"if" Statements

We've learned about comparison operators, so we can test whether the user wants to order 50 or more cans, or 100 or more. Now we just need to add code that applies a discounted price if those conditions are true.

• Like most programming languages, C# has an if statement that executes some code only if a condition expression is true.
• Starts with if keyword
  • followed by condition, which in these examples are the boolean values true and false
  • followed by an opening curly brace
  • followed by one or more lines of code (like a method body, these are usually indented to make them easier to read, although this isn't enforced)
  • followed by closing curly brace
• Because condition for first if always true, code it contains will always be run.
• Because condition for second if always false, code it contains will never be run.
  • We get an error when attempting to run it: "warning CS0162: Unreachable code detected"
  • This isn't an actual error, so the program will run anyway, but C# does try to warn you that you've written code that won't be run under any circumstances.

        if (true)
        {
            // This code will always be run.
            Console.WriteLine("true");
            Console.WriteLine("additional code here");
        }
        if (false)
        {
            // This code will never be run.
            Console.WriteLine("false");
            Console.WriteLine("additional code here");
        }

• Just putting the values true and false isn't very useful, though.
• Actual programs will use expressions that evaluate to true or false, like comparison operators.
  • Again, notice that we get a warning about the middle if statement. Because 75 is always going to be less than 100, the condition will always evaluate to false, and the code will never be run.

        if (75 > 50)
        {
            // This code will always be run.
            Console.WriteLine("75 > 50");
        }

        if (75 > 100)
        {
            // This code will never be run.
            Console.WriteLine("75 > 100");
        }

        if (50 == 50)
        {
            // This code will always be run.
            Console.WriteLine("50 == 50");
        }

• Actual programs aren't likely to compare hard-coded numbers, either. Usually, they'll test the value of a variable.
• Notice that this time, the warning goes away.
  • Because we're using a variable in our comparisons, we could theoretically change its value to something else: int number = 50;
  • If we do that, if conditions that previously evaluated to false could change to true, and vice-versa, which is how if statements usually work in practice. So there's nothing to warn us about.

        int number = 75;
        if (number > 50)
        {
            // This code will always be run.
            Console.WriteLine("number > 50");
        }

        if (number > 100)
        {
            // This code will never be run.
            Console.WriteLine("number > 100");
        }

        if (number == 50)
        {
            // This code will never be run.
            Console.WriteLine("number == 50");
        }

• Let's try using if in our program:

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

• Whoops! We get errors:

Program.cs(10,13): error CS0103: The name 'pricePerUnit' does not exist in the current context [/home/treehouse/workspace/workspace.csproj]
Program.cs(14,13): error CS0103: The name 'pricePerUnit' does not exist in the current context [/home/treehouse/workspace/workspace.csproj]
Program.cs(18,13): error CS0103: The name 'pricePerUnit' does not exist in the current context [/home/treehouse/workspace/workspace.csproj]
Program.cs(20,27): error CS0103: The name 'pricePerUnit' does not exist in the current context [/home/treehouse/workspace/workspace.csproj]

• That's right, we forgot to declare the pricePerUnit variable before assigning to it.
• I'll go up to the first instance and make it a declaration:

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

• Hmm, that's strange. Now we get three errors and a warning:

Program.cs(14,13): error CS0103: The name 'pricePerUnit' does not exist in the current context [/home/treehouse/workspace/workspace.csproj]                       
Program.cs(18,13): error CS0103: The name 'pricePerUnit' does not exist in the current context [/home/treehouse/workspace/workspace.csproj]                       
Program.cs(20,27): error CS0103: The name 'pricePerUnit' does not exist in the current context [/home/treehouse/workspace/workspace.csproj]                       
Program.cs(10,20): warning CS0219: The variable 'pricePerUnit' is assigned but its value is never used [/home/treehouse/workspace/workspace.csproj]

• This last warning especially is interesting: "The variable 'pricePerUnit' is assigned but its value is never used"
  • We use the pricePerUnit variable right here when we're calculating a return value! Why is it saying it's never used?
• Remember we showed you that variables declared within a function are only in scope within that function?
  • The same is true for if statements. If you declare a variable within an if statement's block, it's only in scope within that block.
• The solution is to declare the variable before any of the if blocks. Then it will be in scope for all of them, as well as afterward.

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

• But we still get an error when we use pricePerUnit to calculate a return value. It says "Use of unassigned local variable 'pricePerUnit'".
  • Why is that? We assign values to pricePerUnit in the lines above!
  • The problem is that we assign to pricePerUnit in the if blocks. What if none of the if blocks had a true condition?
  • None of them would run, and we'd get to the return statement with no value assigned to pricePerUnit!
  • Now, you and I know that's not actually possible. No matter what value is assigned to quantity, one of those three if statements is going to be true. A value will be assigned to pricePerUnit no matter what.
  • But C# doesn't know that. All it sees is a possibility that we'll reach the return statement with no value assigned to pricePerUnit. And the result is an error.
  • So we need a way to say that one of those three if statements will be evaluated, no matter what.
• There's an additional problem here, too.
  • This code works fine for quantity >= 50 and quantity < 50, but if quantity >= 100, price_per_unit gets set to 1.75 because 100 is also >= 50.

Up next, we'll look at else if and else clauses for if statements, which will help us fix these issues.