Fuel Up

Available Commands

Hints

• The total amount of energy the character has at any given time is stored in a variable called energy.
• The initial value of energy should be enough to reach the pixie dust. For example, energy = 20.
• Don't give your character too much energy! The amount of energy should be exactly what's needed to complete the puzzle.
• Each time the character moves forward one space, they use up 10 units of energy.

Stockpile

Available Commands

Hints

• The total amount of energy the character has at any given time is stored in a variable called energy.
• The initial value of energy should be enough to reach the pixie dust. For example, energy = 20.
• Don't give your character too much energy! The amount of energy should be exactly what's needed to complete the puzzle.
• Each time the character moves forward one space, they use up 10 units of energy.

Rocket Jump

Available Commands

Hints

• The total amount of energy the character has at any given time is stored in a variable called energy.
• The initial value of energy should be enough to reach the pixie dust. For example, energy = 20.
• Don't give your character too much energy! The amount of energy should be exactly what's needed to complete the puzzle.
• Each time the character moves forward one space, they use up 10 units of energy.
• Each time the character jumps over a gap, it uses up 20 units of energy.

What Are Strings?

In Python, you can use variables to store text, not just numbers. We call text based information strings. A string is just a series of characters, like "Hello" or "It's cold outside." or "Game over!"

You create a string by surrounding text with a pair of single quotes ' or double quotes ".

Programmers call numbers and strings data types, which describe the nature of the information stored in a variable. You've also seen True and False, which are boolean values, another data type.

Using Quotes in Your Strings

If you want to use a quote inside a string, make sure to use a different quote to enclose the string. For example, you could write:

Programmers call this concatenation.

Numbers and Strings Aren't The Same

Even though it seems like you can "add" two strings together, you can't add a number and a string together.

This won't work! Why not? 32 and "Hello" are two different data types. One is a string and the other is a number. To combine a number and a string, you'll need to convert the number to a string using str() first.

That function, str(), converts the number 32 into a string that holds the string "32".

What happens when you use the * instead of +, though? Try it out!

String Methods

Let's look at some other methods (a special kind of function) that are useful when working with strings. Suppose that we have a variable called string1, let's take a look at what we can perform on that string:

Hack the Trap 1

Hint

• Don't forget to use quote marks when assigning a string value to a variable.

How Do You Define Multiple Variables?

In a hurry? We can define many variables to many different values all at once, in a single statement.

Defining Multiple Variables

To define multiple variables in a single statement, separate the variables you want to define with a comma ,. Then put a = operator and the values you want to assign to the variables in corresponding order, also separated by commas. Let's see it in action.

Here, x, y, and z are assigned values in a single line of code.

The same syntax works with strings, too:

The values don't have to all be the same type, either. You can assign variables of different types on the same line.

Note: You can't reference the value of variables in terms of variables you're defining in that line, though.

For example, this line of code won't work, because b is defined in terms of a, and c is defined in terms of a and b:

Let Loose

Available Commands

Hints

• You can't jump over the enemies. They can only be destroyed by firing arrows.
• This enemy has a number that displays its health. This is the number of arrows needed to destroy it!
• Use the fire() command to launch an arrow that travels 5 steps and destroys the enemies. Each fire() command uses one unit of power.
• The total amount of power for the character is stored in variable power. The initial value of power should be enough to destroy all the enemies. For example, power = 2.
• Remember to set both energy and power variables before you send the commands to the character.
• The initial values of energy and power should be exactly enough to complete the puzzle.

It's a Toughie

Available Commands

Hints

• You can't jump over the enemies. They can only be destroyed by firing arrows.
• Each enemy has a number that displays its health. This is the number of arrows needed to destroy it! Use the get_enemy_health() command to see how much health an enemy in front of you has.
• To destroy a tough enemy, you could use get_enemy_health() to get its initial health, store the value in a local variable, then call fire() in a "for" loop.
• You could also destroy a tough enemy with a while loop. Have the loop run while get_enemy_health() is greater than 0, then have the character fire() inside the loop.
• Use the fire() command to launch an arrow that travels 5 steps and destroys the enemies. Each fire() command uses one unit of the power.
• The total amount of power for the character is stored in variable power. The initial value of power should be enough to destroy all the enemies. For example, power = 2.
• Remember to set both energy and power variables before you send the commands to the character.
• The initial values of energy and power should be exactly enough to complete the puzzle.

Hack the Trap 2

Obstacle Course

Available Commands

Hints

• You can't jump over the enemies. They can only be destroyed by firing arrows.
• Each enemy has a number that displays its health. This is the number of arrows needed to destroy it! Use the get_enemy_health() command to see how much health an enemy in front of you has.
• To destroy a tough enemy, you could use get_enemy_health() to get its initial health, store the value in a local variable, then call fire() in a "for" loop.
• You could also destroy a tough enemy with a while loop. Have the loop run while get_enemy_health() is greater than 0, then have the character fire() inside the loop.
• Use the fire() command to launch an arrow that travels 5 steps and destroys the enemies. Each fire() command uses one unit of power.
• The total amount of power for the character is stored in variable power. The initial value of power should be enough to destroy all the enemies. For example, power = 2.
• Remember to set both energy and power variables before you send the commands to the character.
• The initial values of energy and power should be exactly enough to complete the puzzle.

What Are Expressions?

You've already seen that expressions can assign a value to a variable.

For example, this expression assigns the literal value of 10 to the variable x.

Some expressions do not assign a value to a variable, but simply evaluate to a value. Some examples below:

An expression can evaluate to any kind of data type, such as a string, boolean, or integer, and can appear in your Python code wherever a value is expected.

Many Kinds of Expressions

You've already seen many types of operators that can be used in an expression, such as +, *, =, and so on. The kinds of expressions used in an expression hint at what kind of expression it might evaluate to. Let's take a closer look:

• Arithmetic expressions evaluate to a number and use mathematical operators like +, -, *, /, %, and more.
• Logical expressions evaluate to true or false and use logical operators like not, and, or.
• String expressions evaluate to a character or string and use string operators like concatenation +, and the combined concatenation and assignment operator +=.

Trickier Trap 1

Hints

• The multiplication operator is *.

• The exponent operator is **.

What Are Numbers?

Number Assignment

In Python, numbers can be assigned to a variable without specifying their type:

Data Type Conversions

Python has functions that can be used to convert a value to a specific type, if possible.

What is Operator Precedence?

Operator Precedence

Expressions will evaluate operator in this order, from first to last (highest precedence to lowest):

Trickier Trap 2

Hints

Here's a reminder of the common arithmetic operators:

• The addition operator is +.

• The multiplication operator is *.

• The exponent operator is **.

What Are Lists?

The Python syntax for defining a list is as follows:

item1, item2, and so on are called the elements of the list. And elements within a list can be of any data type: numbers, strings, booleans, and so on.

For example:

To declare an empty list, simply use the square brackets without any items in it.

In Python, while defining a list, any spaces and line breaks are ignored, so your list definition can span multiple lines as shown below.

Not every element within the list needs to be the same type, either. For example, you can store first name, last name, and the player's score with a mix of string and number data types. You can even store a list within another list!

Why Use Lists?

Let's say you need to store five different color names in your program. Without lists, you would have to store each value in five different variables, like this.

This works well enough when you only need to store a few values, but what if you needed to store hundreds or even thousands of values? You would have to create many, many variables, and your code would become difficult to change or debug.

The solution to this issue is to use a list.

Accessing List Items

You need to use an index in order to access the elements inside the list. An index is an int that refers to the relative position of each element in the list. (You cannot use a float as a list index!)

In Python, indexes are "zero-based" which means the first element is at index 0, rather than 1. That means that the second element is at index 1, the third element is at index 2,..., and the nth item is at index n-1.

To access an element of the list, you write:

For example, let's say you wanted to access the third element of the colors list, and store it in a new variable called color_yellow.

You can also update an item in the list using its index. For example, to change "yellow" to "violet" in our colors list, we'd write:

This changes the third list item.

Length

You can use the same len() method to get the length of a list as you did with a string.

For example,

Variables Review

In programming, variables are used to store data.

The value on the right is stored in the variable on the left. You can change the value of your variable at any time, and Python will keep track of it for you!

Variables allow you to access the same value in multiple instructions, making it easier to change the information your program uses.

When a variable is reassigned to a new value, the old value is discarded and replaced with a new one.

To define multiple variables in a single statement, separate the variables you want to define with a comma ,. Then put a = operator and the values you want to assign to the variables in corresponding order, also separated by commas. The values assigned can have different types.

You can join variables with literal strings to add labels or additional characters around the variable's value.

In Python, we use strings to store text. Strings are enclosed within quotes. You can use a single quote ' or a double quote ".

The Python syntax for declaring strings is as follows:

The string data type has many methods (a special kind of function) that can be used to access and manipulate the data.

Expressions and Lists Review

Numbers

Python has separate data types for different kinds of numbers. In Python, integers are values with type int, and floating-point numbers are values with type float.

You can use int(), float(), and str() functions to convert between datatypes, to perform calculations, or display numbers as text, as needed.

Expressions

An expression is any valid combination of variables, literals, operators, and function calls that compute to a single value.

Using Operators

Operators such as +, *, and = can be used within expressions.

Example Expressions

Expressions can assign a value to a variable. For example, the expression:

assigns the literal value of 10 to the variable x.

Some expressions don't assign but simply evaluate to a value. Some examples below:

Types of Expressions

An expression can return a string, boolean, or number.

Arithmetic expressions evaluate to a number and use arithmetic operators such as +, -, *, /, % .

Logical expressions evaluate to a true or false and use logical operators like not, and, or.

String expressions evaluate to a character or string and use string operators like concatenation + and assignment operators like +=.

Operator Precedence

Operator precedence is the order in which the operators in an expression are evaluated to get the expression's value. You can use parentheses to override precedence of operators.

Operator Precedence (highest to lowest)

Lists

Lists are used to store lots of data in one variable. The Python syntax for defining a list is:

item1, item2 , and so on are the elements of the list. The elements in a list can be of any data type: numbers, strings, booleans, or even other lists. The items in the list are separated by commas.

To declare an empty list, simply use the square brackets without any items in it.

In Python, while defining a list, any spaces and line breaks are ignored, so your list definition can span multiple lines.

You can access specific items in the list using an index. An index is an integer that refers to the relative position of each element in the list. You cannot use a float as a list index!

You can use the len() method to get the length of a list in the same way that you did with a string.