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Lesson 7

What comes next?

We will now go into one of the hardest parts to learn in basic Dart. Some topics may be confusing, but I am sure you guys can get through them.

If there are any questions along the way, ask David or any veteran members for clarification.

Step 5: Conditionals

What is a conditional?

An easy way to think about a conditional is that the program will run based on whether a question is true or false.

A real-life example might be:

Is it raining 🌧️? If yes, bring an umbrella ☂️. If not, then do not bring an umbrella.

Do you notice the word if being bolded?

That is because if is the code we will use.

This is an example of a conditional statement:

void main() {
  bool isRaining = true;
 
  if (isRaining) {
    print('Bring an umbrella');
  } else {
    print('Do not bring an umbrella');
  }
}

First, run the code.

Let’s go line by line.

First, we have a boolean variable. bool is how we represent booleans.

Then, we have:

if (isRaining) {}

Like the main function, whatever is inside the curly braces {} is inside the if statement.

The code inside the curly braces will run only if what is inside the parentheses () results in a boolean value of true.

In this case, isRaining is true, so the if statement runs and prints:

Bring an umbrella

Now, we have:

else {}

This will run if the condition in the if statement is false.

Try it yourself

Change the value of the variable isRaining from true to false.

Now you should notice that the program prints:

Do not bring an umbrella

Multiple Conditions

What if there are multiple conditions?

For example, a real-life situation could be an elevator where:

  • If the floor level is 1, you get on the elevator.
  • If you are at floor 2, you do not get off.
  • If you are at floor 3, you get off the elevator.

Let’s see what this looks like in code:

void main() {
  int floor = 1;
 
  if (floor == 1) {
    print("Get on the elevator");
  } else if (floor == 2) {
    print("Do not get off the elevator");
  } else {
    print("Get off the elevator!");
  }
}

Try changing the value of floor from 1 to 2 and then to 3.

Everything seems familiar, but there are some differences from before.

First, you will notice:

floor == 1

What could that mean?

In Dart, if you use double equal signs ==, you are comparing the first value to the second value.

In this case, you are comparing the value of the variable floor to 1.

Make sure you know the difference between = and ==.

A single equal sign = assigns a value to a variable.

A double equal sign == compares two values.

You will also notice the else if.

This is another condition.

If the first if statement results in false, the else if statement will be checked. If that also results in false, the else statement will run.

You can have as many else if statements as you need.

Multiple Conditions That Must Be True

What do you do when you need multiple conditions to be true?

A real-life example might be:

There is a roller coaster, but you must be over 150 cm and over 13 years old to ride it.

Let’s see how this looks in code:

void main() {
  int age = 15;
  int height = 160;
 
  if (age >= 13 && height >= 150) {
    print("You can ride the roller coaster");
  } else {
    print("You cannot get on the roller coaster");
  }
}

Try running this and see what you get as the result.

Now, change the values of the variables age and height to something else that would print a different result.

Now let’s break it down.

Whenever you want the if statement to run only when 2 or more conditions are met, use double ampersands &&.

For example, if the rider must also be under 250 pounds, you could write:

age >= 13 && height >= 150 && weight < 250

You would also need to create a new variable called weight.

When Only One of Two Conditions Needs to Be True

How about when you only want 1 of 2 things to be true?

An example of this could be:

If either Oreos or Chips Ahoy are in your snack cabinet, you will drink milk.

Let’s see how it looks in code:

void main() {
  String snack1 = "chips ahoy";
 
  if (snack1 == "oreo" || snack1 == "chips ahoy") {
    print("I will drink milk");
  } else {
    print("I will not drink milk");
  }
}

Everything seems the same as before, but instead of using double ampersands, we use double pipes ||.

The double pipes mean OR.

This means that if either condition is true, the if statement will run.

Step 6: Loops

There are only 2 more big concepts you will need to know before moving on to Module 2 of Non-Robot Programming Training!

What we will learn next is called loops.

What are loops?

Well, it’s a loop 😂.

You are going to write code that will continuously repeat, or loop through, until you want it to stop.

There are mainly 2 types of loops we will use:

  • for loops
  • while loops

For Loops

Let’s learn about for loops first.

Try this code:

void main() {
  for (int i = 0; i < 10; i++) {
    print(i);
  }
}

What do you get as the output?

You should see a series of numbers being printed from 0-9.

Let’s see what is happening.

We first wrote:

for () {}

Inside the parentheses (), we created an integer variable i and set it to 0.

Then, after a semicolon, we wrote that we want to run this loop while:

i < 10

Then, after another semicolon, we said we want to increment i by 1.

Finally, inside the curly braces {}, we printed i.

So, the three parts inside a for loop are:

for (starting value; condition; increment)

In our example:

for (int i = 0; i < 10; i++)
  • int i = 0 → where the loop starts
  • i < 10 → how long the loop runs
  • i++ → how much i increases each time

Try changing the increment

Try this code, but before running it, predict what the output will be and compare it to the actual output:

void main() {
  for (int i = 0; i < 10; i = i + 2) {
    print(i);
  }
}

Notice that instead of increasing by 1, we increased by 2, so we only got even numbers as outputs.

But why did it not print 10?

That is because the loop will run while i is less than 10.

When i reaches 10, the condition i < 10 becomes false, so the loop stops.

But when we do this:

void main() {
  for (int i = 0; i <= 10; i = i + 2) {
    print(i);
  }
}

The program also prints 10 because the loop will run while i is less than or equal to 10.

While Loops

Now let’s look at while loops.

Here is an example of a while loop:

void main() {
  int i = 0;
 
  while (i < 10) {
    print(i);
    i++;
  }
}

Try running this program.

What is the result?

It is the same as the first for loop we ran.

So what is different?

We ran what is called a while loop.

Inside the parentheses for this kind of loop, you are only going to put the condition for when the loop should continue.

Instead of putting the incrementation inside the parentheses, you put it inside the curly braces.

Putting Everything Together

Before moving on, look at this code and predict the outcome.

This code includes:

  • A boolean variable
  • An if statement
  • A while loop
void main() {
  bool isSeven = false;
  int i = 0;
 
  while (isSeven == false) {
    if (i == 7) {
      print("Number is 7!");
      isSeven = true;
    } else {
      print("Number is not 7");
      i++;
    }
  }
}

Try predicting what the result will be and then run the code.

In this example, we have a boolean variable called isSeven, and it starts as false.

We want it to become true when the value of i is 7.

Then, we have a while loop that will continuously run while isSeven is false.

We have an if statement inside the while loop to check if the value of i is 7.

If it is not 7, we add 1 to i every time.

Once the value of i is 7, the boolean variable isSeven becomes true, which ultimately ends the while loop.

Step 7: Functions

Finally, let’s learn about functions.

Before we explain what they are and what they do, let’s look at the code first:

void main() {
  add(2, 3);
  print(add2(2, 4));
}
 
void add(int a, int b) {
  print(a + b);
}
 
int add2(int a, int b) {
  return a + b;
}

You can see that we have code outside the main function.

What are they doing outside of it?

Like the main function, these functions contain code that can be run.

However, these functions will not run unless you call them inside the main function.

That is why the main function is so important.

When you press Run in your IDE, the program will first go into the main function and start from there.

Inside the main function, it finds:

add(2, 3);

So now it will go and find the add function outside the main function.

The program finds:

void add(int a, int b) {
  print(a + b);
}

The add function is a void function, which means it does not return a value.

Inside the parentheses, there are 2 variables:

int a
int b

These are called parameters.

Inside the function, these values are added together and printed.

Parameters vs. Arguments

Let’s go back to the function call:

add(2, 3);

The 2 numbers inside the parentheses are the values that will be given to a and b.

These values are called arguments.

So:

add(2, 3);

means:

  • a = 2
  • b = 3

The function then adds them together and prints 5.

Returning a Value

Let’s look at the code again:

print(add2(2, 4));

Why is this function call inside a print() statement while the other one isn’t?

Let’s go to where the function is written:

int add2(int a, int b) {
  return a + b;
}

The add2 function is not a void function.

Instead, it is an int function.

That means the function needs to return an int value back to the main function.

The function returns:

a + b

Since 2 + 4 = 6, the function returns 6.

Then, the print() statement prints that returned value.

Dart Coding Challenge: Amusement Park Ride

Now that we’ve learned the basics of Dart, let’s test your knowledge with a small coding challenge.

Challenge

Create a simple program that determines whether someone can ride a roller coaster at an amusement park.

Requirements

Your code must have these variables in the main() function:

  • An int variable called age.
  • A double variable called height.
  • A bool variable called isRaining.
  • A String variable called name.
  • An int variable called time and set it to 13, representing 1:00 PM.

Roller Coaster Function

Create a boolean function called rollerCoaster.

The function should have these four parameters:

  • An int for age
  • A double for height
  • A bool for whether it is raining
  • A String for the person’s name

Inside the function:

  • Check if the person is 13 or older.
  • Check if the person is 160 cm or taller.
  • Check that it is not raining.
  • If all three conditions are true, print a message using their name saying they can ride the roller coaster and return true.
  • Otherwise, print a message using their name saying they cannot ride the roller coaster and return false.

Calling the Function

In main():

  • Call the rollerCoaster function using the variables you created.
  • If the function returns true, add 2 to time and print the new time.
  • If the function returns false, print the original time without changing it.

Example

If:

int age = 15;
double height = 173.2;
bool isRaining = false;
String name = "David";
int time = 13;

The person should be allowed to ride, and the time should change from 13:00 to 15:00.

Once done, take a screenshot of the code and the output results and send it to David through Discord or show it to him at a meeting.

Example Output

Variables:

int age = 15;
double height = 173.2;
bool isRaining = false;
String name = "David";

Output:

David can ride the roller coaster!
Rode the roller coaster so time is now 15:00

Congrats!

Now you are ready to move onto Module 2 of Non-Robot Programming!

Lesson 6G201 Calculator