 end function func4
end interface end interface
! Overloaded functions can be defined using named interfaces. ! Overloaded functions can be defined using named interfaces.
interface myabs interface myabs
! Can use 'module procedure' keyword to include functions already ! Can use 'module procedure' keyword to include functions already
! defined within the module. ! defined within the module.
module procedure real_abs, complex_abs module procedure real_abs, complex_abs
end interface end interface
! Derived Data Types ! Derived Data Types
! ================== ! ==================
! Can create custom structured data collections. ! Can create custom structured data collections.
@ -394,19 +394,19 @@ module fruity
real :: dimensions(3) !i.e. length-width-height (metres). real :: dimensions(3) !i.e. length-width-height (metres).
character :: colour character :: colour
end type car end type car
type(car) :: mycar !declare a variable of your custom type. type(car) :: mycar !declare a variable of your custom type.
! See create_mycar() routine for usage. ! See create_mycar() routine for usage.
! Note: There are no executable statements in modules. ! Note: There are no executable statements in modules.
contains contains
subroutine create_mycar(mycar) subroutine create_mycar(mycar)
! Demonstrates usage of a derived data type. ! Demonstrates usage of a derived data type.
implicit none implicit none
type(car),intent(out) :: mycar type(car),intent(out) :: mycar
! Access type elements using '%' operator. ! Access type elements using '%' operator.
mycar%model = "Ford Prefect" mycar%model = "Ford Prefect"
mycar%colour = 'r' mycar%colour = 'r'
@ -414,7 +414,7 @@ contains
mycar%dimensions(1) = 5.0 !default indexing starts from 1! mycar%dimensions(1) = 5.0 !default indexing starts from 1!
mycar%dimensions(2) = 3.0 mycar%dimensions(2) = 3.0
mycar%dimensions(3) = 1.5 mycar%dimensions(3) = 1.5
end subroutine end subroutine
real function real_abs(x) real function real_abs(x)
@ -425,7 +425,7 @@ contains
real_abs = x real_abs = x
end if end if
end function real_abs end function real_abs
real function complex_abs(z) real function complex_abs(z)
complex :: z complex :: z
! long lines can be continued using the continuation character '&' ! long lines can be continued using the continuation character '&'