Para algunas funciones puede ser necesario devolver más de un valor. Esto se puede lograr al devolver un vector de valores, pero muchas veces resulta conveniente pasar una referencia a una variable. Puede pasar una referencia a una variable a una función, y la función fijará el valor, eliminando una referencia. Aunque éste es el principal uso de las referencias, no es el único.

When using functions that return values through references in the argument list, just pass the variable name with an ampersand. For example the following code will compute an eigenvalue of a matrix A with initial eigenvector guess x, and store the computed eigenvector into the variable named v:

RayleighQuotientIteration (A,x,0.001,100,&v)

La manera como funcionan las referencias y la sintaxis que utilizan son similares al lenguaje C. El operador & hace referencia a una variable y * la elimina. Ambos pueden aplicarse sólo a un identificador, por lo que **a no es una expresión legal en GEL.

References are best explained by an example:

a=1;
b=&a;
*b=2;

now a contains 2. You can also reference functions:

function f(x) = x+1;
t=&f;
*t(3)

gives us 4.

The first time a variable is referenced, it is created in the current context unless it exists. So if we make a function that sets the variable to 2, such as function f(k) = (*k=2), then the call f(&n) will declare a new variable n then the function f will run and set it to 2. So there is no need to write something like n=0;f(&n). A new variable in the current context is created even if a variable exists in a higher context, such as in the global context. So for example after

n=3;
function f(k) = (*k=2);
function g() = (f(&n);print(n));
g();

the variable n is still 3 and not 2. That is because inside g, when n is referenced, it is a new variable in the context of g. The function f then sets this n to 2, and then 2 is printed, but the n in the global context is not touched. So we can use references to get values out of functions and we do not need to explicitly declare them, nor do we have to worry about messing up global variables that the user might be using for something else.