Parrot Babysteps 03 - Simple Control Structures

goto

A goto instruction sends Parrot execution to a specific labelled location in your code.

goto <LABEL_IDENTIFIER>

When it follows a goto, Parrot picks up by executing the first instruction following the label. In this example, it creates an infinite loop which can only be interrupted by pressing Control-C.

# example-03-03.pir
.sub main :main
    .const string PROMPT = "What is your name? "
    .local string name
    .local string response
    .local pmc    stdin

    stdin = getstdin

GET_NAME:
    name = stdin.'readline_interactive'(PROMPT)

    goto GET_NAME

    response = "Hello, " . name
    say response
.end

Infinite loops have their occasional uses, but most of the time they’re just annoying.

$ parrot example-03-03.pir
What is your name? Brian
What is your name? Brian
What is your name? I said "Brian!"
What is your name? <Control-C>

if

The if operation lets us perform specific instructions when a particular condition is true by branching control. At our level of expertise, that branching is performed with goto. So for us, the general if syntax looks like this:

if <conditional> goto <LABEL>

Let’s see if in action by adding a little control logic to our name prompt.

# example-03-04.pir
.sub 'main' :main
    .const string PROMPT = "What is your name? "
    .local string name
    .local string response
    .local pmc    stdin

    stdin = getstdin

    name = stdin.'readline_interactive'(PROMPT)
    if name == "Brian" goto GREETING_FOR_BRIAN
    response = "Hello, " . name
    goto SAY_IT

  GREETING_FOR_BRIAN:
    response = "Hey, Brian!"

  SAY_IT:
    say response

.end

I admit it. This example is all about making me feel good. One greeting is prepared for users named Brian, while another greeting is prepared for everybody else. The program accomplishes this by examining the value of name. If name is "Brian", the program is instructed to goto the label GREETING_FOR_BRIAN. Otherwise it continues on, preparing a generic response and going to the SAY_IT label. GREETING_FOR_BRIAN prepares a custom response and follows up to the next instruction, which happens to be the SAY_IT code. Remember that labels are just markers. They don’t cut the labeled code off from the rest of your code.

# example-03-05.pir
.sub main :main
    .local int    a
    .local int    b
    .local pmc    stdin

    stdin = getstdin

    a = stdin.'readline_interactive'('a: ')
    b = stdin.'readline_interactive'('b: ')

    if a > b goto A_IS_GREATER
    if b > a goto B_IS_GREATER
    goto BOTH_EQUAL

A_IS_GREATER:
    say "a is greater"
    goto END
B_IS_GREATER:
    say "b is greater"
    goto END
BOTH_EQUAL:
    say "a and b are the same"
    goto END

END:
    say "Wasn't that fun?"
.end

$ parrot example-03-05.pir
a: 5
b: 10
b is greater
Wasn't that fun?

# example-03-06.pir
.sub main :main
    .const int MINIMUM = 10
    .const int MAXIMUM = 100
    .local int input
    .local int input_is_valid
    .local pmc stdin

    stdin = getstdin

GET_NUMBER:
    print "Enter a number ("
    print MINIMUM
    print " - "
    print MAXIMUM
    print ")"
    input = stdin.'readline_interactive'(': ')
    $I0 = input >= MINIMUM
    $I1 = input <= MAXIMUM
    input_is_valid = and $I0, $I1
    if input_is_valid goto VALID_INPUT
    say "That is not in the acceptable range!"
    goto END_PROGRAM

  VALID_INPUT:
    say "That is in the acceptable range."

  END_PROGRAM:
    say "Thank you!"
.end

$ parrot example-03-06.pir
Enter a number (10 - 100): 990
That is not in the acceptable range!
Thank you!
$ parrot example-03-06.pir
Enter a number (10 - 100): 82
That is in the acceptable range.
Thank you!

  $I0 = input >= MINIMUM
  $I1 = input <= MAXIMUM
  input_is_valid = and $I0, $I1
  if input_is_valid goto VALID_INPUT
  say "That is not in the acceptable range!"
  goto END_PROGRAM

VALID_INPUT:
  say "That is in the acceptable range."

END_PROGRAM:
  say "Thank you!"

if ($input >= $MINIMUM && $input <= $MAXIMUM) {
    say "That is in the acceptable range.";
} else {
    say "That is not in the acceptable range.";
}

say "Thank you!";

# example-03-07.pir
.sub 'main' :main
    .const int MINIMUM = 10
    .const int MAXIMUM = 100
    .local int input
    .local int input_is_invalid
    .local pmc stdin

    stdin = getstdin

GET_NUMBER:
    print "Enter a number ("
    print MINIMUM
    print " - "
    print MAXIMUM
    print ")"
    input = stdin.'readline_interactive'(': ')
    $I0 = input <= MINIMUM
    $I1 = input >= MAXIMUM
    input_is_invalid = or $I0, $I1
    if input_is_invalid goto WARN_USER_ABOUT_INPUT
    goto END_PROGRAM

WARN_USER_ABOUT_INPUT:
    say "That is not in the acceptable range!"

END_PROGRAM:
    say "Thank you!"
.end

unless

Sometimes the normal if test does not clearly describe your needs. You may only want to branch if a test fails. You can do this with if, but it’s awkward. Parrot provides the unless test for exactly this situation. unless looks similar to if:

unless <conditional> goto <LABEL>

I can use it in the user prompt program to streamline the description of the application’s response to anyone but me.

# example-03-08.pir
.sub main :main
    .const string PROMPT = "What is your name? "
    .local string name
    .local string response
    .local pmc    stdin

    stdin = getstdin

    name = stdin.'readline_interactive'(PROMPT)
    unless name == "Brian" goto GENERIC_GREETING
    response = "Hey, Brian!"
    goto SAY_IT

  GENERIC_GREETING:
    response = "Hello, " . name

  SAY_IT:
    say response
.end

The program will run the same after this little change. Try it and see.

Usage of unless is a personal choice. I like it because it lets me describe my program a little more concisely. Others don’t like to use unless because it adds to the mental load of reading application code: "let’s see, unless means '`if` this is not true." Both points of view are valid, and ultimately it’s up to you whether unless belongs in your code.

Repetition

We saw the infinite loop earlier when we first looked at goto. Let’s explore more controlled loops.

Believe it or not, the combination of if and goto provide us with the core features we need for a wide range of control structures. Things can get interesting now that we’ve added these tools to our kit.

How about a countdown?

# example-03-09.pir
.sub 'main' :main
    .const int start = 10
    .const int stop  = 0
    .local int current

    current = start
  COUNTDOWN:
    if current < stop goto LIFTOFF
    say current
    current -= 1
    goto COUNTDOWN

  LIFTOFF:
    say "Liftoff!"
.end

This presents a simple count-controlled loop. We set a loop counter current to a reasonable start value before we start looping. We check the value of the loop counter each time we start the loop, quitting if current is less than our stopping value. After our check, we display the value, subtract one, and run the loop again.

A condition-controlled loop is easy, too.

# example-03-10.pir
.sub main :main
    .const string PROMPT = "What is your name? "
    .local string name
    .local string response
    .local pmc    stdin

    stdin = getstdin

GET_NAME:
    name = stdin.'readline_interactive'(PROMPT)
    if name goto GREET_USER
    goto GET_NAME

GREET_USER:
    response = "Hello, " . name
    say response
.end

Our script will now continue to check to see if the user actually entered anything for a name. If the user has entered a name, control goes to the GREET_USER label. Otherwise, control is sent backwards to the GET_USER label.

$ parrot example-03-10.pir
What is your name?
What is your name? Brian
Hello, Brian

Collection loops such as list iterators will have to wait until we examine PMCs.