"else if" and "else"

• If the condition of an if statement is false, the code in its body won't run.

        bool ifCondition = false;
        if (ifCondition)
        {
            Console.WriteLine("if clause");
        }

• If you have some code that you want to run when the if statement doesn't run, you can add an else clause.
• You add the else keyword right after the if statement's block, followed by another curly brace block with one or more lines of code that you want to run instead of the if statement's code.
• C# doesn't require it, but it's conventional to have the else keyword aligned with the if keyword, and the else code indented one level. Again, it just makes the code easier to read.

        else
        {
            Console.WriteLine("else clause");
        }

• If the if statement's condition is true, then the else clause won't run: bool ifCondition = true; [then change back]
• If you have other conditions that you want to test only when the if condition is false, you can add one or more else if clauses between the if and else clauses.
• You add the keywords else and if, another condition expression, and some code you want to run if that condition is true.
• As with the else clause, the else if keywords should be lined up with the if keyword, and the code following it should be indented one level.

        bool ifCondition = false;
        // new
        bool elseIfCondition = true;
        // old
        if (ifCondition)
        {
            Console.WriteLine("if clause");
        }
        // new
        else if (elseIfCondition)
        {
            Console.WriteLine("else if clause");
        }
        //old
        else
        {
            Console.WriteLine("else clause");
        }

• If the if clause is true, then neither the elsif clauses nor the else clause will be run.
• If the if condition and all else if conditions are false, then the else clause will be run.
• The else clause isn't required. If all conditions are false and there's no else clause, then no code will be run.

Let's use what we've learned to fix our store program's Price function.

    static double Price(int quantity)
    {
        double pricePerUnit;
        if (quantity >= 100)
        {
            pricePerUnit = 1.5;
        }
        else if (quantity >= 50) // Add "else"
        {
            pricePerUnit = 1.75;
        }
        else if (quantity < 50) // Add "else"
        {
            pricePerUnit = 2;
        }
        return quantity * pricePerUnit;
    }

• This addresses the issue where quantities over 100 would be assigned a pricePerUnit of $1.50 and then get pricePerUnit reassigned to $1.75.
  • The else if condition only gets evaluated when the if condition is not true.
  • So if quantity is 100 or more, the if condition will be true.
  • pricePerUnit will get assigned $1.50.
  • And although a quantity of 100 is also greater than a quantity of 50, the else if condition will never even get evaluated, because the if condition was true.
• So that's one problem fixed.
• But we're still getting the error "Use of unassigned local variable 'pricePerUnit'".
• That's because as far as C# can tell, it's still possible to reach the return statement, where pricePerUnit gets accessed, without having assigned to it.
• We need to set up our code in such a way that pricePerUnit will get assigned to, no matter what.
• You and I can actually tell that pricePerUnit will be assigned no matter what.
  • If the quantity isn't greater than or equal to 100...
  • And it also isn't greater than or equal to 50...
  • Then it for sure will be less than 50.
  • So one of these three branches is always going to be run.
  • But C# can't tell this. And so it's giving us an error.
• To fix this, let's try removing the if and the condition from this last else if clause.

    static double Price(int quantity)
    {
        double pricePerUnit;
        if (quantity >= 100)
        {
            pricePerUnit = 1.5;
        }
        else if (quantity >= 50)
        {
            pricePerUnit = 1.75;
        }
        else
        {
            pricePerUnit = 2;
        }
        return quantity * pricePerUnit;
    }

• We really don't need it, because if quantity isn't greater than or equal to 50, it must be less than 50.
  • So now all we have is an else clause. Even if the if and the else if clauses don't run, the else clause will.
  • Let's try running this now.
  • The error goes away, and our program appears to be working correctly.
  • Since pricePerUnit gets assigned in all three of these scenarios, C# can tell that pricePerUnit is going to have a value when it gets accessed, no matter what.