thp-lang.org/md/learn/basics/variables.md

Variables

Variables allows us to store values under a name, so that we can use them later.

For example, in a program we could have the name of our user, and use it multiple times:

print("Hello, where's Joe?")
print("How old is Joe?")
print("Do you know if Joe has kids?")

We are using the same value Joe many times. Each time we use it we have to type Joe. But what happens if we needed to use a different name? We'd have to change the name everywhere in our code!

print("Hello, where's Jane?")
print("How old is Jane?")
print("Do you know if Jane has kids?")

Variables to the rescue

With a variable we can store values so we can use them later, or use them in multiple times.

In the previous code, we can use a variable to store the person's name, and then use it everywhere.

// This is the variable
val person = "John"

print("Hello, where's {person}?")
print("How old is {person}?")
print("Do you know if Joe has {person}?")

Now, instead of writing "John" every time, we write the name of the variable instead.

If we wanted to change the person's name to "Jane", we just need to change it in one place: the variable

// We change this
val person = "Jane"

// And all these lines will use the new value
print("Hello, where's {person}?")
print("How old is {person}?")
print("Do you know if Joe has {person}?")

Variable rules

To use a variable we do the following:

• Write the special word val
• Write the name of our variable
• Write the equal sign =
• Write the value of our variable

    val person = "Jane"
/*  --- ------ - ------
     |    |    |   +- The value of our variable
     |    |    +----- The equal sign
     |    +---------- The name of our variable
     +--------------- The special word (keyword) val
*/

The value can be anything: ints, floats, string, bools, even other variables and operations!

val variable_1 = 322
val variable_2 = 123.456
val variable_3 = "a text"
val variable_4 = false
val variable_5 = variable_1 + variable 2

Variable name rules

• Starts with a lowercase letter (a-z) or underscore (_)
• Then can have any letter (a-zA-Z), underscore (_) or number (0-9)
• Cannot have spaces
• Cannot have the same name as a keyword (for example, the val keyword)

Some examples of valid variable names:

val name = ...
val age = ...
val my_name = ...
val many_words_joined_by_underscores = ...
val person_2 = ...
val person_3 = ...

Some invalid variables and why they are invalid:

val 1name = ...         // Invalid: starts with a number
val 123_person = ...    // Invalid: starts with a number
val val = ...           // Invalid: same name as a keyword (val)
val Person = ...        // Invalid: starts with an uppercase letter
val person name = ...   // Invalid: contains whitespace
val +@name = ...        // Invalid: contains non-letters (+@)
val name🫠 = ...        // Invalid: contains emoji (🫠)

Variable reassignment

When you create a new variable with the same name of an old variable, the old is "replaced" with the new one.

val person_name = "John"
print(person_name)  // Will print "John"

val person_name = "Jane"
print(person_name)  // Will print "Jane"

This will have some implications on the future, but for now you should now that you will always use the value of the last variable you define.