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Norwid Behrnd
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@ -5,8 +5,8 @@ contributors:
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filename: learnfortran.f90
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---
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Fortran is one of the oldest computer languages. It was developed in the 1950s
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by IBM for numeric calculations (Fortran is an abbreviation of "Formula
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Fortran is one of the oldest computer languages. It was developed in the 1950s
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by IBM for numeric calculations (Fortran is an abbreviation of "Formula
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Translation"). Despite its age, it is still used for high-performance computing
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such as weather prediction. However, the language has changed considerably over
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the years, although mostly maintaining backwards compatibility; well known
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@ -30,29 +30,29 @@ program example !declare a program called example.
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! Declaring Variables
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! ===================
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! All declarations must come before statements and expressions.
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implicit none !prevents dynamic declaration of variables (recommended!)
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! Implicit none must be redeclared in every function/program/module...
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! IMPORTANT - Fortran is case insensitive.
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real z
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REAL Z2
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real :: v,x ! WARNING: default initial values are compiler dependent!
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real :: v,x ! WARNING: default initial values are compiler dependent!
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real :: a = 3, b=2E12, c = 0.01
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integer :: i, j, k=1, m
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real, parameter :: PI = 3.1415926535897931 !declare a constant.
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logical :: y = .TRUE. , n = .FALSE. !boolean type.
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complex :: w = (0,1) !sqrt(-1)
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character (len=3) :: month !string of 3 characters.
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real :: array(6) !declare an array of 6 reals.
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real, dimension(4) :: arrayb !another way to declare an array.
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integer :: arrayc(-10:10) !an array with a custom index.
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real :: array2d(3,2) !multidimensional array.
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! The '::' separators are not always necessary but are recommended.
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! many other variable attributes also exist:
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@ -65,8 +65,8 @@ program example !declare a program called example.
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! in functions since this automatically implies the 'save' attribute
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! whereby values are saved between function calls. In general, separate
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! declaration and initialisation code except for constants!
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! Strings
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! =======
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@ -75,7 +75,7 @@ program example !declare a program called example.
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character (len = 30) :: str_b
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character (len = *), parameter :: a_long_str = "This is a long string."
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!can have automatic counting of length using (len=*) but only for constants.
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str_b = a_str // " keyboard" !concatenate strings using // operator.
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@ -98,7 +98,7 @@ program example !declare a program called example.
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! Other symbolic comparisons are < > <= >= == /=
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b = 4
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else if (z .GT. a) then !z greater than a
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! Text equivalents to symbol operators are .LT. .GT. .LE. .GE. .EQ. .NE.
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! Text equivalents to symbol operators are .LT. .GT. .LE. .GE. .EQ. .NE.
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b = 6
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else if (z < a) then !'then' must be on this line.
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b = 5 !execution block must be on a new line.
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@ -145,32 +145,32 @@ program example !declare a program called example.
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cycle !jump to next loop iteration.
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enddo
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! Goto statement exists but it is heavily discouraged though.
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goto 10
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goto 10
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stop 1 !stops code immediately (returning specified condition code).
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10 j = 201 !this line is labeled as line 10
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! Arrays
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! ======
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array = (/1,2,3,4,5,6/)
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array = [1,2,3,4,5,6] !using Fortran 2003 notation.
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arrayb = [10.2,3e3,0.41,4e-5]
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array2d = reshape([1.0,2.0,3.0,4.0,5.0,6.0], [3,2])
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! Fortran array indexing starts from 1.
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! (by default but can be defined differently for specific arrays).
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v = array(1) !take first element of array.
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v = array2d(2,2)
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print *, array(3:5) !print all elements from 3rd to 5th (inclusive).
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print *, array2d(1,:) !print first column of 2d array.
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array = array*3 + 2 !can apply mathematical expressions to arrays.
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array = array*array !array operations occur element-wise.
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!array = array*array2d !these arrays would not be compatible.
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! There are many built-in functions that operate on arrays.
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c = dot_product(array,array) !this is the dot product.
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! Use matmul() for matrix maths.
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@ -180,13 +180,13 @@ program example !declare a program called example.
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c = size(array)
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print *, shape(array)
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m = count(array > 0)
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! Loop over an array (could have used Product() function normally).
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v = 1
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do i = 1, size(array)
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v = v*array(i)
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end do
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! Conditionally execute element-wise assignments.
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array = [1,2,3,4,5,6]
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where (array > 3)
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@ -196,30 +196,30 @@ program example !declare a program called example.
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elsewhere
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array = 0
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end where
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! Implied-DO loops are a compact way to create arrays.
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array = [ (i, i = 1,6) ] !creates an array of [1,2,3,4,5,6]
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array = [ (i, i = 1,12,2) ] !creates an array of [1,3,5,7,9,11]
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array = [ (i**2, i = 1,6) ] !creates an array of [1,4,9,16,25,36]
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array = [ (4,5, i = 1,3) ] !creates an array of [4,5,4,5,4,5]
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! Input/Output
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! ============
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print *, b !print the variable 'b' to the command line
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! We can format our printed output.
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print "(I6)", 320 !prints ' 320'
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print "(I6.4)", 3 !prints ' 0003'
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print "(I6.4)", 3 !prints ' 0003'
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print "(F6.3)", 4.32 !prints ' 4.320'
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! The letter indicates the expected type and the number afterwards gives
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! The letter indicates the expected type and the number afterwards gives
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! the number of characters to use for printing the value.
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! Letters can be I (integer), F (real), E (engineering format),
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! Letters can be I (integer), F (real), E (engineering format),
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! L (logical), A (characters) ...
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print "(I3)", 3200 !print '***' since the number doesn't fit.
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! we can have multiple format specifications.
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print "(I5,F6.2,E6.2)", 120, 43.41, 43.41
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print "(3I5)", 10, 20, 30 !3 repeats of integers (field width = 5).
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@ -230,7 +230,7 @@ program example !declare a program called example.
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read "(2F6.2)", v, x !read two numbers
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! To read a file.
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open(unit=11, file="records.txt", status="old")
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open(unit=11, file="records.txt", status="old")
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! The file is referred to by a 'unit number', an integer that you pick in
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! the range 9:99. Status can be one of {'old','replace','new'}.
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read(unit=11, fmt="(3F10.2)") a, b, c
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@ -241,39 +241,39 @@ program example !declare a program called example.
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write(12, "(F10.2,F10.2,F10.2)") c, b, a
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close(12)
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! There are more features available than discussed here and alternative
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! There are more features available than discussed here and alternative
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! variants due to backwards compatibility with older Fortran versions.
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! Built-in Functions
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! ==================
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! Fortran has around 200 functions/subroutines intrinsic to the language.
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! Examples -
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! Examples -
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call cpu_time(v) !sets 'v' to a time in seconds.
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k = ior(i,j) !bitwise OR of 2 integers.
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v = log10(x) !log base 10.
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i = floor(b) !returns the closest integer less than or equal to x.
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v = aimag(w) !imaginary part of a complex number.
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! Functions & Subroutines
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! =======================
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! A subroutine runs some code on some input values and can cause
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! side-effects or modify the input values.
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call routine(a,c,v) !subroutine call.
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! A function takes a list of input parameters and returns a single value.
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! However the input parameters may still be modified and side effects
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! However the input parameters may still be modified and side effects
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! executed.
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m = func(3,2,k) !function call.
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! Function calls can also be evoked within expressions.
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Print *, func2(3,2,k)
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Print *, func2(3,2,k)
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! A pure function is a function that doesn't modify its input parameters
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! or cause any side-effects.
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m = func3(3,2,k)
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@ -297,7 +297,7 @@ contains ! Zone for defining sub-programs internal to the program.
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function func2(a,b,c) result(f) !return variable declared to be 'f'.
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implicit none
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integer, intent(in) :: a,b !can declare and enforce that variables
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integer, intent(in) :: a,b !can declare and enforce that variables
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!are not modified by the function.
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integer, intent(inout) :: c
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integer :: f !function return type declared inside the function.
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@ -328,14 +328,14 @@ contains ! Zone for defining sub-programs internal to the program.
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end program example ! End of Program Definition -----------------------
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! Functions and Subroutines declared externally to the program listing need
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! Functions and Subroutines declared externally to the program listing need
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! to be declared to the program using an Interface declaration (even if they
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! are in the same source file!) (see below). It is easier to define them within
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! the 'contains' section of a module or program.
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elemental real function func4(a) result(res)
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! An elemental function is a Pure function that takes a scalar input variable
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! but can also be used on an array where it will be separately applied to all
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! but can also be used on an array where it will be separately applied to all
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! of the elements of an array and return a new array.
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real, intent(in) :: a
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res = a**2 + 1.0
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@ -345,7 +345,7 @@ end function func4
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! Modules
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! =======
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! A module is a useful way to collect related declarations, functions and
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! A module is a useful way to collect related declarations, functions and
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! subroutines together for reusability.
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module fruit
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@ -358,7 +358,7 @@ end module fruit
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module fruity
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! Declarations must be in the order: modules, interfaces, variables.
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! (can declare modules and interfaces in programs too).
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use fruit, only: apple, pear ! use apple and pear from fruit module.
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implicit none !comes after module imports.
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@ -367,7 +367,7 @@ module fruity
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public :: apple,mycar,create_mycar
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! Declare some variables/functions private to the module (redundant here).
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private :: func4
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! Interfaces
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! ==========
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! Explicitly declare an external function/procedure within the module
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@ -377,14 +377,14 @@ module fruity
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real, intent(in) :: a
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end function func4
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end interface
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! Overloaded functions can be defined using named interfaces.
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interface myabs
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! Can use 'module procedure' keyword to include functions already
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! defined within the module.
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module procedure real_abs, complex_abs
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end interface
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end interface
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! Derived Data Types
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! ==================
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! Can create custom structured data collections.
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@ -394,19 +394,19 @@ module fruity
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real :: dimensions(3) !i.e. length-width-height (metres).
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character :: colour
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end type car
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type(car) :: mycar !declare a variable of your custom type.
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! See create_mycar() routine for usage.
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! Note: There are no executable statements in modules.
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contains
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subroutine create_mycar(mycar)
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! Demonstrates usage of a derived data type.
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implicit none
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type(car),intent(out) :: mycar
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! Access type elements using '%' operator.
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mycar%model = "Ford Prefect"
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mycar%colour = 'r'
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@ -414,7 +414,7 @@ contains
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mycar%dimensions(1) = 5.0 !default indexing starts from 1!
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mycar%dimensions(2) = 3.0
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mycar%dimensions(3) = 1.5
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end subroutine
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real function real_abs(x)
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@ -425,7 +425,7 @@ contains
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real_abs = x
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end if
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end function real_abs
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real function complex_abs(z)
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complex :: z
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! long lines can be continued using the continuation character '&'
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