Explain Data Dictionary

A data dictionary lists all data items appearing in the DFD model of a system. The data items listed include all data flows and the contents of all data stores appearing on the DFDs in the DFD model of a system. A data dictionary lists the purpose of all data items and the definition of all composite data items in terms of their component data items. For example, a data dictionary entry may represent that the data grossPay consists of the components regularPay and overtimePay.
grossPay = regularPay + overtimePay
For the smallest units of data items, the data dictionary lists their name and their type. Composite data items can be defined in terms of primitive data items using the following data definition operators:

+: denotes composition of two data items, e.g. a+b represents data a and b.
[,,]: represents selection, i.e. any one of the data items listed in the brackets can occur. For example, [a,b] represents either a occurs or b occurs.
(): the contents inside the bracket represent optional data which may or may not appear. e.g. a+(b) represents either a occurs or a+b occurs.
{}: represents iterative data definition, e.g. {name}5 represents five name data. {name}* represents zero or more instances of name data.
=: represents equivalence, e.g. a=b+c means that a represents b and c. /* */: Anything appearing within /* and */ is considered as a comment.

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