The Delphi/Pascal Sentence

...or "When/Where to use semicolons".

Apologies to all you long time Pascal gurus, but in the last few days I was pondering on the advice I gave a few students learning Pascal and felt that Today's post should be directed at those learning the language.

Many new-comers find it difficult to grasp when they should be using a semicolon. The rules seem complex and inconsistant to them, when in fact they are reasonably consistant. Consider this...

"Pascal is a sentence"

Writing in Delphi Pascal is writing sentances, its that simple. Consider the following simple piece of code...

if x > 0 then
  ShowMessage('x is worth something')
else
  ShowMessage('x is worthless');

That equates to a sentence of "If x is greater than zero then x is worth something otherwise x is worthless.". Some people call that a Pascal statement, but I like to think of it as a sentence. Sentences are normally ended in a full stop, but in Pascal, sentances are ended in a semicolon and only the program is ended in a full stop. in the above Pascal sentence, the semicolon comes at the end as it should, and not half way through. You wouldn't say "If x is greater than zero. Then x is worth something. Otherwise. X is worthless." would you? That's 4 parts of a sentence that are seperated into 4 sentences that are meaningless.

So why can't we just place a semicolon before the else? Well think of it. If someone is talking to you in fluent english and finishes the sentence, then a little later (when you are thinking of something else), states "otherwise...", you would say "Huh? what do you mean 'Otherwise'". The Delphi Pascal compiler would state the equivelent of "Huh?" at that point and totally fail to understand you.

"OK-ish", I hear you saying "but what about all that begin/end stuff, nobody says that in real life", and normally you'd be right. Consider the following...

if x > 0 then
begin
  y := x;
  ShowMessage('x and y are the same');
  y := y * 10;
  ShowMessage('and now they are not')
end;

How does that equate to a sentence? Well, there is an overriding sentence there that says something like "If x is greater than zero then do some things...". (There's more to that sentence that I'll add soon).

You can think of begin/end as brackets if you like, that's what C and some other languages do, but Pascal is ever so slightly different. When speaking fluent Pascal (Pascal is a language after all), and you want to say some other sentences in the middle of your main sentence, you will need to let the listener know that by saying 'begin' and 'end'.

Each of the lines ending in a semicolon are their own sentences: "make y equal to x"; "say 'x and y are the same'"; "make y equal to y times 10". Being sentences in their own right, they can even have their own begin/ends for their sentences if they need them.

What about that last ShowMessage() statement? Well, here's where the Pascal sentence comes in. The overriding sentence actually says "If x is greater than zero then do some things and say 'and now they are not'.". More correctly, in fluent Pascal the true sentence is "If x is greater than zero then begin do some things and say 'and now they are not' end."

When speaking Pascal you do have to say the 'begin' and 'end' words in a sentence.

Now I can also hear you saying "but I have seen a semicolon before the 'end' many times". Yes, and pascal is smart enough to understand that you are saying nothing. Placing a semicolon before the 'end' is the equivilent of stating an empty sentence before finishing the outer sentence. Sweet nothings.

So what about Procedures and Functions? Procedures and functions are just teaching the Pascal language new words to use. For example...

Procedure Swap(var x, y: integer);

...says "I'm going to teach you the word 'Swap' and it will involve 2 whole numbers". Here's the full procedure which takes 2 numbers and swaps their values so that x equals what y did and y equals what x did (normally used in something like a simple shell sort) ...

Procedure Swap(var x, y: integer);
var i : integer;
begin
  i := x;
  x := y;
  y := i
end;

In this case, the sentence doesn't flow unless you expand it a little.

"Here's a new Pascal Word 'Swap' that takes 'x' and 'y' as variable parameters.
'i' is a new variable just for the word 'Swap'.
begin do some things and then make y equal i end.

The 'do some things' were some other sentences as you can see.

Are you now more confused than ever? Don't worry, that's normal, you are becoming a programmer after all :-)