Difference between revisions of "LlGetListLength"
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(Mono's llGetListLength is, in testing, 100% faster than list!=[]. list!=[] is still faster for LSL2 however.) |
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===Optimizations=== | ===Optimizations=== | ||
A faster and lighter (in bytecode) way to determine the length of a list is to do a not-equals compare with a null list. This works because the list not-equals compare returns the difference between the lengths, meaning that it returns the same result as <code>llGetListLength()</code>, minus the overhead in bytecode, and performance penalty of calling a non-native function. | A faster and lighter (in bytecode) way to determine the length of a list is to do a not-equals compare with a null list. This works because the list not-equals compare returns the difference between the lengths, meaning that it returns the same result as <code>llGetListLength()</code>, minus the overhead in bytecode, and performance penalty of calling a non-native function. Note: This optimization only is much less beneficial in Mono. Mono's llGetListLength function is almost twice as fast. | ||
<lsl> | <lsl> | ||
list in; | list in; | ||
Revision as of 20:42, 31 July 2012
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Summary
Function: integer llGetListLength( list src );| 0.0 | Forced Delay |
| 10.0 | Energy |
Returns an integer that is the number of elements in the list src.
| • list | src |
Caveats
- The LSL compiler will not optimize the condition, it will not hoisting this function out of the condition. Unless strictly necessary all work with strings, keys and lists should be manually hoisted and performed outside of the loop. See Best Practices for more details.
Examples
<lsl>//Basic usage default {
state_entry()
{
list l = ["one", "two", "three"];
integer i = llGetListLength(l);
llOwnerSay("there are " + (string)i + " entries in the list");
}
}</lsl>
Notes
Best Practices
When using list length to help you loop through a list, it is better to determine the length first, then start your loop:
<lsl> integer i = 0; integer length = llGetListLength(mylist); for (; i < length; ++i) {
llSay(0, llList2String(mylist, i));
} </lsl>
The following example is to illustrate what not to do, it calculates the length in the "for" loop and is inefficient because the length gets recalculated at each pass through the loop. This should only ever be done if the list is in fact changing (in length) with each iteration of the loop[1].
<lsl> integer i; for (i = 0; i < llGetListLength(mylist); ++i) {
llSay(0, llList2String(mylist, i));
} </lsl>
Optimizations
A faster and lighter (in bytecode) way to determine the length of a list is to do a not-equals compare with a null list. This works because the list not-equals compare returns the difference between the lengths, meaning that it returns the same result as llGetListLength(), minus the overhead in bytecode, and performance penalty of calling a non-native function. Note: This optimization only is much less beneficial in Mono. Mono's llGetListLength function is almost twice as fast.
<lsl>
list in;
integer len_in = llGetListLength(in);
integer flen_in = (in != []);
//flen_in and len_in will be the same
integer neg_len_in = -llGetListLength(in); integer fneg_len_in = ([] != in); //fneg_len_in and neg_len_in will be the same </lsl>
See Also
Functions
| • | llListStatistics | – | LIST_STAT_NUM_COUNT – Returns the number of integers and floats in the list | |
| • | llStringLength | – | Returns the number of characters in a string. |
Deep Notes
Source
'linden\indra\lscript\lscript_library\lscript_alloc.cpp' @ lsa_cmp_lists
'linden\indra\lscript\lscript_execute\lscript_execute.cpp' @ list_list_operation
Footnotes
- ^ Recalculating the length is slow because the VM duplicates the entire list (including the values) when it is pushed on the stack (so it can be popped off the stack when the length is calculated).
Signature |
|---|
function integer llGetListLength( list src ); |