2.1.8 Writing if/else if Statements

Turn a Requirement into Ordered Checks

A Zoo vending response must give one reason for the result of a purchase attempt. The rule says: report unavailable food first; otherwise report insufficient payment; otherwise report that the purchase can proceed. Writing the chain begins with those requirements, before choosing comparison operators.

Identify the Inputs and Priority

Use portionsAvailable for the inventory count and payment for the amount offered. The price in this teaching example is 2.00. Both values are parameters, so a caller can supply different attempts without editing the method body.

The priority matters when the machine is empty and payment is also insufficient. The requirement chooses the unavailable-food response. Write that condition first. Reaching the next check means food was available.

From requirement to condition
Requirement	C# test	Meaning when true
No portion can be sold	portionsAvailable <= 0	Choose the unavailable response
Payment is below the price	payment < 2.00m	Choose the insufficient response
Neither rejection applies	else	Choose the ready response

Build and Verify the Method

Write One Branch at a Time

Implement the ordered response rules

public string GetPurchaseMessage(decimal payment, int portionsAvailable)
{
    string message;
    if (portionsAvailable <= 0)
    {
        message = "No food available";
    }
    else if (payment < 2.00m)
    {
        message = "More payment needed";
    }
    else
    {
        message = "Ready to purchase";
    }

    return message;
}

Start with the first rejection and its message. Add the next rejection as an else if rather than an independent if. Finish with else, because everything that reaches it already passed the earlier checks. All branches assign message, so the return has a result to use.

The method belongs with the object that owns the vending rule. It is a small response example rather than a replacement for the project's full purchase operation. A real successful purchase would also perform the required money and food changes.

Use the Flow to Review the Code

First check whether portionsAvailable is zero or lower. Otherwise check whether payment is below 2.00. Choose no food, more payment, or ready, merge the paths, and return the chosen message. — Review the requirement order

Compare each code condition with the corresponding decision. The messages are actions; the expressions are questions. A selected message goes directly to the shared return rather than continuing through the remaining checks.

Test the Boundary and Priority

Small verification set
Payment	Available portions	Expected message	What it checks
2.00	0	No food available	Inventory rejection
1.99	1	More payment needed	Just below the price
2.00	1	Ready to purchase	Exact price succeeds
1.00	0	No food available	First rule takes priority

Call the method separately for each row. Predict the result first, compare the returned text, and change only the condition or order responsible for a mismatch. Do not change the expected result merely to agree with faulty code.

Preserve the Meaning During Revision

An else if chain chooses among alternatives at the same decision level. A nested if inside a selected branch would ask another question after that branch had already been entered. Use that additional nesting only when the requirement actually asks for a decision within a decision.