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function return and a simple if statement

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Posted by Nick Gammon Australia (23,173 posts) Bio Forum Administrator

Date Reply #15 on Sun 04 Mar 2007 04:26 AM (UTC)

Message

Quote:

if string.find (defs, "p") ~= nil then aff.prone = 1 else aff.prone = 0 end

Apart from the problem with 0 and 1, you can rewrite that more neatly:



aff.prone = string.find (defs, "p") ~= nil

- Nick Gammon

www.gammon.com.au, www.mushclient.com

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Posted by

Gore (207 posts) Bio

Date

Reply #16 on Mon 05 Mar 2007 08:49 AM (UTC)

Message

I'm assuming that it sets the variable to true or false, but how exactly does it work?

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Posted by David Haley USA (3,881 posts) Bio

Date Reply #17 on Mon 05 Mar 2007 09:24 AM (UTC)

Message

The first thing that happens is figuring out the value of the following:

string.find (defs, "p") ~= nil

For simplicity's sake, let's call the result of string.find "result".

If result is equal to nil, then result == nil is true, so result ~= nil is false. Therefore, the whole expression is false, so we put false into aff.prone.

However if result is not nil, then result == nil is false so result ~= nil is true. So the expression is true, and aff.prone becomes true.

In general, you have:
variable = expression

So, you first figure out the value of expression, and you put it into variable.

Personally I would have written:

aff.prone = (string.find (defs, "p") ~= nil)

To me, adding in the parentheses makes the assignment clearer.

David Haley aka Ksilyan
Head Programmer,
Legends of the Darkstone

http://david.the-haleys.org

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Posted by

Gore (207 posts) Bio

Date

Reply #18 on Mon 05 Mar 2007 10:53 AM (UTC)

Message

I thought that was what was happening, but I didn't want to assume.

That's a very neat shortcut, thanks a lot -g-
I wonder what other shortcuts are nice to use. Lua is amazing!

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Posted by David Haley USA (3,881 posts) Bio

Date Reply #19 on Mon 05 Mar 2007 11:07 AM (UTC)

Message

Actually it's a fairly common trick across languages. Almost all languages assign an expression to a value (in some, the assignment itself is an expression!). Then, any kind of expression is fair game.

Syntactically, the number "4" alone is an expression, and so is, say, "4+2". But, you can also have relational operators: things that compare two values and return true if the values are in that relation. You could have, say in C++:

bool b = 1 < 2;

This would be always true.

Once you accept this, it's just one small step to allow for function calls as parts of that expression. And then you can have things like what Nick showed you, or arbitrarily complicated expressions the value of which will be assigned to a variable.

Consider this, in C(++):

int i;

int j;



i = (j = (5>3 ? 24 : 5));

This is somewhat contrived, but what this means is this:

If 5>3, assign 24 to j; otherwise assign 5.
Assign to i the value of j's assignment. (In other words, assign j to i.)

One common thing to see is something like this:

char * p;
if ( (p = get_the_pointer()) != NULL ) {
// do something interesting with p
}

In this case, we first get a value for p. Then, we check p's value against NULL; if it's not null, we do whatever's inside the if block, with a value already assigned to p. It is shorter than, but basically the same as, the following:


if ( get_the_pointer() != NULL ) {
  char * p = get_the_pointer();
  // do something interesting with p
}

(It's not the same thing when the function get_the_pointer has side effects.)

David Haley aka Ksilyan
Head Programmer,
Legends of the Darkstone

http://david.the-haleys.org

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Posted by

Gore (207 posts) Bio

Date

Reply #20 on Tue 20 Mar 2007 09:04 PM (UTC)

Message

Sweet, thanks a bunch :>

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