Difference between revisions of "If"
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Revision as of 16:12, 24 October 2008
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if ( condition ) branch
| • | condition | – | If this executes as true then branch is executed. | |
| • | branch | – | Can be either a single statement, a block statement, or a null statement. |
if ( condition ) branch_true else branch_false
| • | condition | – | If this executes as true then branch_true is executed otherwise branch_false is executed. | |
| • | branch_true | – | Can be either a single statement, a block statement, or a null statement. | |
| • | branch_false | – | Can be either a single statement, a block statement, or a null statement. |
It is important to know that there is an artificial limit to the number of else-if's that can be chained together. A syntax error will be thrown if this limit is exceeded. This is caused by the stack depth limit on the compiler and depending upon how many globals and locals in the scope are on the stack change the number of else-if's that can be chained together. 23 is about as many as can be reasonably expected but fewer is not unheard of. VWR-811
Specification
| Type | Condition |
|---|---|
| integer | True if it is not zero. |
| float | True if it is not zero.[1] |
| string | True if its length is not zero. |
| key | True only if it is a valid key and not NULL_KEY. |
| vector | True if the vector is not ZERO_VECTOR. |
| rotation | True if the rotation is not ZERO_ROTATION. |
| list | True if the length is not zero. Note that correct behavior is only seen with Mono-compiled scripts; LSO-compiled scripts incorrectly resolve to false if the list is non-empty: BUG-230728 |
Operators in LSL do NOT short circuit. For example:
| <lsl>// A simple method to say if the method was called.
integer test() { llOwnerSay("Test method called!");
return TRUE;
} default { touch_start(integer total_number)
{
if (FALSE && test())
{ // If short-circuit optimized then the test() should never be called.
// Will never get here.
}
}
}</lsl> |
In a short-circuited language if the left-hand side of a logical AND (&&) is FALSE the right-hand side is never tested since the entire test would always return a FALSE output no matter what the value of the right-hand side. Since LSL is not short circuited, both the left and the right-hand sides are tested all the time. This can be simulated by using a nested if structure:
<lsl>default {
touch_start(integer total_number)
{
if (FALSE)
{
if (test())
{
// Will never get here.
}
}
}
}</lsl>
Examples
Simple inline if statement: <lsl>if (a == 1) c = b;</lsl>
Simple block if statement: <lsl>if (a == 1) {
// Do something here.
}</lsl>
Complex if/else block (only one line of text will be said by this example) <lsl>if (a == "Loren") {
llSay(0, "Lorem ipsum sic amet!");
} else if (a == "Bob") {
llSay(0, "Babble dabble rabble rouse.");
} else {
llSay(0, "Gobbledygook? or English?");
}</lsl>
Compounded if statement: <lsl>if (a == 1 && b == c) {
// more here.
}</lsl>
Nested if statements: <lsl>if (a == 1) {
if (b == c)
{
// more here.
}
}</lsl>
Watch out for misplaced semicolons. They can, and will, cause strange things to happen. For instance: If you place a semicolon between an if statement and a code block, the if statement will not control the execution of the block. <lsl>if (a == "Loren"); {
llSay(0, "Lorem ipsum sic amet!");
}</lsl>
This code will execute in the following order:
- The line
if (a == "Loren");will execute; the semicolon will tell LSL that this is a simple inline if statement. - Next it meets the start of the code block and will execute the contents, in this case saying "Lorem ipsum sic amet!", no matter what the outcome of the above if statement.
Another problem happens when someone forgets to use the curly braces. <lsl>if (a == "Loren")
llSay(0, "Lorem ipsum sic amet!"); llSay(0, "I don't know what it means either...");</lsl>
The assumption is that because the indentation makes it "look" like both llSay commands are controlled by the if statement, that they are. In fact, if a was equal to "Joe" then all that would be said is "I don't know what it means either..."
Deep Notes
Footnotes
- ^ The OpenSim LSL compiler will not do this implicitly. You will need to use an explicit check.