INTRODUCTION
Essential Lahey Fortran 90 is a Programming Language just like C/C++. It is essentially a Compiler that interprets sequences of Fortran language statements and has them executed by the computer. After successful execution, results are output on the screen. Fortran is DOS based and may not come with any text editor for writing the source codes. But this can easily be overcome since all operating systems have some kind of text editor. The procedure for opening a text editor in MS Windows will be explained shortly.
Because of it’s DOS based, Fortran does not easily plot graphs like Matlab. So it is recommended to export output data to other applications (e.g. excel) for graphical manipulation.
GETTING STARTED WITH FORTRAN 90
From Start on MS Windows OS, select Essential Lahey Fortran 90. The window of Figure 1 below appears
Figure 1: Fortran Startup Window
As the figure shows, Fortran 90 startup window does nothing but point to the default directory of the system. Fortran 90 does not open any special text editor for you to start inserting your program codes. The Start up window’s main purpose is to compile your source code once it’s been written. But it provides three useful tips to the novice to get started That is:
 Type elf90 to see all options &switches
 Usage: elf90 filenames [switches]
 To quit, type exit
NOTE: It is recommended that you work on two separate windows:
 An MSDOS window for editing your source codes (programs)
 The Essential Lahey Fortran 90 Startup window just for compiling, running, and executing your programs.
OPENING A TEXT EDITOR
 Select Start/Command prompt .
 type edit at the MSDOS prompt to bring up an MSDOS window. See procedure on Figure 2.
If done successfully, a DOS  based text editor appears. Insert your program codes and saved the file to the appropriate directory. See saving procedure below.
Figure 2: Opening a text editor in DOS
SAVING A FORTRAN 90 SOURCE CODE FILE
A Fortran 90 file is saved with extension “.f90”. Example: my_first_program.f90
COMPILING A SOURCE CODE
From the Fortran window prompt:
 Locate the source file you want to compile. Use dos commands to do so as follows:
 dir to list the files in the current (default) directory.
 Type the drive name where your source file was saved, e. g. a:\..
 use cd command to move to the directory where you saved the source code. Example: if your source code was saved on a:\ drive under the directory name fortran_directory, type cd fortran_directory. Type dir to list files under fortran_directory and make sure my_first_program.f90 source file is there
 compile the source code. For instance, to compile my_first_program.f90, type:
elf90 my_first_program.f90
IF my_first_program.f90 compiles correctly, Fortran will create a few other files. Among them, an executable named my_first_program.exe. That is the file you will actually run. To do so, type my_first_program. You may exclude the .exe extension. Follow the procedures as prompted by the program.
DATA TYPES NOTATION
Type

Notation

Example

Integer

INTEGER

INTEGER :: a

Real

REAL

REAL :: a

Character

CHARACTER

CHARACTER :: s

Complex Assignment

COMPLEX

COMPLEX :: b

Complex output

(real_a, imaginary_b)

(a,b)

Operator

Notation

Example

Addition

+

a + b

Subtraction



a  b

Multiplication

*

a*b

Division

/

a/b

Exponentiation

**

a**b (a to power of b)

RELATIONAL LOGIC OPERATORS
Operator

Notation

Example

Equal to

= =

a= =b

Not equal to

/=

a/=b

Greater than

>

a>b

Greater than or equal

>=

a>=b

Less than

<

a 
Less than or equal

<=

a<=b

COMBINATIONAL LOGIC OPERATORS
Operator Function

Notation

Example

Logical AND

.AND.

a .AND. b

Logical OR

.OR.

a .OR. b

Logical Equivalence

.EQV.

a .EQV. b

Logical Nonequivalence

.NEQV.

a .NEQV. b

Logical NOT

.NOT.

.NOT. a
 SOME COMMON FORTRAN INBEDDED FUNCTIONS
Function

Notation & Argument

Comment

Square Root

SQRT(x)

Square root of x

Absolute Value

ABS(x)

Absolute value of x

Sine Value

SIN(x)

Sine of x

Cosine Value

COS(x)

Cosine of x

Tangent Value

TAN(x)

Tangent of x

Arc Sine Value

ASIN(x)

Inverse sine of x

Arc Cosine Value

ACOS(x)

Inverse cosine of x

Arc tangent

ATAN(x)

Inverse Tangent of x

Exponential e^{x}

EXP(x)

e to the x power

Base e logarithm

LOG(x)

Natural log of x

Base 10 logarithm

LOG10(x)

Base 10 log of x

Modulo

MOD(a,b)

Remainder of a/b

Maximum

MAX(a,b)

Returns max of a, b

Minimum

MIN(a,b)

Returns min of a, b

BASIC FORTRAN PROGRAMMING STRUCTURE
The basic Fortran Programming structure will be illustrated with a simple example.
EXAMPLE 1:
Write a program that prompts the user to enter two integers to standard input (keyboard). The program then multiplies the two integers and displays the result to standard output (screen).
Program source code:
PROGRAM mul
IMPLICIT NONE
INTEGER :: i, j, k
WRITE (*,*) 'This program multiplies two integers'
WRITE (*,*) 'Enter the integers: '
READ (*,*) i, j
k = i*j ! Take the product of the two numbers
WRITE (*, *) 'Result =',k ! Output result to screen
STOP
END PROGRAM mul
The source code starts with PROGRAM mul and ends with STOP then END PROGRAM mul. mul is the name of the program. All Fortran programs follow this basic structure. IMPLICIT NONE is used to turn off default typing in Fortran. Must be used to prevent errors in a program where variables types must explicitly be declared.
Note also that comments are preceded by “!”
COMPILING mul.f90 source code
The procedure below compiles mul.f90 source code:
Z:\FORTRAN>elf90 mul.f90
The line above indicates that mul.f90 was saved in z:\ drive under a directory created by the user, and named FORTRAN.
elf90 mul.f90 is the command that compiles the source code.
Note that Z:\FORTRAN directory was set in the Fortran window before compiling the program. See section “Compiling a Source Code” for details.
RUNNING A PROGRAM
The procedure below runs mul.f90 source code if it was compiled successfully:
Z:\FORTRAN> mul
PROGRAM OUTPUT
This program multiplies two integers
Enter the integers: 57 13
Result = 741
Program completed
Press Enter to Continue.
MORE PROGRAMMING EXAMPLES
Compute the square root of the product of two real numbers. If one of the input numbers is negative, the program must announce that the result will be a complex number.
The program uses IF statement as well as logical .AND., .OR. statements. If one of the input numbers is negative, the program announces that the result will be a complex number.
NOTE: Fortran does not compute the square root of a negative number to obtain a complex number. For this reason, the program switches to a slightly different computation method (i.e. multiplying the arguments of SQRT by 1)
See program code on next page
PROGRAM ff
! Purpose: Compute the square root of the product of two real numbers
IMPLICIT NONE
REAL :: x, y
REAL :: z ! Square Root of product(x,y)
WRITE (*,*) 'Please enter the two real values: '
READ (*,*) x, y
IF (((x < 0) .AND. (y>0)).OR. ((y<0) .AND. (x>0))) THEN
WRITE (*,*) 'The product is negative ==> Complex Answer'
z = SQRT(x*y)
WRITE (*,*) 'Result =',z,'j'
END IF
! Multiply the two integers and
z = SQRT(x*y) ! take the square root of the product
WRITE (*, *) 'Result =',z ! Output result to screenWS
STOP
END PROGRAM ff
EXAMPLES OF SQRT RESULTS OUTPUT
a)
Please enter the two real values: 12.33 4.7
The product is negative ==> Complex Answer
Result = 7.61256 j
SQRT argument negative (see "SQRT Function" in the Essential Lahey Fortran 90
Reference).
Error occurred at line 18 of file ff.f90.
Program completed
Note the error message in the result although the argument of SQRT() was made positive.
b)
Please enter the two real values: 8.67 13
Result = 10.6165
Program completed
Press Enter to Continue.
c)
Please enter the two real values: 98 33.49
Result = 57.2889
Program completed
Press Enter to Continue.
EXAMPLE 3: SINGLE PHASE POWER COMPUTATION
Compute the current, real, reactive, apparent power, and the power factor supplied to the load of the single phase circuit below.
I
+
Vs_{S }Z = R+jX (Z_{Z})_{ }

Source Code:
PROGRAM power
! Purpose: Compute the current, real, reactive, apparent power, and
! the power factor of supplied to a load.
IMPLICIT NONE
! Declare input parameters
REAL :: Vsource, Theta_source, Theta_sc ! Voltage and angle of source
COMPLEX :: Z_load ! To be input in the form (R,X) > R+jX
! Declare Output values
REAL :: I_current, Z, PF, Preal, Preactive, Papparent
REAL :: Theta, Theta_z, Theta_i
! Initialize input parameters
WRITE (*,*) 'Please input the source voltage '
WRITE (*,*) 'and angle (in degrees) respectively.'
WRITE (*,*) '(Example: 250 0) '
READ (*,*) Vsource, Theta_source
WRITE (*, *) 'Input complex components of load Z of the form'
WRITE (*,*) 'Z = R+jX (input example: 1200, 75) '
READ (*,*) Z_load
! Compute output results
Theta_sc = Theta_source*3.141593/180 !Degrees to radians
Theta_z = ATAN(AIMAG(Z_load)/REAL(Z_load)) ! Angle due to load
Theta_i = Theta_sc  Theta_z ! Current angle
Theta = Theta_sc  Theta_i ! p.f. angle
PF = COS(Theta) ! Power Factor
Z = ABS(Z_load) ! Load Magnitude
I_current = Vsource/Z ! Current Magnitude
Preal = Vsource*I_current*PF ! Real Power
Preactive = Vsource*I_current*SIN(Theta) ! Reactive Power
Papparent = Vsource*I_current ! Apparent Power
! Output computed results
WRITE (*,*) 'Load Current: ', I_current,'Amps'
WRITE (*,*) 'Real Power: ', Preal,'Watts'
WRITE (*,*) 'Reactive Power: ', Preactive,'Vars'
WRITE (*,*) 'Apparent Power: ', Papparent,'VA'
WRITE (*,*) 'Power factor: ', PF
WRITE (*,*) 'At an angle of: ', Theta*180/3.141593,'degrees'
STOP
END PROGRAM power
REMARK: Note the computation of Theta_z in the program. AIMAG(Z_load) is the imaginary part of Z_load REAL(Z_load), the real part.
Z:\FORTRAN>power
Please input the source voltage
and angle (in degrees) respectively.
(Example: 250 0) 450 30
Input complex components of load Z of the form
Z = R+jX (input example: 1200, 75) 2100, 173
Load Current: 0.213562 Amps
Real Power: 95.7786 Watts
Reactive Power: 7.89033 Vars
Apparent Power: 96.1030 VA
Power factor: 0.996624
At an angle of: 4.70945 degrees
Program completed
Press Enter to Continue.
EXAMPLE 4: DELTATOY YTODELTA IMPEDANCES CONVERSION
Zab
a b a b
Za Zb
Zca Zbc
Zc
c
c
 Y Y 
Source code:
PROGRAM convert
IMPLICIT NONE
REAL :: Zab, Zbc, Zca ! Impedances in a Delta Connection
REAL :: Za, Zb, Zc ! Inpedance in a Y Connection
CHARACTER (len=1) :: num ! Selection identifier
WRITE (*,*) 'Convert Threephase impedances:'
WRITE (*,*) 'Delta to Y equivalent impedances and vice versa.'
WRITE (*,*) ' '
DO
WRITE (*,*) 'Press 1 to convert from Delta to Y'
WRITE (*,*) 'Press 2 to convert from Y to Delta: '
WRITE (*,*) 'Press ANY OTHER KEY to quit.'
WRITE (*,*) 'Choice: '
READ (*,*) num
SELECT CASE (num)
CASE ('1')
WRITE (*,*) 'Enter the values of Zab, Zbc, Zca: '
READ (*,*) Zab, Zbc, Zca
Za = (Zab*Zca)/(Zab+Zbc+Zca)
Zb = (Zbc*Zab)/(Zab+Zbc+Zca)
Zc= (Zbc*Zca)/(Zab+Zbc+Zca)
WRITE (*,*) 'Yconnection equivalent impedances:'
WRITE (*,*) 'Za =',Za
WRITE (*,*) 'Zb =',Zb
WRITE (*,*) 'Zc =',Zc
WRITE (*,*) ' '
CASE ('2')
WRITE (*,*) 'Enter the values of Za, Zb, Zc: '
READ (*,*) Za, Zb, Zc
Zab = (Za*Zb + Zb*Zc + Zc*Za)/Zc
Zbc = (Za*Zb + Zb*Zc + Zc*Za)/Za
Zbc = (Za*Zb + Zb*Zc + Zc*Za)/Za
Zca = (Za*Zb + Zb*Zc + Zc*Za)/Zb
WRITE (*,*) 'Deltaconnection equivalent impedances:'
WRITE (*,*) 'Zab =',Zab
WRITE (*,*) 'Zbc =',Zbc
WRITE (*,*) 'Zca =',Zca
WRITE (*,*) ' '
CASE DEFAULT
WRITE (*,*) 'You have chosen to quit. Good bye...'
EXIT
END SELECT
END DO
STOP
END PROGRAM convert
A CONVERSION COMPUTATION EXAMPLE
Z:\FORTRAN>convert
Convert Threephase impedances:
Delta to Y equivalent impedances and vice versa.
Press 1 to convert from Delta to Y
Press 2 to convert from Y to Delta:
Press ANY OTHER KEY to quit.
Choice: 1
Enter the values of Zab, Zbc, Zca: 200 350 80
Yconnection equivalent impedances:
Za = 25.3968 ohms
Zb = 111.111 ohms
Zc = 44.4444 ohms
Press 1 to convert from Delta to Y
Press 2 to convert from Y to Delta:
Press ANY OTHER KEY to quit.
Choice: 2
Enter the values of Za, Zb, Zc: 250 250 250
Deltaconnection equivalent impedances:
Zab = 750.000 ohms
Zbc = 750.000 ohms
Zca = 750.000 ohms
Press 1 to convert from Delta to Y
Press 2 to convert from Y to Delta:
Press ANY OTHER KEY to quit.
Choice: k
You have chosen to quit. Good bye...
Program completed
Press Enter to Continue.
TABLE OF CONTENTS
INTRODUCTION 1
GETTING STARTED WITH FORTRAN 90 1
OPENING A TEXT EDITOR 2
SAVING A FORTRAN 90 SOURCE CODE FILE 2
COMPILING A SOURCE CODE 2
RUNNING THE EXECUTABLE FILE 3
DATA TYPES NOTATION 3
ARITHMETIC OPERATORS 3
RELATIONAL LOGIC OPERATORS 3
COMBINATIONAL LOGIC OPERATORS 3
SOME COMMON FORTRAN INBEDDED FUNCTIONS 4
BASIC FORTRAN PROGRAMMING STRUCTURE 4
EXAMPLE 1: 4
COMPILING mul.f90 source code 5
RUNNING A PROGRAM 5
EXAMPLE 2: SQRT COMPUTATION 5
EXAMPLES OF SQRT RESULTS OUTPUT 6
EXAMPLE 3: SINGLE PHASE POWER COMPUTATION 7
POWER COMPUTATION OUTPUT EXAMPLE 8
EXAMPLE 4: DELTATOY YTODELTA IMPEDANCES CONVERSION 9
A CONVERSION COMPUTATION EXAMPLE 10
TABLE OF FIGURES
Figure 1: Fortran Startup Window 1
Figure 2: Opening a text editor in DOS 2
