{"id":3416,"date":"2024-02-09T11:07:26","date_gmt":"2024-02-09T10:07:26","guid":{"rendered":"https:\/\/oopm.org\/?page_id=3416"},"modified":"2024-12-20T09:16:38","modified_gmt":"2024-12-20T08:16:38","slug":"6-2-statements","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=3416","title":{"rendered":"6.2 Statements"},"content":{"rendered":"
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A statement describes a computation. Execution of the statement implies <\/mark>that the computation is performed by the computer. Such a computation may be a simple operation or consists of a sequence of operations. We have the following kind of statements: assignment statements, conditional statements, loop statements, break statements, method invocations, and object instantiation.<\/p>\n\n\n\n

Conditional statements, loop statements, and break statements are often referred to as control structures<\/em> since they control the order in which the\u00a0statements\u00a0of a program are executed based on the state of the objects – this is often referred to as the control flow <\/em>of the program.<\/p>\n\n\n\n

Some of the statements are primitives of the qBeta language and others are defined in the qBeta library. We return to this is in the last section on this chapter. <\/p>\n\n\n\n

The assignment statement<\/h2>\n\n\n\n

In the previous chapters, the assignment statement<\/em> has been used to describe operations carried out by methods like addInterest<\/code>, deposit<\/code> and withdraw<\/code>. The assignment statement has the form:<\/p>\n\n\n\n

VariableDataItem<\/em> := Expression<\/em> <\/code><\/pre>\n\n\n\n
The left side of the statement must be the name of a variable data-item such as balance<\/code> defined within the various descriptions of accounts. A variable data-item is declared using var<\/strong><\/code> or ref<\/strong><\/code>.<\/p>\n\n\n\n
The right-side of the statement is an expression<\/em> which describes the computation of a datum. An expression may be an arithmetic expression like balance - amount<\/code>, as described in the previous section. This expression evaluates to a primitive value of type float. The statement below assigns this value to the variable balance<\/code>:<\/p>\n\n\n\n
balance := balance + amount<\/code><\/pre>\n\n\n\n
Conditional statements<\/h2>\n\n\n\n
Sometimes the computations of a method depends on the values of data-items of the object or other conditions. One example is withdraw<\/code> where you may not be able to withdraw an amount that is greater than the balance. We may describe this using a conditional statement like an if:then<\/code> statement or an if:then:else<\/code>-statement.<\/p>\n\n\n\n
The if:then statement<\/h3>\n\n\n\n
An if:then<\/code> statement describes a conditional execution of a sequence of items. An example of an if:then<\/code> statement is shown in section