1.2.4 Loops

Most loops in Yorick programs are implicit; remember that operations between array arguments produce array results. Whenever you write a Yorick program, you should be suspicious of all explicit loops. Always ask yourself whether a clever use of array syntax could have avoided the loop.

To illustrate an appropriate Yorick loop, let's revise q_out to write several values of Q in a single call. Incidentally, this sort of incremental revision of a function is very common in Yorick program development. As you use a function, you notice that the surrounding code is often the same, suggesting a savings if it were incorporated into the function. Again, a careful job leaves all of the previous behavior of q_out intact:

func q_out(Q, file) { if (structof(file)==string) file = create(file); n = numberof(Q); for (i=1 ; i<=n ; ++i) { write, file, "Q = "+pr1(Q(i)); write, file, " theta amplitude"; write, file, theta, damped_wave(theta,Q(i)); } return file; }

Two new features here are the numberof function, which returns the length of the array Q, and the array indexing syntax Q(i), which extracts the i-th element of the array Q. If Q is scalar (as it had to be before this latest revision), numberof returns 1, and Q(1) is the same as Q, so scalar Q works as before.

But the most important new feature is the for loop. It says to initialize i to 1, then, as long as i remains less than or equal to n, to execute the loop body, which is a compound of three write statements. Finally, after each pass, the increment expression ++i is executed before the i<=n condition is evaluated. ++i is equivalent to i=i+1; --i means i=i-1.

A loop body may also be a single statement, in which case the curly braces are unecessary. For example, the following lines will write the Q=3, Q=2, and Q=1 curves to the file `q.out', then plot them:

q_list = [3,2,1] q_out, q_list, "q.out" fma; for(i=1;i<=3;++i) plg, damped_wave(theta,q_list(i)), theta

A for loop with a plg body is the easiest way to overlay a series of curves. After the three simple statement types, for statements see the most frequent direct use from the keyboard.

1.2.4.1 The while and do while statements
1.2.4.2 The for statement
1.2.4.3 Using break, continue, and goto		How to break, continue, and goto from a loop body.