6 Loops¶

In the first five chapters of this book, your code ran from the top of the main function to the bottom, and then it was finished. With the addition of if statements in the previous chapter, you gave your code the opportunity to make decisions. However, it’s still running from top to bottom, albeit following different branches.

Rather than just running through a set of instructions once, it’s often useful to repeat tasks. Think about all the repetitious things you do every day:

Breathing: Breathe in, breathe out, breathe in, breathe out…
Walking: Right leg forward, left leg forward, right leg forward, left leg forward…
Eating: Spoon up, spoon down, chew, chew, chew, swallow, repeat…

Computer programming is just as full of repetitive actions as your life is. The way you can perform these actions is by using loops. Dart, like many programming languages, has while loops and for loops. You’ll learn how to make them in the following sections.

While Loops¶

A while loop repeats a block of code as long as a Boolean condition is true. You create a while loop like so:

while (condition) {
  // loop code
}

The loop checks the condition on every iteration. If the condition is true, then the loop executes and moves on to another iteration. If the condition is false, then the loop stops. Just like if statements, while loops introduce a scope because of their curly braces.

The simplest while loop takes this form:

while (true) { }

This is a while loop that never ends because the condition is always true. Of course, you would never write such a while loop, because your program would spin forever! This situation is known as an infinite loop, and while it might not cause your program to crash, it will very likely cause your computer to freeze.

If you actually tried to run that infinite loop in VS Code and can’t figure out how to make it stop, press the Stop button:

Here’s a somewhat more useful example of a while loop:

var sum = 1;
while (sum < 10) {
  sum += 4;
  print(sum);
}

Run that to see the result. The loop executes as follows:

Before 1st iteration: sum = 1, loop condition = true
After 1st iteration: sum = 5, loop condition = true
After 2nd iteration: sum = 9, loop condition = true
After 3rd iteration: sum = 13, loop condition = false

After the third iteration, the sum variable is 13, and therefore the loop condition of sum < 10 becomes false. At this point, the loop stops.

Do-While Loops¶

A variant of the while loop is called the do-while loop. It differs from the whileloop in that the condition is evaluated at the end of the loop rather than at the beginning. Thus, the body of a do-while loop is always executed at least once.

You construct a do-while loop like this:

do {
  // loop code
} while (condition)

Whatever statements appear inside the braces will be executed. Finally, if the whilecondition after the closing brace is true, you jump back up to the beginning and repeat the loop.

Here’s the example from the last section, but using a do-while loop:

var sum = 1;
do {
  sum += 4;
  print(sum);
} while (sum < 10);

In this example, the outcome is the same as before.

Comparing While and Do-While Loops¶

It’s possible to only use while loops, but in some cases, your code will be cleaner with a do-while loop. Take the following while loop as an example:

var sum = (1 + 3 - 2 * 4 + 8);
while (sum < 10) {
  sum += (1 + 3 - 2 * 4 + 8);
}
print('while loop sum: $sum');

All that math in the parentheses is there merely to represent a complex operation that you need to run at least once, and then run again on every iteration of the loop.

Note

In Chapter 7, “Functions”, you’ll learn about grouping related code into a single complex operation called a function. Since you haven’t studied functions yet, though, this chapter represents the idea by using a series of mathematical operations.

The problem with the while loop above is that you need to repeat (1 + 3 - 2 * 4 + 8) both to initialize the variable and on every iteration of the loop. Using a do-whileloop eliminates the need for repetition:

var sum = 0;
do {
  sum += (1 + 3 - 2 * 4 + 8);
} while (sum < 10);
print('do-while loop sum: $sum');

Here, you only wrote the complex operation code once. Less repetition makes for cleaner code!

Breaking Out of a Loop¶

Sometimes you’ll need to break out of a loop early. You can do this using the breakstatement, just as you did from inside the switch statement earlier. This immediately stops the execution of the loop and continues on to the code that follows the loop.

For example, consider the following while loop:

sum = 1;
while (true) {
  sum += 4;
  if (sum > 10) {
    break;
  }
}

Here, the loop condition is true, so the loop would normally iterate forever. However, the break means the while loop will exit once the sum is greater than 10.

You’ve now seen how to write the same loop in different ways. This demonstrates that in computer programming there are often many ways to achieve the same result. You should choose the method that’s easiest to read and that conveys your intent in the best way possible. This is an approach you’ll internalize with enough time and practice.

Exercise¶

Create a variable named counter and set it equal to 0.
Create a while loop with the condition counter < 10.
The loop body should print out “counter is X” (where X is replaced with the value of counter) and then increment counter by 1.

For Loops¶

In addition to while loops, Dart has another type of loop called a for loop. This is probably the most common loop you’ll see, and you use it to run a block of code a set number of times.

Here’s a simple example of a C-style for loop in Dart:

for (var i = 0; i < 5; i++) {
  print(i);
}

If you have some prior programming experience, this C programming language style for loop probably looks very familiar to you. If not, though, the first line would be confusing. Here’s a summary of the three parts between the parentheses and separated by semicolons:

var i = 0 (initialization): Before the loop starts, you create a counter variable to keep track of how many times you’ve looped. You could call the variable anything, but i is commonly used as an abbreviation for index. You then initialize it with some value; in this case, 0.
i < 5 (condition): This is the condition that the for loop will check before every iteration of the loop. If it’s true, then it will run the code inside the braces. But if it’s false, then the loop will end.
i++ (action): The action runs at the end of every iteration, usually to update the loop index value. It’s common to increment by 1 using i++ but you could just as easily use i += 2 to increment by 2 or i-- to decrement by 1.

Run the previous code and you’ll see the following output:

The counter index i started at 0 and continued until it equaled 5. At that point, the for loop condition i < 5 was false, so the loop exited before running the printstatement again.

The Continue Keyword¶

Sometimes you want to skip an iteration only for a certain condition. You can do that using the continue keyword. Have a look at the following example:

for (var i = 0; i < 5; i++) {
  if (i == 2) {
    continue;
  }
  print(i);
}

This example is similar to the last one, but this time, when i is 2, the continuekeyword will tell the for loop to immediately go on to the next iteration. The rest of the code in the block won’t run on this iteration.

This is what you’ll see:

No 2 here!

More For Loops¶

There are two more kinds of for loops that you’ll learn about later:

for-in loops
for-each loops

You’ll study for-in loops in Chapter 12, “Lists”, and for-each loops in the “Anonymous Functions” chapter of Dart Apprentice: Beyond the Basics.

Exercise¶

Write a for loop starting at 1 and ending with 10 inclusive.
Print the square of each number.

Challenges¶

Before moving on, here are some challenges to test your knowledge of loops. It’s best if you try to solve them yourself, but solutions are available in the challenge folder if you get stuck.

Challenge 1: Next Power of Two¶

Given a number, determine the next power of two above or equal to that number. Powers of two are the numbers in the sequence of 2¹, 2², 2³, and so on. You may also recognize the series as 1, 2, 4, 8, 16, 32, 64…

Challenge 2: Fibonacci¶

Calculate the nth Fibonacci number. The Fibonacci sequence starts with 1, then 1 again, and then all subsequent numbers in the sequence are simply the previous two values in the sequence added together (1, 1, 2, 3, 5, 8…). You can get a refresher here: https://en.wikipedia.org/wiki/Fibonacci_number

Challenge 3: How Many Times?¶

In the following for loop, what will be the value of sum, and how many iterations will happen?

var sum = 0;
for (var i = 0; i <= 5; i++) {
  sum += i;
}

Challenge 4: The Final Countdown¶

Print a countdown from 10 to 0.

Challenge 5: Print a Sequence¶

Print the sequence 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0.

Key Points¶

while loops perform a certain task repeatedly as long as a condition is true.
do-while loops always execute the loop at least once.
for loops allow you to perform a loop a set number of times.
The break statement lets you break out of a loop.
The continue statement ends the current iteration of a loop and begins the next iteration.