Programs: talking to a computer

You now know that a computer follows an algorithm: a clear list of steps. But a computer cannot read English. How do we give it the steps? We write a program.

From steps to a program

A program is an algorithm written in a language that a computer can follow. These are called programming languages. Some famous ones are Python, JavaScript and Scratch.

A computer's processor only understands bits, patterns of 1s and 0s called machine code. Writing those by hand would be slow and painful. So programmers write in a friendlier language, and a special program (an interpreter or compiler) translates it into machine code for the processor.

Here is the "find the biggest number" algorithm from the last lesson, written in Python:

numbers = [4, 9, 2, 7]

biggest = numbers[0]
for n in numbers:
    if n > biggest:
        biggest = n

print(biggest)

Even if you have never programmed, you can probably guess what it does. Read it out loud:

• Make a list of numbers.
• Say the biggest is the first one.
• For each number n in the list: if n is bigger than the biggest, make it the new biggest.
• Show the biggest.

It prints 9.

The main ideas

Nearly every language has the same few ideas.

Variables: labelled boxes. A variable is a box with a name, for storing a value. biggest = numbers[0] puts the first number in a box called biggest. Later you can change what is in the box.

Choice: if. Do something only when a test is true.

if score > 10:
    print("You win!")
else:
    print("Keep trying")

Repetition: loops. Repeat steps.

for count in range(3):
    print("Hello")

This prints "Hello" three times. Imagine writing that for a million repeats by hand! A loop does it in two lines.

Functions: named recipes. A function is a set of steps with a name, so you can use it again.

def double(x):
    return x * 2

print(double(21))

This prints 42. The program gives the steps a name, double, and now it can be used anywhere.

Even a puppy can be programmed

Here is a tiny piece of a puppy's world, in the same style:

while not at_treat:
    move("right")

"Keep moving right until you reach the treat." Train the Puppy goes further: instead of the programmer writing every step, the puppy learns its steps. But the learning itself is also a program, with a loop and a few lines of arithmetic.

Bugs

When a program does the wrong thing, it has a bug. Bugs come in two main kinds:

• Syntax errors. You broke the grammar rules, for example by forgetting a bracket. The computer usually refuses to run the program and points at the mistake.
• Logic errors. The program runs, but it does not do what you wanted. For example, using < instead of > would find the smallest number instead.

The second kind is trickier, because the computer is not complaining. It is doing exactly what you said. That is the golden rule of programming:

A computer does what you tell it, not what you meant.

Remember the puppy and the sneaky toy? It did exactly what the rewards said, not what the designer meant. Same lesson, many years later and in a new form.

Finding and fixing bugs is called debugging, and it is a big part of every programmer's day. A good method is to read your program slowly, line by line, and check what each step really does.

Where programs run

The games on this site are programs. They are written in a language called TypeScript (a version of JavaScript), and they run in your web browser. When you move a slider in Launch Lab, a function runs, does some sums, and draws the answer.