2.7 Method returning a value

For completeness, we add a method for withdrawing an amount from the account. The name of the method is withdraw and it has a parameter amount being the amount to be withdrawn from the account.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.

account_1010: obj
   owner: val "John Smith"
   balance: var float
   interestRate: var float
   addInterest:
      balance := balance + (balance * interestRate) / 100
   deposit(amount: var float):
      balance := balance + amount
   withdraw(amount: var float) -> newB: var float:
      balance := balance - amount
      newB := balance

The arrow (->) in -> newB: var float specifies that the value of the data-item newB is returned by withdraw.

As can be seen, withdraw contains two assignment statements. First amount is deducted from balance (balance := balance - amount) and next, the updated value of balance is assigned to newB, which is returned as the value of withdraw.

The method withdraw may be invoked as shown here:

newBalance: var float
newBalance := account_1010.withdraw(140)

1. We have declared a variable newBalance of type float. 
2. The assignment statement 
   newBalance := account_1010.withdraw(140) is executed.
3. The method withdraw of account_1010 is executed with the parameter amount holding the value 140.
4. Assuming that balance is 450.56 as in the example above, the new value of balance will then be 310.56. 
5. This new value of balance is returned by withdraw. 
6. After the execution of newBalance := account_1010.withdraw(140), the variable newBalance holds the value 310.56.

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 We can do this by using an if:then statement to test if balance is greater than the amount to be withdrawn:

   withdraw(amount: var float) -> newB: var float:
      if (balance >= amount) :then
         balance := balance - amount
      newB := balance

The if:then statement has a condition (balance >= about), which is evaluated and results in a Boolean value that is either true or false. The operator >= means greater than or equal. If balance is greater than or equal amount then (balance >= amount) evaluates to true otherwise it evaluates to false.

If the condition evaluates to true, then and only then is the part after :then executed – in this case the statement balance := balance - amount.

The if:then statement thus ensures that a withdraw is only possible if there is sufficient money on the account. With the above solution, withdraw only does not update balance – in practice one has to take some action to inform about the withdraw not being possible. We return to that in section .