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the left. If no justification is specified, the data is packed (if it is incoming data) or will be packed—there will be no blank spaces between successive items.
Packing makes efficient use of storage space, but it may make it more difficult for the program to obtain the items independently of each other, since the processor may have to provide for "unpacking." However, packed items can usually be read or written more rapidly, since the system will not have to process non-significant information. In general, if the programmer must be economical of storage space, or if the program calls extensively for reading or writing long sequences of data, it may be more efficient to pack the data. If a great many of the items must be obtained individually , however, and/or if scanning long sequences of data is not a common operation , justification of data may be more efficient. The programmer should evaluate each case on its particular merits.
Alignment of alphameric information, such as lists of names, cannot be specified in the field pictorial. If alignment of such data is required, it can be accomplished by the use of the "Justify" column.
The symbols L and R are used in Column 37 to indicate left and right justification respectively. If neither symbol is used, the data will be packed, as has already been noted.
Description (Col. 38-71)
The "Description" columns are used to show the length of each field of data, together with its format. They may also be used to specify constants and the names of data and procedures, as explained below. Specifically, the following kinds of information may appear in Columns 38 through 71:
- Format characters. These are shown and described in the table below.
- Constants.
- Data names associated with the type codes REDEF and COPY.
- The word LIBRARY, followed by the name of a data description stored in the library.
- The words QUANTITY IN, followed by the name of a field which will contain a quantity when the object program is run.
In a number of cases, a complete data description entry will require that more than one of these kinds of information be listed on the same line. For example, it is generally necessary to show both the format and the value of a constant. In such a case, the various items should be written in the order shown above, separated by one or more blanks.
Format Characters
The format characters serve two functions: (1) They show the number of character spaces to be occupied by a field. (2) They show the kind of character that will occupy each space. The resulting data representation is known as a "field pictorial."
If the item of data being described is one which will be brought into the system at object time, the format characters must reflect the format of the data as it already exists; changes in input data cannot be effected by the field pictorial. However, if the item is one produced as a result of the operation of the program—as in moving the data or performing arithmetic on it, for example—the field pictorial has a direct effect on the manner in which the data will be handled.
With certain exceptions, which are explained below, one format character is required for each data character for which storage space is to be reserved. The particular format character chosen for each space prepares the system to receive in that space data of the type shown in the table on the following page.