{"id":952,"date":"2020-09-23T03:31:39","date_gmt":"2020-09-23T03:31:39","guid":{"rendered":"http:\/\/learnlearn.uk\/alevelcs\/?page_id=952"},"modified":"2021-04-02T05:12:01","modified_gmt":"2021-04-02T05:12:01","slug":"user-defined-types","status":"publish","type":"page","link":"https:\/\/learnlearn.uk\/alevelcs\/user-defined-types\/","title":{"rendered":"User Defined Types"},"content":{"rendered":"
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Introduction<\/h2>\n
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What are user defined types?<\/h3>\n

Sometimes the use of in-built data types \/ structures are not sufficient to represent the data you are working with. It is therefore necessary to construct your own data types\/structures. User defined types can make programs much easier to code, read and debug.<\/p>\n

Non-composite vs Composite types<\/strong><\/p>\n

Non composite types are user defined types that contain only one data type, for instance enumerators.<\/p>\n

Composite data types are user defined types that contain more than one data type, for instance a record<\/p>\n\n<\/div>

Enumerators<\/h2>\n
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Enumerators<\/h3>\n

Enumerators bind a group of names to constant values and are useful for things such as days of the weeks or months of the year,<\/p>\n

Enumerators in Python<\/strong><\/p>\n

Enumerators were added to Python 3.4+<\/p>\n

 <\/p>\n

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import<\/span> enum<\/span>\r\n\r\nclass<\/span> Months<\/span>(enum.<\/span>Enum):\r\n    JAN =<\/span> 1<\/span>\r\n    FEB =<\/span> 2<\/span>\r\n    MAR =<\/span> 3<\/span>\r\n    APR =<\/span> 4<\/span>\r\n    MAY =<\/span> 5<\/span>\r\n    JUN =<\/span> 6<\/span>\r\n    JUL =<\/span> 7<\/span>\r\n    AUG =<\/span> 8<\/span>\r\n    SEP =<\/span> 9<\/span>\r\n    OCT =<\/span> 10<\/span>\r\n    NOV =<\/span> 11<\/span>\r\n    DEC =<\/span> 12<\/span>\r\n\r\nprint<\/span>(Months.<\/span>JAN)\r\n# prints <Months.JAN: 1><\/span>\r\n\r\nprint<\/span>(Months['JAN'<\/span>])\r\n# also prints <Months.JAN: 1>  \/\/ useful for passing variable references<\/span>\r\n    \r\nprint<\/span>(Months(1<\/span>))\r\n also prints <<\/span>Months.<\/span>JAN: 1<\/span>><\/span>  \/\/<\/span> find a month from<\/span> its<\/span> number<\/span>\r\n\r\nprint<\/span>(Months.<\/span>JAN.<\/span>value)\r\n# prints 1   \/\/useful if you need to find a month's number<\/span>\r\n\r\nprint<\/span>(Months(1<\/span>).<\/span>name)\r\n# prints the month's name <\/span>\r\n\r\nprint<\/span>(Months['JAN'<\/span>].<\/span>value ><\/span> Months['FEB'<\/span>].<\/span>value)\r\n# Prints False \/\/Useful for finding out which month has a higher value<\/span>\r\n<\/pre>\n<\/div>\n\n<\/div>
Pointers<\/h2>\n
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Pointers<\/h3>\n
A pointer is a datatype that refers to a location in memory. It is a form of indirect referencing<\/a>. Obtaining the value stored at the memory location is known as dereferencing.<\/p>\n
Python note<\/strong><\/p>\n
Python does not use pointers, though many other languages (such as C) do. For a discussion why Python doesn’t use pointers, take a look at this article<\/a>.<\/p>\n\n<\/div>
Records<\/h2>\n
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Records<\/h3>\n
Records are composite data types that contain references to at least 1 other datatype within them. The advantage of records is that all related data is tied together in one place. They are incredibly useful and popular in many languages.<\/p>\n
<\/p>\n
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#Works for Python 3.3+<\/span>\r\n\r\nimport<\/span> types<\/span>\r\n\r\ngiraffe =<\/span> types.<\/span>SimpleNamespace()\r\n\r\ngiraffe.<\/span>name =<\/span> \"Gerry\"<\/span>\r\ngiraffe.<\/span>height =<\/span> 350<\/span>\r\ngiraffe.<\/span>age =<\/span> 15<\/span>\r\ngiraffe.<\/span>origin =<\/span> \"Africa\"<\/span>\r\n<\/pre>\n<\/div>\n
CIE Pseudocode & Records<\/h4>\n
In the exam the main use of records will be when reading & writing records from\/to random files.<\/p>\n
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Sets<\/h2>\n
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Sets<\/strong><\/p>\n
Sets are a composite data type that are unordered, iterable, mutable and contain no duplicate elements.<\/p>\n
Python sets are in built and are incredibly useful for certain tasks where you need to compare 2 sets in some way.<\/p>\n
The main functions are:<\/p>\n