Simple Procedural Languages: FORTRAN and C

Lecture 9:

Dolores Zage

Procedural Languages Are the closest thing to “typical” programming

language for demonstrating common language features.

Language consists of a series of procedures that execute when called

Each procedure consists of a sequence of statements

Each statement manipulates data that either is local to the procedure, a parameter passed in from the calling procedure, or defined globally

Procedural Languages

Local data for each procedure are stored in an activation record associated with that procedure

data stored in such activation records typically have relatively simple types, such as integer, real, character, and Boolean

FORTRAN and C (examples) both have similar virtual computer and

execution characteristics

FORTRAN and C

FORTRAN is characterized by static storage allocation (activation records can be created during language translation) FORTRAN 90 changes this

C is characterized by dynamic activation record creation.

Both were designed for run-time efficiency

FORTRAN

Widely used for scientific and engineering computation

A text editor is used to create the program A FORTRAN compiler translates the

program into executable form. A linker is used to merge subprograms, the

main program, and run-time library routines into one executable program

Hello World in FORTRANPROGRAM TRIVIAL

INTEGER I

I = 2

IF( I .GE. 2 ) CALL PRINTIT

STOP

END

SUBROUTINE PRINTIT

PRINT *, “Hello World”

RETURN

END

FORTRAN Overview No run-time management is provided All storage is allocated statically before program

execution begins Only a restricted set of data types provided: four

types of numeric data( integer, real, complex, and double-precision real), Boolean data (called logical), arrays, character strings, and files

An extensive set of arithmetic operations and mathematical functions is provided

FORTRAN Overview

Relational and Boolean operations are provided

Simple selection from arrays using subscripting

Both sequential and direct-access files are supported

Flexible set of input-output facilities and format specification features are available

FORTRAN’s weakest point

Restricted data structuring facilities Essentially arrays and character strings of

fixed length No facilities for type definitions or data

abstraction. Subprograms provide the only abstraction

mechanism

Other weakness Statement sequence relies heavily on statement labels

and GOTO statements although each revision has added more nested control structures FORTRAN 77 added the IF … THEN … ELSE

Only two levels of referencing environment are provided: local and global the global environment may be divided into separate common

environments (called COMMON blocks) No special features are included to support construction of

large programs beyond the provision for independent compilation

FORTRAN example to sum an array PROGRAM MAIN

PARAMETER (MAXSIZ=99)

REAL A(MAXSIZ)

10 READ(5, 100, END=999) K

100 FORMAT(I5)

IF( K .LE. 0 .OR. K .GT. MAXSIZ) STOP

READ *, (A(I), I=1,K)

PRINT *, (A(I), I=1,K)

PRINT *, “SUM=“, SUM(A,K)

GO TO 10

999 PRINT *, “ALL DONE”

STOP

END

Labels are placed in columns 2 - 5

A C in column 1 indicates a comment

All statements require one line

unless column 6 has a nonblank

character, indicating continuation of

previous statement

Statements begin in column 7

Notes on FORTRAN main MAXSIZ is a programmer defined constant. The

value of 99 is actually what the translator uses. By default arrays begin at 1. K is undeclared so it is an integer 5 is used for keyboard input, 6 is used for display

output optional END=999 refers to end-of-file exception * denotes a list directed read which parses numbers

sequentially from the input stream FORTRAN 77 has no WHILE construct

Notes on FORTRAN main Variable names are 1 to 6 characters long,

begin with a letter, and contain letters and digits FORTRAN 90 - 31 chars long and also the _

FORTRAN is case insensitive blanks are ignored ( all 3 represent the same

statement) DO 20 I = 1, 20 DO20I=1,20 D O 2 0 I = 1 , 2 0

The FORTRAN Function SUM

C SUMMATION SUBPROGRAM

FUNCTION SUM(V,N)

REAL V(N)

SUM = 0.0

DO 20 I = 1, N

SUM = SUM + V(I)

20 CONTINUE

RETURN

END

Notes on FORTRAN SUM

Information from the main program is not used to pass information to the compiler. The erroneous line:

FUNCTION SUM (V, N, M) would also compile but may fail when the loader

tries to merge this subprogram with MAIN Although array is given as V(N), it refers to the

statically allocated parameter, real array A(99)

Language Evaluation Has amazing staying power Compiling techniques for arithmetic expression

evaluation, sparse matrix calculations and large array processing are excellent

numerical calculations that rely on computing derivatives, numerical integration, fluid dynamics, and solving for roots of complex equations - FORTRAN is as efficient as any language

Language Evaluation

Individual lines are easily understandable overall structure is opaque because the

heavy use of statement labels and GOTOs restriction of identifiers

C

Developed in 1972 by Dennis Ritchie and Ken Thompson of AT&T Bell Telephone Laboratories

Compact syntax and efficient execution characteristics have made it a popular systems programming language, even though it is a general-purpose language

C Hello World #include <stdio.h>

void main()

{

int i;

i=2;

if(i >= 2) printit();

}

void printit()

{ printf(“Hello World”);}

Semicolons now terminate statements - need no continuation marker

The C concept

You generally cannot just look at grammar, but at

the C language the C preprocessor the C interface assumptions (the h files) the C library

A C module

Consists of global declarations and a sequence of functions

multiple modules are linked together to form one executable program

each function may invoke other functions and access data local to that function or global

each function has a local activation record which is dynamic and allows for recursion

C function

Each function has access to global data there is no real block, thus activation records

do not need static links, only dynamic links

Data items Each data item has an efficient implementation Multi-dim arrays are built from one dimensional one dimensional have indices beginning with 0

which avoids the need for descriptors (attributes of data type)

the starting address of the array is the same as the virtual origin which avoids all complex array calculations

Data items C has pointers there is an equivalence between arrays and

pointers, permitting programs to use whichever method of access is most appropriate

Strings are implemented as arrays of characters, completely transparent so that strings may be access as strings, as arrays, or as pointers to individual characters

Other features

C has a large set of arithmetic operators C has a full set of control structures with very

flexible semantics C has a flexible type definition C has always been closely tied to OS

functionality ( in UNIX, example: malloc - OS function for allocating dynamic storage, but you use and include to do this)

Language Evaluation

Crisp, clear syntax close association of C data objects and

underlying machine architecture permits very efficient programs

large library

C Weakness Does permit very sloppy and error-prone

programming all enumerated types are integers, no strong typing break and switch are error-prone the == mistake use of call-by-value parameter passing when

pointers are needed to implement call-by-reference C has no real encapsulation

C popularity

It is flexible (any application) it is efficient (low-level semantics of

language) it is available (C compiler on most every

computer it is portable (you can write C program that

can execute on multiple platforms)

C assignment