{"id":6562,"date":"2024-08-02T13:01:25","date_gmt":"2024-08-02T11:01:25","guid":{"rendered":"https:\/\/oopm.org\/?page_id=6562"},"modified":"2025-05-09T12:24:37","modified_gmt":"2025-05-09T10:24:37","slug":"2-4-the-first-program","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=6562","title":{"rendered":"2.4 The first program"},"content":{"rendered":"
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A description written in a programming language is called a computer program<\/em> or just a program<\/em> but is also referred to as source code<\/em> or simply code<\/em>.<\/p>\n\n\n\n

The description of account_1010<\/code> in the previous section may be considered an example of a program, although it is not very useful since it is not used for any purpose.<\/p>\n\n\n\n

Here we show an example of a program where money is deposited and withdrawn, represented as actions changing the balance of account_1010<\/code>.<\/p>\n\n\n\n

We may assign values to balance<\/code> by an using assignment statements as shown here:<\/p>\n\n\n\n

account_1010.balance := 350.56<\/code><\/pre>\n\n\n\n
The expression account_1010.balance<\/code> access the attribute balance<\/code> of the object account_1010<\/code> and is an example of an attribute accessor<\/em>.<\/p>\n\n\n\n
Execution of this statement implies that balance<\/code> in account_1010<\/code> now holds the value 350.56.<\/p>\n\n\n\n
The operator :=<\/code> specifies that the variable account_1010.balance<\/code> on the left-hand side gets a new value as specified by the expression<\/em> 350.56 at the right-side of :=<\/code>. <\/p>\n\n\n\n
The operation described by the assignment statement may represent a deposit of 350.56 on John Smiths account.<\/p>\n\n\n\n
The assignment statement as shown above is just a program fragment and must be placed in an object in order to be executed. The next example shows such an object – our first example of a program<\/em> carrying out operations:<\/p>\n\n\n\n
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myFirstProgram: obj<\/strong> \n   account_1010: obj<\/strong>\n      owner: val<\/strong> \"John Smith\"\n      balance: var<\/strong> float \n   account_1010.balance := 350.56<\/code><\/pre>\n<\/div>\n\n\n\n
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As said, a program<\/em> is an object descriptor like the one for myFirstProgram<\/code>. The opposite is not the case since an object descriptor is not necessarily a program. We return to this in chapter