Up to now, every program ran every line, top to bottom, no matter what. Real programs make choices: if the player has no lives left, show "Game over"; otherwise, keep playing. In this lesson your programs learn to decide.

✦Asking questions

A decision starts with a question whose answer is yes or no: a Boolean, ok (true) or ort (false). These comparison operators ask questions about two values:

bongSlanhOun "questions";
akthe age = 11;
jongyeytha age == 11;
jongyeytha age != 11;
jongyeytha age >= 13;
jongyeytha "mango" == "mango";

ok
ort
ort
ok

Remember: one = puts a value in a box, two == asks whether two things are equal.

✦ber: if

ber means if. It runs a block of code only when a question comes out ok:

bongSlanhOun "if";
akthe temperature = 36;
ber (temperature > 30) {
    jongyeytha "It's hot! Drink some water.";
}
jongyeytha "Have a nice day.";

It's hot! Drink some water.
Have a nice day.

The question goes inside round brackets ( ), and the code to run goes inside curly braces { }. Everything between a { and its matching } is called a block. We indent (push to the right with spaces) the lines inside a block. steav2 doesn't care about the spaces, but they make it much easier for humans to see what's inside what.

Change 36 to 20 and run it again: the first message disappears, but "Have a nice day." is still printed, because it's outside the block.

✦minjengte: else

minjengte means else (or "otherwise"). It gives ber a second block, for when the answer is ort:

bongSlanhOun "else";
akthe lives = 0;
ber (lives > 0) {
    jongyeytha "Keep playing!";
} minjengte {
    jongyeytha "Game over :(";
}

Game over :(

Exactly one of the two blocks runs, never both, never neither.

✦More than two choices

To choose between several options, put another ber straight after minjengte. steav2 checks each question in order and runs the first block whose answer is ok:

bongSlanhOun "grades";
akthe score = 72;
ber (score >= 90) {
    jongyeytha "A: amazing!";
} minjengte ber (score >= 70) {
    jongyeytha "B: great job!";
} minjengte ber (score >= 50) {
    jongyeytha "C: you passed!";
} minjengte {
    jongyeytha "Let's practise some more.";
}

B: great job!

72 is also bigger than 50, but only the first matching block runs.

✦Combining questions: ng, reu and !

Sometimes one question isn't enough. You can combine them:

• ng means and: a ng b is ok only if both are ok.
• reu means or: a reu b is ok if at least one is ok.
• ! means not: it flips ok into ort and back.

bongSlanhOun "combine";
akthe age = 15;
akthe hasTicket = ok;
ber (age >= 13 ng hasTicket) {
    jongyeytha "Enjoy the movie!";
}
akthe day = "Saturday";
ber (day == "Saturday" reu day == "Sunday") {
    jongyeytha "It's the weekend!";
}
ber (!hasTicket) {
    jongyeytha "Please buy a ticket first.";
}

Enjoy the movie!
It's the weekend!

The last message isn't printed: hasTicket is ok, so !hasTicket is ort.

✦Questions must really be questions

Some languages let you write ber (5) and guess what you meant. steav2 doesn't guess: the thing inside ber ( ) must be a Boolean.

bongSlanhOun "not a question";
akthe coins = 3;
ber (coins) {
    jongyeytha "I have coins";
}

[line 3] Error at 'coins': expected Boolean but got LekThom

What you probably meant was ber (coins > 0), which is a real yes-or-no question.

✦Your turn

1. Make a variable number and print positive, negative or zero depending on its value. Hint: ber, minjengte ber, minjengte.
2. A theme park ride needs you to be at least 120 cm tall and at least 8 years old. Write a program with height and age variables that prints whether you can ride. Hint: ng.
3. Print Nice! if a variable color is "pink" or "purple", and Hmm. otherwise.

Solution for exercise 1:

bongSlanhOun "sign";
akthe number = -4;
ber (number > 0) {
    jongyeytha "positive";
} minjengte ber (number < 0) {
    jongyeytha "negative";
} minjengte {
    jongyeytha "zero";
}

negative