{"id":411,"date":"2023-05-14T13:28:16","date_gmt":"2023-05-14T11:28:16","guid":{"rendered":"https:\/\/oopm.org\/?page_id=411"},"modified":"2025-01-12T13:28:34","modified_gmt":"2025-01-12T12:28:34","slug":"2-6-method-returning-a-value","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=411","title":{"rendered":"2.7 Method returning a value"},"content":{"rendered":"
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For completeness, we add a method for withdrawing an amount from the account. The name of the method is withdraw<\/code> and it has a parameter amount<\/code> being the amount to be withdrawn from the account. In addition to withdrawing an amount, the method returns the updated balance to the invoker of the method.<\/p>\n\n\n\n

account_1010: obj<\/strong>\n   owner: val<\/strong> \"John Smith\"\n   balance: var<\/strong> float\n   interestRate: var<\/strong> float\n   addInterest:\n      balance := balance + (balance * interestRate) \/ 100\n   deposit(amount: var<\/strong> float):\n      balance := balance + amount\n   withdraw(amount: var<\/strong> float) -> newB: var<\/strong> float:\n      balance := balance - amount\n      newB := <\/strong>balance<\/code><\/pre>\n\n\n\n
The arrow (-><\/code>) in -> newB: var<\/strong> float <\/code>specifies that the value of the data-item newB<\/code> is returned by withdraw<\/code>.<\/p>\n\n\n\n
As can be seen, withdraw<\/code> contains two assignment statements. First amount<\/code> is deducted from balance<\/code> (balance := balance - amount<\/code>) and next, the updated value of balance<\/code> is assigned to newB<\/code>, which is returned as the value of withdraw<\/code>.<\/p>\n\n\n\n
The method withdraw<\/code> may be invoked as shown here:<\/p>\n\n\n\n
newBalance: var<\/strong> float\nnewBalance := account_1010.withdraw(140)<\/code><\/pre>\n\n\n\n
    \n
  1. We have declared a variable newBalance<\/code> of type float<\/code>. <\/li>\n\n\n\n
  2. The assignment statement 
    newBalance := account_1010.withdraw(140)<\/code> is executed.<\/li>\n\n\n\n
  3. The method withdraw<\/code> of account_1010 <\/code>is executed with the parameter amount<\/code> holding the value 140<\/code>.<\/li>\n\n\n\n
  4. Assuming that balance<\/code> is 450.56<\/code> as in the example above, the new value of balance<\/code> will then be 310.56<\/code>. <\/li>\n\n\n\n
  5. This new value of balance<\/code> is returned by withdraw<\/code>.  <\/li>\n\n\n\n
  6. After the execution of newBalance := account_1010.withdraw(140)<\/code>, the variable newBalance<\/code> holds the value 310.56<\/code>.<\/li>\n<\/ol>\n\n\n\n
    Suppose we would like to prevent a withdraw if there is not enough money on the account. We can do this by using an if:then<\/code> statement to test if balance<\/code> is greater than the amount<\/code> to be withdrawn:<\/p>\n\n\n\n
       withdraw(amount: var<\/strong> float) -> newB: var<\/strong> float:\n      if (balance >= amount) :then\n         balance := balance - amount\n      newB := <\/strong>balance<\/code><\/pre>\n\n\n\n
    The if:then<\/code> statement has a condition (balance >= about)<\/code>, which is evaluated <\/mark>and results in a Boolean value that is either true or false. The operator >=<\/code> means greater than or equal. If balance<\/code> is greater than or equal amount<\/code> then (balance >= amount)<\/code> evaluates to true otherwise it evaluates to false.<\/p>\n\n\n\n
    If the condition evaluates to true, then and only then is the part after :then<\/code>  executed – in this case the statement balance := balance - amount<\/code>.<\/p>\n\n\n\n
    The if:then<\/code> statement thus ensures that a withdraw<\/code> is only possible if there is sufficient money on the account. With the above solution, withdraw<\/code> only does not update balance<\/code> – in practice one has to take some action to inform about the withdraw<\/code> not being possible. We return to that in section