Converting Strings to Numbers

Cat Food Store Features

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

Now that we understand math operations, let's see if we can calculate a total price for the order. At the base rate, each can is $2, so we need to multiply the number of cans by 2. We'll worry about applying discounts for larger orders later.

• We'll update the call to Console.WriteLine to print the number of cans entered, and then the same number multiplied by 2.

Console.WriteLine($"For {entry} cans, your total is: ${entry * 2}");

• But this generates an error:

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

• The value returned from Console.ReadLine in the Ask method is a string.
• So the value returned from Ask is also a string.
• And you can't multiply a string by a number; it just doesn't make sense.
• We need to convert the string to an actual number before multiplying it.
• We can't use a conversion, though: int dozen = (int)"12";
  • Those only work on numeric types. We'll get an error if we try to use one on a string: "Cannot convert type 'string' to 'int'"
• Instead, all of the built-in types have a static Parse method that we can call to convert string values.
  • I can write the type name I want to convert to, int, a dot, the name of the Parse method, parentheses, and the string value I want to convert: int dozen = int.Parse("12");
  • That will get me a value of the int type.
  • Remember, int is just a shorthand for the System.Int32 type; that's why it shows System.Int32 here.

int dozen = int.Parse("12");
Console.WriteLine(dozen.GetType()); // => System.Int32
Console.WriteLine(dozen); // => 12

• Be sure that you use a string that will actually convert to the type you want.
  • For example, what if I changed this string to hold a floating-point number? int dozen = int.Parse("12.42398");
  • I'll get an error, because it's expecting an integer: "System.FormatException: Input string was not in a correct format."
  • And of course, using a string that doesn't even contain a number will fail as well: int dozen = int.Parse("ADSFAFDS");
• As I mentioned, every built-in type has a Parse method.
  • You just have to be sure to provide a string that will convert to a value of that type.

double pi = double.Parse("3.1415");
Console.WriteLine(pi.GetType());
Console.WriteLine(pi);
bool flag = bool.Parse("true");
Console.WriteLine(flag.GetType());
Console.WriteLine(flag);
char letter = char.Parse("A");
Console.WriteLine(letter.GetType());
Console.WriteLine(letter);

int number = int.Parse(entry);
Console.WriteLine($"For {number} cans, your total is: ${number * 2}");