Fortran tutorial

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AbouttheTutorial
Fortran was originally developed by a team at IBM in 1957 for scientific calculations. Later
developments made it into a high level programming language. In this tutorial, we will
learn the basic concepts of Fortran and its programming code.
Audience
This tutorial is designed for the readers who wish to learn the basics of Fortran.
Prerequisites
This tutorial is designed for beginners. A general awareness of computer programming
languages is the only prerequisite to make the most of this tutorial.
Copyright&Disclaimer
 Copyright 2017 by Tutorials Point (I) Pvt. Ltd.
All the content and graphics published in this e-book are the property of Tutorials Point (I)
Pvt. Ltd. The user of this e-book is prohibited to reuse, retain, copy, distribute or republish
any contents or a part of contents of this e-book in any manner without written consent
of the publisher.
We strive to update the contents of our website and tutorials as timely and as precisely as
possible, however, the contents may contain inaccuracies or errors. Tutorials Point (I) Pvt.
Ltd. provides no guarantee regarding the accuracy, timeliness or completeness of our
website or its contents including this tutorial. If you discover any errors on our website or
in this tutorial, please notify us at contact@tutorialspoint.com

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TableofContents
About the Tutorial····································································································································i
Audience··················································································································································i
Prerequisites············································································································································i
Copyright & Disclaimer·····························································································································i
Table of Contents····································································································································ii
1. FORTRAN ─ OVERVIEW········································································································1
Facts about Fortran·································································································································1
2. FORTRAN ─ ENVIRONMENT SETUP······················································································2
Setting up Fortran in Windows················································································································2
How to Use G95 ······································································································································3
3. FORTRAN ─ BASIC SYNTAX···································································································4
A Simple Program in Fortran ···················································································································4
Basics ······················································································································································5
Identifier ·················································································································································5
Keywords ················································································································································6
4. FORTRAN ─ DATA TYPES······································································································8
Integer Type············································································································································8
Real Type ················································································································································9
Complex Type········································································································································10
Logical Type ··········································································································································11
Character Type······································································································································11
Implicit Typing·······································································································································11

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5. FORTRAN ─ VARIABLES······································································································12
Variable Declaration······························································································································12
6. FORTRAN ─ CONSTANTS····································································································15
Named Constants and Literals···············································································································15
7. FORTRAN ─ OPERATORS····································································································17
Arithmetic Operators ····························································································································17
Relational Operators·····························································································································19
Logical Operators ··································································································································21
Operators Precedence in Fortran ··········································································································23
8. FORTRAN ─ DECISIONS······································································································26
If…then Construct ·································································································································27
If… then… else Construct·······················································································································29
if...else if...else Statement·····················································································································31
Nested If Construct ·······························································································································33
Select Case Construct ····························································································································34
Nested Select Case Construct ················································································································37
9. FORTRAN ─ LOOPS·············································································································39
do Loop·················································································································································40
do-while Loop ·······································································································································43
Nested Loops ········································································································································45
Loop Control Statements·······················································································································46
Exit Statement ······································································································································47
Cycle Statement ····································································································································48
Stop Statement ·····································································································································50

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10. FORTRAN ─ NUMBERS·······································································································51
Integer Type··········································································································································51
Real Type ··············································································································································52
Complex Type········································································································································53
The Range, Precision, and Size of Numbers···························································································55
The Kind Specifier ·································································································································57
11. FORTRAN ─ CHARACTERS ··································································································59
Character Declaration ···························································································································59
Concatenation of Characters·················································································································60
Some Character Functions·····················································································································61
Checking Lexical Order of Characters ····································································································64
12. FORTRAN ─ STRINGS··········································································································66
String Declaration ·································································································································66
String Concatenation·····························································································································67
Extracting Substrings·····························································································································68
Trimming Strings ···································································································································70
Left and Right Adjustment of Strings·····································································································70
Searching for a Substring in a String······································································································71
13. FORTRAN ─ ARRAYS···········································································································73
Declaring Arrays····································································································································73
Assigning Values····································································································································74
Some Array Related Terms····················································································································76
Passing Arrays to Procedures ················································································································76
Array Sections ·······································································································································79
Array Intrinsic Functions ·······················································································································81

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14. FORTRAN ─ DYNAMIC ARRAYS···························································································99
Use of Data Statement ························································································································100
Use of Where Statement·····················································································································102
15. FORTRAN ─ DERIVED DATA TYPES ···················································································104
Defining a Derived data type···············································································································104
Accessing Structure Members ·············································································································104
Array of Structures······························································································································106
16. FORTRAN ─ POINTERS ·····································································································109
Declaring a Pointer Variable················································································································109
Allocating Space for a Pointer ·············································································································109
Targets and Association ······················································································································110
17. FORTRAN ─ BASIC INPUT OUTPUT···················································································114
Formatted Input Output······················································································································114
The Format Statement ························································································································119
18. FORTRAN ─ FILE INPUT OUTPUT······················································································120
Opening and Closing Files····················································································································120
19. FORTRAN ─ PROCEDURES································································································127
Function··············································································································································127
Subroutine ··········································································································································129
Recursive Procedures··························································································································131
Internal Procedures·····························································································································133
20. FORTRAN ─ MODULES·····································································································135
Syntax of a Module ·····························································································································135
Using a Module into your Program······································································································135

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Accessibility of Variables and Subroutines in a Module·······································································137
21. FORTRAN ─ INTRINSIC FUNCTIONS··················································································140
Numeric Functions ······························································································································140
Mathematical Functions······················································································································143
Numeric Inquiry Functions ··················································································································145
Floating-Point Manipulation Functions ·······························································································145
Bit Manipulation Functions ·················································································································146
Character Functions ····························································································································147
Kind Functions·····································································································································148
Logical Function ··································································································································148
22. FORTRAN ─ NUMERIC PRECISION····················································································149
The Kind Attribute·······························································································································149
Inquiring the Size of Variables·············································································································150
Obtaining the Kind Value ····················································································································151
23. FORTRAN ─ PROGRAM LIBRARIES····················································································153
24. FORTRAN ─ PROGRAMMING STYLE·················································································154
25. FORTRAN ─ DEBUGGING PROGRAM················································································155
The gdb Debugger ·······························································································································155
The dbx Debugger ·······························································································································156

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Fortran, as derived from Formula Translating System, is a general-purpose, imperative
programming language. It is used for numeric and scientific computing.
Fortran was originally developed by IBM in the 1950s for scientific and engineering
applications. Fortran ruled this programming area for a long time and became very popular
for high performance computing, because.
It supports:
 Numerical analysis and scientific computation
 Structured programming
 Array programming
 Modular programming
 Generic programming
 High performance computing on supercomputers
 Object oriented programming
 Concurrent programming
 Reasonable degree of portability between computer systems
FactsaboutFortran
 Fortran was created by a team, led by John Backus at IBM in 1957.
 Initially the name used to be written in all capital, but current standards and
implementations only require the first letter to be capital.
 Fortran stands for FORmula TRANslator.
 Originally developed for scientific calculations, it had very limited support for character
strings and other structures needed for general purpose programming.
 Later extensions and developments made it into a high level programming language
with good degree of portability.
 Original versions, Fortran I, II and III are considered obsolete now.
 Oldest version still in use is Fortran IV, and Fortran 66.
 Most commonly used versions today are : Fortran 77, Fortran 90, and Fortran 95.
1. FORTRAN ─ OVERVIEW

Fortran
8
 Fortran 77 added strings as a distinct type.
 Fortran 90 added various sorts of threading, and direct array processing.

Fortran
9
SettingupFortraninWindows
G95 is the GNU Fortran multi-architechtural compiler, used for setting up Fortran in Windows.
The windows version emulates a unix environment using MingW under windows. The installer
takes care of this and automatically adds g95 to the windows PATH variable.
You can get the stable version of G95 from here :
2. FORTRAN ─ ENVIRONMENT SETUP

Fortran
10
HowtoUseG95
During installation, g95 is automatically added to your PATH variable if you select the option
“RECOMMENDED”. This means that you can simply open a new Command Prompt window
and type “g95” to bring up the compiler. Find some basic commands below to get you started.
Command Description
g95 –c hello.f90 Compiles hello.f90 to an object file named hello.o
g95 hello.f90 Compiles hello.f90 and links it to produce an
executable a.out
g95 -c h1.f90 h2.f90 h3.f90 Compiles multiple source files. If all goes well,
object files h1.o, h2.o and h3.o are created
g95 -o hello h1.f90 h2.f90 h3.f90 Compiles multiple source files and links them
together to an executable file named 'hello'
Command line options for G95:
-c Compile only, do not run the linker.
-o Specify the name of the output file, either an object file or the executable.
Multiple source and object files can be specified at once. Fortran files are indicated by names
ending in ".f", ".F", ".for", ".FOR", ".f90", ".F90", ".f95", ".F95", ".f03" and ".F03". Multiple
source files can be specified. Object files can be specified as well and will be linked to form
an executable file.

Fortran
11
A Fortran program is made of a collection of program units like a main program, modules,
and external subprograms or procedures.
Each program contains one main program and may or may not contain other program units.
The syntax of the main program is as follows:
program program_name
implicit none
! type declaration statements
! executable statements
end program program_name
ASimplePrograminFortran
Let’s write a program that adds two numbers and prints the result:
program addNumbers
! This simple program adds two numbers
implicit none
! Type declarations
real :: a, b, result
! Executable statements
a = 12.0
b = 15.0
result = a + b
print *, 'The total is ', result
3. FORTRAN ─ BASIC SYNTAX

Fortran
12
end program addNumbers
When you compile and execute the above program, it produces the following result:
The total is 27.0000000
Please note that:
 All Fortran programs start with the keyword program and end with the keywordend
program, followed by the name of the program.
 The implicit none statement allows the compiler to check that all your variable types
are declared properly. You must always use implicit none at the start of every
program.
 Comments in Fortran are started with the exclamation mark (!), as all characters after
this (except in a character string) are ignored by the compiler.
 The print * command displays data on the screen.
 Indentation of code lines is a good practice for keeping a program readable.
 Fortran allows both uppercase and lowercase letters. Fortran is case-insensitive,
except for string literals.
Basics
The basic character set of Fortran contains:
 the letters A ... Z and a ... z
 the digits 0 ... 9
 the underscore (_) character
 the special characters = : + blank - * / ( ) [ ] , . $ ' ! " % & ; < > ?
Tokens are made of characters in the basic character set. A token could be a keyword, an
identifier, a constant, a string literal, or a symbol.
Program statements are made of tokens.
Identifier
An identifier is a name used to identify a variable, procedure, or any other user-defined item.
A name in Fortran must follow the following rules:
 It cannot be longer than 31 characters.
 It must be composed of alphanumeric characters (all the letters of the alphabet, and
the digits 0 to 9) and underscores (_).

Fortran
13
 First character of a name must be a letter.
 Names are case-insensitive
Keywords
Keywords are special words, reserved for the language. These reserved words cannot be used
as identifiers or names.
The following table, lists the Fortran keywords:
Non-I/O keywords
allocatable allocate assign assignment block data
call case character common complex
contains continue cycle data deallocate
default do double precision else else if
elsewhere end block data end do end function end if
end interface end module end program end select end subroutine
end type end where entry equivalence exit
external function go to if implicit
in inout integer intent interface
intrinsic kind len logical module
namelist nullify only operator optional
out parameter pause pointer private

Fortran
14
program public real recursive result
return save select case stop subroutine
target then type type() use
Where While
I/O related keywords
backspace close endfile format inquire
pen print read rewind Write

Fortran
15
Fortran provides five intrinsic data types, however, you can derive your own data types as
well. The five intrinsic types are:
 Integer type
 Real type
 Complex type
 Logical type
 Character type
IntegerType
The integer types can hold only integer values. The following example extracts the largest
value that can be held in a usual four byte integer:
program testingInt
implicit none
integer :: largeval
print *, huge(largeval)
end program testingInt
When you compile and execute the above program it produces the following result:
2147483647
Note that the huge() function gives the largest number that can be held by the specific
integer data type. You can also specify the number of bytes using the kind specifier. The
following example demonstrates this:
program testingInt
implicit none
!two byte integer
4. FORTRAN ─ DATA TYPES

Fortran
16
integer(kind=2) :: shortval
!four byte integer
integer(kind=4) :: longval
!eight byte integer
integer(kind=8) :: verylongval
!sixteen byte integer
integer(kind=16) :: veryverylongval
!default integer
integer :: defval
print *, huge(shortval)
print *, huge(longval)
print *, huge(verylongval)
print *, huge(veryverylongval)
print *, huge(defval)
end program testingInt
32767
2147483647
9223372036854775807
170141183460469231731687303715884105727
2147483647
RealType
It stores the floating point numbers, such as 2.0, 3.1415, -100.876, etc.
Traditionally there are two different real types, the default real type and double
precisiontype.

Fortran
17
However, Fortran 90/95 provides more control over the precision of real and integer data
types through thekindspecifier, which we will study in the chapter on Numbers.
The following example shows the use of real data type:
program division
implicit none
! Define real variables
real :: p, q, realRes
! Define integer variables
integer :: i, j, intRes
! Assigning values
p = 2.0
q = 3.0
i = 2
j = 3
! floating point division
realRes = p/q
intRes = i/j
print *, realRes
print *, intRes
end program division
When you compile and execute the above program it produces the following result:
0.666666687
0

Fortran
18
ComplexType
This is used for storing complex numbers. A complex number has two parts, the real part and
the imaginary part. Two consecutive numeric storage units store these two parts.
For example, the complex number (3.0, -5.0) is equal to 3.0 – 5.0i
We will discuss Complex types in more detail, in the Numbers chapter.
LogicalType
There are only two logical values: .true. and .false.
CharacterType
The character type stores characters and strings. The length of the string can be specified by
len specifier. If no length is specified, it is 1.
For example,
character (len=40) :: name
name = “Zara Ali”
The expression, name(1:4) would give the substring “Zara”.
ImplicitTyping
Older versions of Fortran allowed a feature called implicit typing, i.e., you do not have to
declare the variables before use. If a variable is not declared, then the first letter of its name
will determine its type.
Variable names starting with i, j, k, l, m, or n, are considered to be for integer variable and
others are real variables. However, you must declare all the variables as it is good
programming practice. For that you start your program with the statement:
implicit none
This statement turns off implicit typing.

Fortran
19
A variable is nothing but a name given to a storage area that our programs can manipulate.
Each variable should have a specific type, which determines the size and layout of the
variable's memory; the range of values that can be stored within that memory; and the set
of operations that can be applied to the variable.
The name of a variable can be composed of letters, digits, and the underscore character. A
name in Fortran must follow the following rules:
 It cannot be longer than 31 characters.
 It must be composed of alphanumeric characters (all the letters of the alphabet, and
the digits 0 to 9) and underscores (_).
 First character of a name must be a letter.
 Names are case-insensitive.
Based on the basic types explained in previous chapter, following are the variable types:
Type Description
Integer It can hold only integer values.
Real It stores the floating point numbers.
Complex It is used for storing complex numbers.
Logical It stores logical Boolean values.
Character It stores characters or strings.
VariableDeclaration
Variables are declared at the beginning of a program (or subprogram) in a type declaration
statement.
Syntax for variable declaration is as follows:
5. FORTRAN ─ VARIABLES

Fortran
20
type-specifier :: variable_name
For example,
integer :: total
real :: average
complex :: cx
logical :: done
character(len=80) :: message ! a string of 80 characters
Later you can assign values to these variables, like,
total = 20000
average = 1666.67
done = .true.
message = “A big Hello from Tutorials Point”
cx = (3.0, 5.0) ! cx = 3.0 + 5.0i
You can also use the intrinsic function cmplx, to assign values to a complex variable:
cx = cmplx (1.0/2.0, -7.0) ! cx = 0.5 – 7.0i
cx = cmplx (x, y) ! cx = x + yi
Example
The following example demonstrates variable declaration, assignment and display on screen:
program variableTesting
implicit none
! declaring variables
integer :: total
real :: average
complex :: cx
logical :: done

Fortran
21
character(len=80) :: message ! a string of 80 characters
!assigning values
total = 20000
average = 1666.67
done = .true.
message = "A big Hello from Tutorials Point"
cx = (3.0, 5.0) ! cx = 3.0 + 5.0i
Print *, total
Print *, average
Print *, cx
Print *, done
Print *, message
end program variableTesting
When the above code is compiled and executed, it produces the following result:
20000
1666.67004
(3.00000000, 5.00000000 )
T
A big Hello from Tutorials Point

Fortran
22
The constants refer to the fixed values that the program cannot alter during its execution.
These fixed values are also called literals.
Constants can be of any of the basic data types like an integer constant, a floating constant,
a character constant, a complex constant, or a string literal. There are only two logical
constants : .true. and .false.
The constants are treated just like regular variables, except that their values cannot be
modified after their definition.
NamedConstantsandLiterals
There are two types of constants:
 Literal constants
 Named constants
A literal constant have a value, but no name.
For example, following are the literal constants:
Type Example
Integer constants 0 1 -1 300 123456789
Real constants 0.0 1.0 -1.0 123.456 7.1E+10 -52.715E-30
Complex constants (0.0, 0.0) (-123.456E+30, 987.654E-29)
Logical constants .true. .false.
Character constants "PQR" "a" "123'abc$%#@!"
" a quote "" "
'PQR' 'a' '123"abc$%#@!'
' an apostrophe '' '
6. FORTRAN ─ CONSTANTS

Fortran
23
A named constant has a value as well as a name.
Named constants should be declared at the beginning of a program or procedure, just like a
variable type declaration, indicating its name and type. Named constants are declared with
the parameter attribute. For example,
real, parameter :: pi = 3.1415927
Example
The following program calculates the displacement due to vertical motion under gravity.
program gravitationalDisp
! this program calculates vertical motion under gravity
implicit none
! gravitational acceleration
real, parameter :: g = 9.81
! variable declaration
real :: s ! displacement
real :: t ! time
real :: u ! initial speed
! assigning values
t = 5.0
u = 50
! displacement
s = u * t - g * (t**2) / 2
! output
print *, "Time = ", t
print *, 'Displacement = ',s
end program gravitationalDisp

Fortran
24
When the above code is compiled and executed, it produces the following result:
Time = 5.00000000
Displacement = 127.374992

Fortran
25
An operator is a symbol that tells the compiler to perform specific mathematical or logical
manipulations. Fortran provides the following types of operators:
 Arithmetic Operators
 Relational Operators
 Logical Operators
Let us look at all these types of operators one by one.
ArithmeticOperators
Following table shows all the arithmetic operators supported by Fortran. Assume
variable Aholds 5 and variable B holds 3 then:
Operator Description Example
+ Addition Operator, adds two operands. A + B will give 8
- Subtraction Operator, subtracts second operand
from the first.
A - B will give 2
* Multiplication Operator, multiplies both operands. A * B will give 15
/ Division Operator, divides numerator by de-
numerator.
A / B will give 1
** Exponentiation Operator, raises one operand to the
power of the other.
A ** B will give 125
Example
Try the following example to understand all the arithmetic operators available in Fortran:
program arithmeticOp
7. FORTRAN ─ OPERATORS

Fortran
26
! this program performs arithmetic calculation
implicit none
! variable declaration
integer :: a, b, c
! assigning values
a = 5
b = 3
! Exponentiation
c = a ** b
! output
print *, "c = ", c
! Multiplication
c = a * b
! output
print *, "c = ", c
! Division
c = a / b
! output
print *, "c = ", c
! Addition
c = a + b
! output
print *, "c = ", c

Fortran
27
! Subtraction
c = a - b
! output
print *, "c = ", c
end program arithmeticOp
c = 125
c = 15
c = 1
c = 8
c = 2
RelationalOperators
Following table shows all the relational operators supported by Fortran. Assume
variable Aholds 10 and variable B holds 20, then:
Operator Equivalent Description Example
== .eq. Checks if the values of two operands are equal
or not, if yes then condition becomes true.
(A == B) is
not true.
/= .ne. Checks if the values of two operands are equal
or not, if values are not equal then condition
becomes true.
(A != B) is
true.
> .gt. Checks if the value of left operand is greater
than the value of right operand, if yes then
condition becomes true.
(A > B) is
not true.
< .lt. Checks if the value of left operand is less than
the value of right operand, if yes then
condition becomes true.
(A < B) is
true.

Fortran
28
>= .ge. Checks if the value of left operand is greater
than or equal to the value of right operand, if
yes then condition becomes true.
(A >= B) is
not true.
<= .le. Checks if the value of left operand is less than
or equal to the value of right operand, if yes
then condition becomes true.
(A <= B) is
true.

Fortran
29
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