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Compound data type: Difference between revisions
→{{header|Lua}}: add solution in Lua.
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Access is via normal list operations like FIRST and BUTFIRST (BF). X is FIRST point, Y is LAST point. For example, a simple drawing program which exits if mouse X is negative:
<lang logo>until [(first mousepos) < 0] [ifelse button? [pendown] [penup] setpos mousepos]</lang>
=={{header|Lua}}==
==== Simple Table ====
Lua could use a simple table to store a compound data type Point(x, y):
<lang lua>
a = {x = 1; y = 2}
b = {x = 3; y = 4}
c = {
x = a.x + b.x;
y = a.y + b.y
}
print(a.x, a.y) --> 1 2
print(c.x, c.y) --> 4 6
</lang>
==== Prototype Object ====
Furthermore, Lua could create a prototype object (OOP class emulation) to represent a compound data type Point(x, y) as the following:
<lang lua>
cPoint = {} -- metatable (behaviour table)
function newPoint(x, y) -- constructor
local pointPrototype = {} -- prototype declaration
function pointPrototype:getX() return x end -- public method
function pointPrototype:getY() return y end -- public method
function pointPrototype:getXY() return x, y end -- public method
function pointPrototype:type() return "point" end -- public method
return setmetatable(pointPrototype, cPoint) -- set behaviour and return the pointPrototype
end--newPoint
</lang>
In the above example, the methods are declared inside the constructor so that they could access the closured values <code>x</code> and <code>y</code> (see usage example). The <code>pointPrototype:type</code> method could be used to extend the original <code>type</code> function available in Lua:
<lang lua>
local oldtype = type; -- store original type function
function type(v)
local vType = oldtype(v)
if (vType=="table" and v.type) then
return v:type() -- bypass original type function if possible
else
return vType
end--if vType=="table"
end--type
</lang>
The usage of metatable <code>cPoint</code> which stores the behavior of the <code>pointPrototype</code> enables additional behaviour to be added to the data type, such as:
<lang lua>
function cPoint.__add(op1, op2) -- add the x and y components
if type(op1)=="point" and type(op2)=="point" then
return newPoint(
op1:getX()+op2:getX(),
op1:getY()+op2:getY())
end--if type(op1)
end--cPoint.__add
function cPoint.__sub(op1, op2) -- subtract the x and y components
if (type(op1)=="point" and type(op2)=="point") then
return newPoint(
op1:getX()-op2:getX(),
op1:getY()-op2:getY())
end--if type(op1)
end--cPoint.__sub
</lang>
Usage example:
<lang lua>
a = newPoint(1, 2)
b = newPoint(3, 4)
c = a + b -- using __add behaviour
print(a:getXY()) --> 1 2
print(type(a)) --> point
print(c:getXY()) --> 4 6
print((a-b):getXY()) --> -2 -2 -- using __sub behaviour
</lang>
=={{header|MAXScript}}==
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