6.7 Variable initializers

A variable can be assigned a value when it is declared, as in int x=3; where the variable x is assigned the value 3. As well as literal constants such as 3, arbitary expressions can be used as initializers, as in real x=2*sin(pi/2);.

A variable is not added to the namespace until after the initializer is evaluated, so for example, in

int x=2;
int x=5*x;

the x in the initializer on the second line refers to the variable x declared on the first line. The second line, then, declares a variable x shadowing the original x and initializes it to the value 10.

Variables of most types can be declared without an explicit initializer and they will be initialized by the default initializer of that type:

• Variables of the numeric types int, real, and pair are all initialized to zero; variables of type triple are initialized to O=(0,0,0).
• boolean variables are initialized to false.
• string variables are initialized to the empty string.
• transform variables are initialized to the identity transformation.
• path and guide variables are initialized to nullpath.
• pen variables are initialized to the default pen.
• frame and picture variables are initialized to empty frames and pictures, respectively.
• file variables are initialized to null.

The default initializers for user-defined array, structure, and function types are explained in their respective sections. Some types, such as code, do not have default initializers. When a variable of such a type is introduced, the user must initialize it by explicitly giving it a value.

The default initializer for any type T can be redeclared by defining the function T operator init(). For instance, int variables are usually initialized to zero, but in

int operator init() {
  return 3;
}
int y;

the variable y is initialized to 3. This example was given for illustrative purposes; redeclaring the initializers of built-in types is not recommended. Typically, operator init is used to define sensible defaults for user-defined types.

The special type var may be used to infer the type of a variable from its initializer. If the initializer is an expression of a unique type, then the variable will be defined with that type. For instance,

var x=5;
var y=4.3;
var reddash=red+dashed;

is equivalent to

int x=5;
real y=4.3;
pen reddash=red+dashed;

var may also be used with the extended for loop syntax.

int[] a = {1,2,3};
for (var x : a)
  write(x);

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