Comparison Operators

Cat Food Store Features

• Display welcome message
• Ask for quantity
• Calculate total
• Discount for large orders

Our program always charges users 2 dollars per can. It takes the number of cans the user wants to order, and multiplies it by 2 to get the total cost.

Our last requirement is to provide a discount for large orders of cans. Specifically, if the order is for 100 cans or more, the price should be $1.50 per can. If it's for 50 cans or more, the price should be $1.75 per can. And all other orders should be the full price, $2 per can.

Our first problem here is to figure out which discount we should apply. Are they ordering 100 cans or more? 50 to 99? Less than 50? To do this, we're going to need a way to compare numbers.

• This code will be a little too complicated to embed within a string. So first, let's move the part that calculates a price out to a method.
  • We'll add a Price function that takes the number of cans being ordered in a parameter named quantity.
  • Notice that we're having our function return a double floating-point value, not an int.
  • An integer return value would work right now, because we're only returning whole dollar amounts.
  • But when we add discounts later, we'll be using fractional dollar amounts like $1.75 or $1.50 per can.
  • So we'll prepare for this by setting Price up with a double floating-point return type now.
  • quantity * 2 is actually an int value, but int is a less-precise type and double is a more-precise type, so the conversion from int to double will be performed implicitly.

    static double Price(int quantity)
    {
        return quantity * 2;
    }

• Now that we've set the Price method up, we can make a call to it in the Main method.
  • We pass it the number of cans, and store its return value in a new total variable.
  • Then we update our call to Console.WriteLine to use the new total variable instead of embedding the calculation in the interpolated string.

double total = Price(number);
Console.WriteLine($"For {number} cans, your total is: ${total}");

Now that we've moved our price calculation out to a separate method, we can work towards applying a discount for large orders.

In order to know which discount to apply, we need to be able to tell if the order quantity is over 100, if it's between 50 and 100, or if it's below 50.

C# uses comparison operators to tell whether one value is equal to, greater than, or less than another.

==
!=
<
>
<=
>=

Comparison operators return a boolean value. Boolean values are either true or false. They're represented in C# code by the words true and false. Note that these are not strings, so they're not surrounded by quotes.

true
false

int quantity = 75;
Console.WriteLine(quantity > 50); // => true
Console.WriteLine(quantity < 50); // => false
Console.WriteLine(quantity == 75); // => true

• Don't confuse == with =!

Console.WriteLine(quantity == 99); // => false
Console.WriteLine(quantity); // => 75
Console.WriteLine(quantity = 99); // Reassignment!
Console.WriteLine(quantity); // => 99

• >=, <=

Console.WriteLine(quantity >= 50); // true
Console.WriteLine(quantity <= 50); // false
quantity = 50;
Console.WriteLine(quantity >= 50); // true
Console.WriteLine(quantity <= 50); // true

• !=
  • The exclamation point can be read aloud as "not".
  • So != can be read aloud as "not equal".
  • It's the opposite of ==.

Console.WriteLine(quantity != 50); // => false
Console.WriteLine(quantity == 50); // => true

• Don't try to compare a string to a numeric type! You'll get an error.

Console.WriteLine("3" == 3);
Console.WriteLine("4" > 3);

Operator '==' cannot be applied to operands of type 'string' and 'int'
Operator '>' cannot be applied to operands of type 'string' and 'int'

• You should only do a comparison if you can convert one class to match the other.

Console.WriteLine(int.Parse("3") == 3); // => true
Console.WriteLine(int.Parse("4") > 3); // => true