Page 127 - Standard Handbook Of Petroleum & Natural Gas Engineering

112 Mathematics

their subscripts, the number of subscripts being determined by the dimen-
sionality of the array. An element is referred to by the array name followed
by its parenthesized subscripts, e.g., TEMP( IJ) might refer to the tem-
perature at the Ith time increment and at the Jh pipe node.
3. Record-A collection of data items (fields) of various types, which may also
be records themselves. If EMPl is a record in the master file of employees,
EMP1.NAME may be a character field, EMPl.ZIP an integer field, and
EMPlSAL a real field.
4. File-Collection of records that normally consist of matching types of fields.
Records in a file may be accessed sequentially (the entire file must be read
until the needed record is reached) or randomly (the record contains a
key field, which determines its physical location in storage).
5. Linked lists-Data items linked by pointers. In the general form, each item,
except the first, has one predecessor, and each item, except the last, has
one successor, with pointers linking items to their successors. Doubly linked
lists have pointers to both the predecessor and the successor of an item and
a circular list has a pointer from the final item to the initial item (producing
a predecessor to the initial item and a successor to the final item). Restricted
lists also exist, such as stacks, where items may only be added (pushed) or
deleted (popped) at one end (the top), and queues, where items must be
inserted at one end and deleted from the other. Trees are linked lists in
which each item (node) except the root node has one predecessor, but all
nodes may have any finite number, or zero, successors; graphs contain both
nodes and edges, which connect the nodes and define their relationships.
Program Statements

The statements of which the program consists may be either executable or
nonexecutable. Nonexecutable statements consist of comments, which explain the
data and logic of the program, and declarations, which are orders to the
translator or to other system programs and which usually serve to allocate
memory space for data.
Executable statements, which are translated into machine code, are instructions
by which operations are performed on data or by which the sequence of
execution is changed. Statements producing operations on data are:

1. Assignment-Assign a value, either a constant or a computed value, to a
variable, an array element, a node, or a field.
2. Input-Transfer data from external devices, such as a keyboard or disk file,
to the program.
3. Output-Transfer data from the program to an external device, such as a
printer, screen, or a disk file.
Executable statements affecting the order in which the program instructions
are executed include conditional (branching) statements, iterative (looping)
statements, and statements which call subprogram units.
1. Conditional statements-Change the sequence in which instructions are
executed depending upon the logical relationship( s) between variables and/
or between variable(s) and set value(s).
2. Iterative statements-Force the repetition of instructions depending on preset
conditions.
3. Calling statements-Transfer control to a subprogram unit,