Converting Strings to Numbers

4:14 with Jay McGavren

Now that we understand math operations, let's see if we can calculate a total price for the order.

Teacher's Notes
Video Transcript
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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}");

0:00
Now that we understand math operations,
0:02
let's see if we can calculate a total price for the order.
0:06
At the base rate, each can is $2, so we need to multiply the number of cans by 2.
0:12
We'll worry about applying discounts for larger orders later.
0:15
We'll update the call, the Console.WriteLine to print the number of
0:19
cans entered, and then the same number multiplied by 2.
0:29
But this generates an error.
0:31
Operator cannot be applied to operands of type string and int.
0:39
The value returned from Console.ReadLine in the ask method is a string.
0:44
So the value returned from ask is also a string.
0:47
And you can't multiply a string by a number, it just doesn't make sense.
0:52
We can't use a conversion though, those only work on numeric types.
0:57
We'll get an error if we try to use one on a string.
1:01
Instead, all of the built-in types have a static parse method that we can call
1:06
the convert string values.
1:08
I can write the type name I want to convert to, int, a dot,
1:12
the name of the parse method, parenthesis, and the string value I want to convert.
1:17
That will get me a value of the int type.
1:25
Remember, int is just a shorthand for the System.Int32 type,
1:29
that's why it shows System.Int32 here.
1:32
Be sure that you use a string that will actually convert to the type you want.
1:37
For example, what if I changed this string to hold a floating point number?
1:42
I'll get an error because it's expecting an integer.
1:48
Input string was not in a correct format.
1:57
And of course, using a string that doesn't even contain a number will fail as well.
2:01
I'll just change the string back to 12.
2:10
As I mentioned, every builtin type has a parse method, you just have to
2:14
be sure to provide a string that will convert to a value of that type.
2:18
So the double type has a parse method that will convert floating point values.
2:22
The bool type has a parse method that will convert Boolean values.
2:29
And the char type has a parse method that will convert single letters.
2:40
And here is the results of all the code we added.
2:44
Double.parse gives us say results of type double.
2:47
Bool.parse gives us a result of type Boolean.
2:50
And Char.parse gives us a result of type char.
2:56
So we can use the int.Parse method to fix our cat food store program.
3:02
We'll just take the string entry that we got back from the ask method and
3:07
pass it to int.Parse, that will give us a result of type int
3:13
Then instead of multiplying the string entry variable,
3:17
we multiply the integer number variable.
3:20
Let's save this and try running it.
3:26
We're asked, how many cans we want to order, we'll say 10.
3:28
And the result gets multiplied by 2 to give us a total value of 20.
3:33
In this stage we've covered the basics of working with numbers in C#.
3:38
Check the teacher's notes for ways you can get more practice with numbers and
3:42
math operations.
3:43
Now that you know the basics of math operations and C#,
3:46
calculating a total price is easy.
3:48
Nice work, you've implemented three of the four features our program needs,
3:53
just one more to go.
3:54
Our last requirement is to offer discounts for large orders.
3:58
If they're ordering 50 we charge $1 75 a piece.
4:01
If they're ordering 100 we charge $1 50 a piece.
4:05
We don't really know a way to change our program's behavior under different
4:09
conditions yet.
4:10
In the final stage of this course, we'll show you how.

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C# Basics

Converting Strings to Numbers

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