Lua Get Key Value From Table In C

This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
The fourth edition targets Lua 5.3 and is available at Amazon and other bookstores.
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Programming in Lua
Part I. The LanguageChapter 2. Types and Values

Line 1 - Value of c is 31 Line 2 - Value of c is 11 Line 3 - Value of c is 210 Line 4 - Value of c is 2.1 Line 5 - Value of c is 1 Line 6 - Value of c is 441 Line 7 - Value of c is -21 luaoperators.htm. A Lua table is a group of keyvalue pairs. Think hashes in Perl and Ruby or Dictionaries in Python. But Lua is built from the ground up to be fast and efficient with its tables. Lookup is quick. So unlike Perl, Python and Ruby, with Lua it's efficient to use a table as an array.

2.5 – Tables

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The table type implements associative arrays.An associative array is an arraythat can be indexed not only with numbers,but also with strings or any other value of the language,except nil.Moreover, tables have no fixed size;you can add as many elements as you want to a table dynamically.Tables are the main (in fact, the only) data structuring mechanism in Lua,and a powerful one.We use tables to represent ordinary arrays,symbol tables, sets, records, queues, and other data structures,in a simple, uniform, and efficient way.Lua uses tables to represent packages as well.When we write,we mean 'the read entry from the io package'.For Lua, that means'index the table io using the string 'read' as the key'.

Tables in Lua are neither values nor variables;they are objects.If you are familiar with arrays in Java or Scheme,then you have a fair idea of what we mean.However, if your idea of an array comes from C or Pascal,you have to open your mind a bit.You may think of a table as a dynamically allocated object;your program only manipulates references (or pointers) to them.There are no hidden copies or creation of new tablesbehind the scenes.Moreover, you do not have to declare a table in Lua;in fact, there is no way to declare one.You create tables by means of a constructor expression,which in its simplest form is written as {}:A table is always anonymous.There is no fixed relationship between a variable that holds a tableand the table itself:When a program has no references to a table left,Lua memory management will eventually delete the tableand reuse its memory.

Each table may store values with different types of indicesand it grows as it needs to accommodate new entries:Notice the last line:Like global variables,table fields evaluate to nil if they are not initialized.Also like global variables,you can assign nil to a table field to delete it.That is not a coincidence:Lua stores global variables in ordinary tables.More about this subject in Chapter 14.

To represent records, you use the field name as an index.Lua supports this representation byproviding as syntactic sugar for a['name'].So, we could write the last lines of the previous examplein a cleanlier manner asFor Lua, the two forms are equivalentand can be intermixed freely;but for a human reader,each form may signal a different intention.

A common mistake for beginners is to confuse a.x with a[x].The first form represents a['x'], that is, a table indexed bythe string 'x'.The second form is a table indexed by the value of the variable x.See the difference:

Clash of clans pc download windows 7 32 bit. To represent a conventional array,you simply use a table with integer keys.There is no way to declare its size;you just initialize the elements you need:When you iterate over the elements of the array,the first non-initialized index will result in nil;you can use this value as a sentinel to represent the end of the array.For instance, you could print the lines read in the last examplewith the following code:The basic Lua library provides ipairs,a handy function that allows you to iterate over the elements of an array,following the convention that the array ends at its first nil element.

Since you can index a table with any value,you can start the indices of an array withany number that pleases you.However, it is customary in Lua to start arrays with one(and not with zero, as in C)and the standard libraries stick to this convention.

Because we can index a table with any type,when indexing a tablewe have the same subtleties that arise in equality.Although we can index a table both with thenumber 0 and with the string '0',these two values are different (according to equality)and therefore denote different positions in a table.By the same token, the strings '+1', '01',and '1' all denote different positions.When in doubt about the actual types of your indices,use an explicit conversion to be sure:You can introduce subtle bugs in your programif you do not pay attention to this point.

Copyright © 2003–2004 Roberto Ierusalimschy. All rights reserved.
Next:5.3 Manipulating Lua ObjectsUp:5 The Application Program Previous:Get5.1 Executing Lua Code

5.2 Converting Values between C and Lua

Lua Get Keys From Table

Because Lua has no static type system,all values passed between Lua and C have type

Lua Get Key Value From Table In Code

lua_Object,which works like an abstract type in C that can hold any Lua value.

Lua has automatic memory management, and garbage collection.Because of that, a lua_Object has a limited scope,and is only valid inside the block where it was created.A C function called from Lua is a block, and its parameters are validonly until its end.A good programming practice is to convert Lua objects to C valuesas soon as they are available,and never to store lua_Objects in global variables.

Lua table get value by key

When C code calls Lua repeatedly, as in a loop,objects returned by there calls may accumulate,creating a memory problem.To avoid this,nested blocks can be defined with the functions:After the end of the block,all lua_Object's created inside it are released.

To check the type of a lua_Object,the following function is available:plus the following macros:All macros return 1 if the object has the given type,and 0 otherwise.

The function lua_type can be used to distinguish betweendifferent kinds of user data; see below.

To translate a value from type lua_Object to a specific C type,the programmer can use:lua_getnumber converts a lua_Object to a float.This lua_Object must be a number or a string convertible to number(see Section 4.2); otherwise, the function returns 0.

lua_getstring converts a lua_Object to a string (char *).This lua_Object must be a string or a number;otherwise, the function returns 0 (the null pointer).This function does not create a new string, but returns a pointer toa string inside the Lua environment.Because Lua has garbage collection, there is no guarantee that suchpointer will be valid after the block ends.

lua_getcfunction converts a lua_Object to a C function.This lua_Object must have type CFunction;otherwise, the function returns 0 (the null pointer).The type lua_CFunction is explained in Section 5.5.

lua_getuserdata converts a lua_Object to void*.This lua_Object must have type userdata;otherwise, the function returns 0 (the null pointer).

The reverse process, that is, passing a specific C value to Lua,is done by using the following functions:plus the macro:All of them receive a C value,convert it to a lua_Object,and leave their results on the top of the Lua stack,where it can be assigned to a variable,passed as paramenter to a Lua function, etc (see below). lua_pushliteral is like lua_pushstring,but also puts the string in the Lua literal table.This avoids the string to be garbage collected,and therefore has a better overall performance.As a rule, when the string to be pushed is a literal,lua_pushliteral should be used.

User data can have different tags,whose semantics are defined by the host program.Any positive integer can be used to tag a user data.When a user data is retrieved,the function lua_type can be used to get its tag.

Lua Get Key Value From Table In C Programming

To complete the set,the value nil or a lua_Object can also be pushed onto the stack,with:

Lua Get Key Value From Table In Cursor

Next:5.3 Manipulating Lua ObjectsUp:5 The Application Program Previous:5.1 Executing Lua Code