Page 129 - Standard Handbook Of Petroleum & Natural Gas Engineering

114 Mathematics

With little extra effort, code can be written so as to minimize the difficulty
a reader will encounter in comprehending the program logic. Several con-
siderations in improving readability of a program are
1. Names should be as descriptive as possible, e.g., DEPTH rather than 12.
2. Comments should be liberally used to describe the data and logic of the
program; they should be brief, when placed in the body of a program unit,
but may be longer and more descriptive at the beginning of a program unit.
3. Indentation, when possible, clarifies conditional and iterative constructions,
and spacing improves the general readability of a program.
4. The use of subprogram units allows separation of the various operations
of a program into modules, thereby clearly delineating the program logic.
Specific types of calculations, input, and output may be done in distinct
modules; in many cases, the main unit will consist primarily of calling
statements to a few modules.
The proper handling of certain common errors can improve the run-time
behavior of programs. In most cases, awareness of inherent problems in machine
handling of data and attention to program details can avoid program crashes
due to error. The following should be considered when the program code is
being designed:

1. Input validation statements should be used to automatically check input
data, to produce a clear message when an error is found, and to allow
reentry of erroneous data. Data input should be echoed for user verification
and an opportunity allowed for alteration of specific data items.
2. A method of exiting the program in case of a run-time error, which
produces a message to the user as to the type and location of the error,
should be provided. The possibility of certain errors occurring can be
anticipated, and the use of flags and conditional constructions may provide
a path to exit the program gracefully.
3. It is necessary to avoid predicting the exact value of a real variable, since,
after several operations, it may have been rounded off one or more times.
4. Side effects in subprogram units are unintentional changes in data values
defined in other units. These frequently occur when the scope of a variable
is mistakenly considered, because of insufficient cross-checking, to be local,
when it is, in fact, global.

FORTRAN Language
FORTRAN names (unit, variable, array, etc.) consist of an initial letter (see
defaults for real and integer types in the following) followed by letters or digits,
the maximum length of which is 6 characters.
Data Types. See the section “Statements” for forms of declaration of type.
Integer-Variable names starting with I-N, unless otherwise declared.
Real-Variable names starting with A-H and 0-Z, unless otherwise declared.
Double precision-Must be declared.
Complex-Must be declared.
Logical-Must be declared.
Character-Must be declared with length of string; default length is 1.