Fortran Tutorial 1

Fortran Tutorial #1

Introduction

In this series of short tutorials, we introduce ourselves to the FORTRAN language (FORmula TRANslating). It is a language that is still implemented in high performance algorithms.

This series of examples is based on the informatics course that took place 10 years ago at the engineering university in Rosario, Argentina.

First Lines

There are some compilers out there that allow us to compile our code, in principle we will focus on the syntax of our codes, based mainly on FORTRAN 90.

A FORTRAN code begins and ends with the following lines.

PROGRAM program_name

...

END PROGRAM program_name

FORTRAN does not require us to declare all the variables that we use, if it finds an undeclared variable it sets the type of data that we are assigning to it at that moment.

For the compiler to inform us if there are undeclared variables, the following statement is used.

IMPLICIT NONE

Each line can be up to 136 characters long, with multiple statements separated by semicolons.

Inline comments begin with !

program calc

integer a,b !Hello, I'm a comment

Below we see two examples of codes, an algorithm that calculates the sum of two variables and another that calculates the product of two variables

Example #1

PROGRAM sum ! This program sum two integer numbers.

IMPLICIT NONE

INTEGER a,b,c ! Define the variables a, b and c such as integer.

read(*,*) a ! Read the value of a from the keyboard

read(*,*) b ! Read the value of b from the keyboard

c = a+b ! Assign to variable c the result of the value of a plus the value of b

write(*,*) c ! Show in the screen the value of c

END PROGRAM ! End the program

Example #2

PROGRAM prod ! This program multiplies two numbers and displays the result.

IMPLICIT NONE

REAL num1,num2,resul ! Define the variables num1, num2 and result such as real.

WRITE(*,*) 'Enter two numbers' ! Shows a message on the screen

READ(*,*) num1,num2 ! Read the value of num1 and num2 from the keyboard

resul=num1*num2 ! Assign to variable resul the result of the operation

WRITE(*,*) 'The result is: ',resul ! Show in the screen the value of c

END PROGRAM ! End the program

The characters allowed in FORTRAN are the following: (It is important to note that Fortran is not case sensitive).

0 1 2 3 4 5 6 7 8 9

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

a b c d e f g h i j k l m n o p q r s t u v w x y z

' " ( ) + - * / : = ! & $ ; < > % ? , .

The reserved words (not including library functions) are:

CALL, CASE, CHARACTER, COMPLEX, DEFAULT, DIMENSION, DO, END, ELSE, EXIT, FUNCTION, IF, IMPLICIT, IN, INOUT, INTEGER, INTENT, LOGICAL,OPEN, OUT, PARAMETER, PROGRAM, READ, REAL, RESULT, RETURN SAVE, SELECT, STOP, SUBROUTINE, THEN, TYPE, WHERE, WRITE

Identifiers are names that are used to identify programs, variables, constants, library functions, user functions, etc. Identifiers must begin with a letter, and can be followed by up to 30 characters (letters, digits, and underscore). Identifiers are not case sensitive. It is usual to capitalize the key words of the language (IF, DO, END, ...). For the name of the variables it is common to use lowercase letters, with the first letter capitalized. Often several words are joined by capitalizing the first letter or separating them with an underscore.

In the next tutorial, we will start with data types and perhaps arithmetic expressions.

See you next time!

Fortran Tutorial 1 ("Introduction")