| Term |
Definition |
| Data Type |
The kind of data being stored by a variable or field. A form of validation as it limits the value which can be stored. Examples are String, Real and Boolean. |
| Integer |
Is any whole number. Can be unsigned (only postive) or signed (negative or positive). Typical size 2 or 4 bytes. |
| Real Number |
A number that can be represented with a fractional part. Usually in the form of Float and Double in programming languages. Typical size 4 or 8 bytes. |
| String |
Used to store any textual data which include letters, numbers symbols and punctuation. Typical size one byte per character used. |
| Character |
A data type that stores a single textual character. Typical size 1 byte. |
| Boolean |
Can only store one of two values: True or False. Typical size 1 byte. |
| Date |
Used to represent dates and times. Typical size 4 or 8 bytes. |
| Array |
A set of data items of the same type grouped together using a single identifier. May have many dimensions and are organised in the same way as tables (rows and columns). |
| Array Element |
Each individual piece of data held within an array. Each is address using the arrays identifier and index (subscript) location. |
| Byte |
A group of bits, usually 8. |
| Kilobyte |
1024 bytes. |
| Records |
Collection of data items grouped together. Each item can be of a different data type. Each collection usually refers to a particular individual or object in real life. |
| Field |
A data item stored as part of a record. Usually given a data type. |
| File |
Data stored in a separate file and accessed by the program. The file structure describes the order in which the data is stored, how the data is split and how each item is found. |
| Serial File |
Data is stored in the order in which they were added to the file. To find a particular record the program must read through every record until it is found. If the record is not there the program will not know until the end of the file is reached. |
| Sequential File |
Records are stored in order using a key field. Records can be found by searching for the key field. If the file does not exist it will be known as soon as the logical location of the key is passed. Adding a new file usually requires recreating the file in order to maintain the order. |
| Serial File Access |
Records are read, one at a time, from the physical start of the file, in the order in which they are stored. |
| Sequential File Access |
Records are read, one at a time in key value order, from the logical start of the file. Items are processed in a known, predictable, order making large amounts of data easier to process. |
| Direct File Access (Random Access) |
Records can be retrieved immediately, provided its position in the file is known. This often means that items must have a known length so that the program can calculate where in the file the record is located. |
| Indexed Access |
Uses additional files that contain information about the location of each record. The index is searched first to find the location of the record. |
| Indexed Sequential File Access |
Uses additional Index files containing indexes to the records in a sequential file. Allows both sequential and indexed approaches to be used. The index is searched to find the section (e.g. the start of surnames beginning with D) of the file where the record maybe and then sequential search is used to find the record from there. |
| File Operations |
Actions that can be performed on an existing data file. Include Reading, Writing, Updating, Inserting, Appending and Overwriting. |
| File Locking |
Used to prevent the contents of a data file being changed if the file is already being used. This includes Read-only access, which allows the file to be accessed and viewed but not changed; Record locking, which prevents access to the current record being updated by others but not the rest of the file; Field locking which prevents access to the current field being updated. |
| Estimating File Size |
Determine maximum size of each field, add each together, multiply by the number of records, add 10% for overheads and convert into sensible unit (KB or MB) |