Fortran
Jordan Martin
Steven Devine
Background
•Developed by IBM in the 1950s
•Designed for use in scientific and engineering
fields
•Originally written as FORTRAN
–Convention is caps up to FORTRAN77
–Title caps for Fortran 90 forward
Background
•Brainchild of John Backus as a more friendly
and useable alternative to assembly language
•Fortran’s compiler, released in 1957, was the
first optimizing compiler.
Hello World
program hello
print *, "Hello World!"
end program hello
Current Usage
•Scientific Community
•Engineers
•Super Computing
Readability
•Pros
–Formulas and functions are easily recognizable
–Strongly typed.
–Looping and control statements work in familiar
ways in later versions of Fortran, using do as the
primary key word.
Readability
•Cons
–Case Insensitive
–Whitespace insensitive
–Many built-in functions and types, lacks high
orthogonality
–GOTO
–Column major order for 2D arrays
Writeability
•Pros
–Case insensitive
–Whitespace insensitive
–Powerful functions and algebraic evaluation
Writeability
•Cons
–GOTO is the basis for control structure in older
versions
–Names restricted to 6 characters
–Variables are in scope only for subroutines
Reliability
- The FORTRAN language has not specified evaluation order for expressions
- Compilers are free to evaluate each line as they please
PROGRAM ARGORD
INTEGER
I, F1, F2, F3, F4
EXTERNAL
F1, F2, F3, F4
WRITE (*,*) 'Evaluation order: '
I = F1(I) + F2(I) + F3(I) + F4(I)
END
Short Circuit Evaluation
-The compiler is free to evaluate boolean expressions in any order,
making short-circuit evaluation unreliable
-This code will either work as intended or crash, depending on the
compiler that was used
INTEGER I
REAL ARRAY(100)
...........................
IF ((I .GE. 1) .AND. (I .LE. 100) .AND. ARRAY(I) .GT. 0) THEN
WRITE (*,*) 'Array element is positive '
ENDIF
Strong Type Checking
- FORTRAN uses static (compile time) type checking
-At run time all variables in memory are bit strings without
data type information
-No type checking is done at run time, variables are just
accessed by the starting address of variable
Portability
-FORTRAN revisions are almost completely backwards-compatible and
have only removed obsolete instructions
-FORTRAN 90 introduced KIND type parameters, which are
variables can be declared with arbitrarily specified ranges and
precisions, eliminating platform-based data type issues
-An example of the KIND type:
real (kind=kind(0.0)) r
-kind(0.0) defaults to the processor's built in size for a real,
or the programmer can specify the number of digits
FORTRAN Exception Handling
- In FORTRAN, pointers cannot overlap in memory
- This is done to allow optimization for greater speed in numerical calculations
- All arrays passed to subroutines are guaranteed not to be aliased, so array
elements can be stored in registers for the duration of the subroutine
void transform (float *output, float * input, float * matrix, int n)
{
int i;
for (i=0; i<n; i++)
{
float x = input[i*2+0];
float y = input[i*2+1];
output[i*2+0] = matrix[0] * x + matrix[1] * y;
output[i*2+1] = matrix[2] * x + matrix[3] * y;
}
}
Cost
- Time spent learning a dying niche language
-There are free compilers available that vary in quality and output.
The expensive compilers offer tools and integration into modern programs
like .NET
-Legacy FORTRAN code can be difficult to read and refactor due to age.
Older FORTRAN programs may not have obeyed any recognizable
methodology. Also, GOTO statements.
Modern Uses
- A majority of supercomputers run programs written in FORTRAN
-Monster.com lists about fifty jobs that require FORTRAN experience.
Twenty of those fifty also require security clearance.
-FORTRAN is still the fastest when it comes to computationallyintensive mathematical models, such as weather prediction,
computational science, air and fluid modeling, etc.
Parting Thoughts
-FORTRAN isn't unique. Everything FORTRAN does can also be done
by more powerful languages, starting with C
- FORTRAN is fast with mathematics, but Moore's Law is faster
-FORTRAN was an important step in programming languages,
but it now only caters to the niche market of High Performance Computing.
Sources
http://www.ibiblio.org/pub/languages/fortran/ch1-8.html
http://www.lahey.com/lookat90.htm
http://chronicle.com/blogs/wiredcampus/supercomputers-often-run-outdated-software/818
http://en.wikipedia.org/wiki/Row-major_order
http://fortran.com/