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1.2.2 Defining Procedures

Any Yorick function may be invoked as a procedure; the return value is simply discarded. Calling damped_wave as a procedure would be pointless, since it has no side effects. The converse case is a function to be called only for its side effects. Such a function need not have an explicit return statement:

func q_out(Q, file_name)
   file = create(file_name);
   write, file, "Q = "+pr1(Q);
   write, file, "   theta       amplitude";
   write, file, theta, damped_wave(theta,Q);

The pr1 function (for "print one value") returns a string representation of its numeric argument, and the + operator between two strings concatenates them.

You would invoke q_out with the line:

q_out, 3, "q.out"

Besides lacking an explicit return statement, the q_out function has two other peculiarities worth mentioning:

First, the variable theta is never defined. But neither are the functions create and write. Any symbol not defined within a function is external to the function. As already noted, parameters and variables defined within the function are local to the function. Here, theta is the 200 element array of phase angles defined before q_out was called. You can change theta (both its dimensions and its values) before the next call to q_out; that is, an external reference may change between calls to a function that uses it.

Second, file is never explicitly closed. When a function returns, its local variables (such as file) disappear; when a file object disappears, the associated file automatically closes.