1.5.14 Writing if/else if Statements

An if/else if chain is useful when one value can belong to one of several alternate categories.

For example, a soccer program might classify a team's goal total:

Only one category should be selected for one value.

Suppose the requirement says:

Before writing C#, identify the questions in order:

Are there at least 3 goals?
If not, are there exactly 2?
If not, are there exactly 1?
Otherwise, no goals remain.

That reasoning becomes the structure of the chain.

Begin with the first condition:

if (goals >= 3)
{
    System.Console.WriteLine("High scoring");
}

This branch handles every value that belongs in the first category.

Now add the second question:

if (goals >= 3)
{
    System.Console.WriteLine("High scoring");
}
else if (goals == 2)
{
    System.Console.WriteLine("Two goals");
}

The second condition is tested only when the first condition was false.

Continue with:

else if (goals == 1)
{
    System.Console.WriteLine("One goal");
}

The complete ordered chain becomes:

if (goals >= 3)
{
    System.Console.WriteLine("High scoring");
}
else if (goals == 2)
{
    System.Console.WriteLine("Two goals");
}
else if (goals == 1)
{
    System.Console.WriteLine("One goal");
}
else
{
    System.Console.WriteLine("No goals");
}

The order of branches affects behavior.

This version is poorly ordered:

if (goals >= 1)
{
    System.Console.WriteLine("At least one goal");
}
else if (goals >= 3)
{
    System.Console.WriteLine("High scoring");
}

When:

goals = 4

the first condition is already true.

The program never reaches the more specific high-scoring branch.

A better order places the higher threshold first:

if (goals >= 3)
{
    System.Console.WriteLine("High scoring");
}
else if (goals >= 1)
{
    System.Console.WriteLine("At least one goal");
}

A final else has no condition:

else
{
    System.Console.WriteLine("No goals");
}

It means:

That makes it useful for the remaining category.

Consider:

starter
substitute
not active

A player should belong to one of those statuses at a time.

That is a natural use for an ordered chain.

By contrast, separate independent conditions may both be allowed to run. Use the structure that matches the requirement.

Suppose an activity diagram shows:

At least 3 goals?
   true  → High scoring
   false → Exactly 2 goals?
               true  → Two goals
               false → Exactly 1 goal?
                           true  → One goal
                           false → No goals

Translate the decision nodes in order into:

if
else if
else if
else

The C# should preserve the same decision sequence.

A branch should perform the action required for that category.

Avoid adding unrelated work into one branch simply because you are already inside a conditional.

The conditional structure should remain easy to explain.

For each branch, say:

If that explanation matches the requirement for every branch, the chain is structured correctly.