The First Program

A program is a list of instructions that the computer carries out in order, from the top of the file to the bottom. This page writes one, runs it, and then grows it a little at a time.

If you haven’t yet, install Emerald first.

Hello, world

Make a file named hello.em containing one line:

print("Hello, world!")

Then open a terminal in the folder where you saved it, and run it:

emerald run hello.em

Output

Hello, world!

That line is a complete program. print is a function built into Emerald: it shows whatever you give it, then moves to a new line. The text between the double quotes is a string, Emerald’s word for a piece of text. The quotes mark where the string starts and ends; they are not printed.

Every Emerald file ends in .em, and emerald run runs one.

One line after another

A program with several lines runs them in order:

print("Ready...")
print("Set...")
print("Go!")

Output

Ready...
Set...
Go!

print can show more than one value at once. Separate them with commas, and it puts a space between each:

print("The answer is", 42)

Output

The answer is 42

42 has no quotes because it isn’t text, it is a number. With nothing between the parentheses, print() prints an empty line.

Asking a question

input shows a prompt, waits for someone to type an answer and press Enter, and gives back what they typed:

const name = input("What is your name? ")
print("Hello, #{name}!")

Terminal

What is your name? Ada
Hello, Ada!

Here Ada is what the person typed. Two new things are at work:

• const name = ... gives the answer a name, so the next line can use it. The page on variables and constants explains names properly.
• #{name} inside a string puts a value into the text. Whatever is between #{ and } is worked out, and the result takes its place. It can be any expression, not only a name:

const apples = 3
print("You have #{apples * 2} apples.")

Output

You have 6 apples.

The space at the end of "What is your name? " is deliberate: without it, the answer would be typed right up against the question mark.

When something is wrong

Everyone makes mistakes when writing programs, and a lot of programming is reading what the computer says about them. Emerald checks the whole program before it runs any of it, and when something is wrong it says what, where, and what to try. Suppose print is misspelled:

pritn("Hello")

Output

hello.em:1:1: [E1001] `pritn` is not defined
  pritn("Hello")
  ^^^^^
Check the spelling, or declare it before this line.

Read a message from the top:

• hello.em:1:1 is where the problem is: the file, then the line, then the column (how many characters along the line).
• Then comes what is wrong, in plain words. [E1001] is the problem’s code; emerald explain E1001 shows a longer explanation with an example.
• The line itself is shown, with ^ marks under the part that is wrong.
• The last line suggests what to do about it.

Because Emerald checks first, nothing runs until every problem is fixed. A program never gets halfway and then stops over a typo on its last line.

Two more mistakes nearly everyone meets early on. A string needs its closing quote:

print("Hello)

Output

hello.em:1:7: this string is never closed
  print("Hello)
        ^
Close it with `"` before the line ends.

And text and numbers can’t be added together with +, because it isn’t clear what adding them would mean:

print("Age: " + 12)

Output

hello.em:1:7: `+` joins two Strings, but this is String and Int
  print("Age: " + 12)
        ^^^^^^^^^^^^
Convert the value with `to_string()`, or use interpolation, as in `"#{name}: #{score}"`.

The suggestion is the usual way to write it: print("Age: #{12}").

Checking without running

emerald check looks for problems without running the program:

emerald check hello.em

Output

No problems found.

It is useful for a program that asks questions or takes a long time, when you only want to know whether it is correct so far.

What you’ve learned

• A program runs its lines in order, top to bottom, and emerald run file.em runs it.
• print shows values; a string is text between double quotes.
• input asks a question and gives back the answer.
• #{...} puts a value into a string.
• Emerald checks the whole program first, and its messages say what is wrong, where, and what to try.

Next, comments: leaving notes in a program for the people who read it.