If

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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.

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if ( condition ) branch_true else branch_false

•	condition	–	If this executes as true then branch 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.

Specification

**Conditional Types**
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

Examples

Simple inline if statement:

if (a == 1) c = b;

Simple block if statement:

if (a == 1)
{
    // Do something here.
}

Compounded if statement:

if (a == 1 && b == c)
{
    // more here.
}

Nested if statements:

if (a == 1)
{
    if (b == c)
    {
        // more here.
    }
}

Notes

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.

if (a == "Loren");
{
    llSay(0, "Lorem ipsum sic amet!");
}

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.

if (a == "Loren")
    llSay(0, "Lorem ipsum sic amet!");
    llSay(0, "I don't know what it means either...");

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..."

Operators in LSL are NOT short circuited. For example:

// 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 the test is short-circuit optimized then the test() method should never be called.
            // Will never get here.
        }
    }
}

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:

default {
    touch_start(integer total_number) {
        if (FALSE) {
            if (test()) {
              // Will never get here.
            }
        }
    }
}

Deep Notes

Search JIRA for related Issues

Footnotes

^ The OpenSim LSL compiler will not do this implicitly. You will need to use an explicit check.

If

If