gosub#

Purpose#

Causes a branch to a subroutine.

Note

This is an advanced function that gives extra flexibility for sophisticated users in some circumstances.

In most cases, it is prefereable to create a procedure (proc).

Format#

gosub label;
...
label:
return;

Examples#

In the program below the name mysub is a label. When the gosub statement is executed, the program will jump to the label mysub and continue executing from there. When the return statement is executed, the program will resume executing at the statement following the gosub.

x = rndn(3, 3);
z = 0;
gosub mysub;
print z;
end;

/* ------ Subroutines Follow ------ */

mysub:
 z = inv(x);
return;

Parameters can be passed to subroutines in the following way (line numbers are added for clarity):

1. gosub mysub(x, y);
2. pop j; /* b will be in j */
3. pop k; /* a will be in k */
4. t = j*k;
5. print t;
6. end;
7.
8. /* ---- Subroutines Follow ----- */
9.
10. mysub:
11. pop b; /* y will be in b */
12. pop a; /* x will be in a */
13.
14. a = inv(b)*b+a;
15. b = a'b;
16. return(a, b);

In the above example, when the gosub statement is executed, the following sequence of events results (line numbers are included for clarity):

1. x and y are pushed on the stack and the program branches to the label mysub in line 10.

11. the second argument that was pushed, y, is pop’ped into b.

12. the first argument that was pushed, x, is pop’ped into a.

14. inv(b)*b+a is assigned to a.

15. a'b is assigned to b.

16. a and b are pushed on the stack and the program branches to the statement following the gosub, which is line 2.

2. the second argument that was pushed, b, is pop’ped into j.

3. the first argument that was pushed, a, is pop’ped into k.

4. j*k is assigned to t.

5. t is printed.

6. the program is terminated with the end statement. Matrices are pushed on a last-in/first-out stack in the gosub and return() statements. They must be pop’ped off in the reverse order. No intervening statements are allowed between the label and the pop or the gosub and the pop. Only one matrix may be pop’ped per pop statement.

Remarks#

For multi-line recursive user-defined functions, see Procedures and Keywords.

When a gosub statement is encountered, the program will branch to the label and begin executing from there. When a return() statement is encountered, the program will resume executing at the statement following the gosub statement. Labels are 1-32 characters long and are followed by a colon. The characters can be A-Z or 0-9, but they must begin with an alphabetic character. Uppercase or lowercase is allowed.

It is possible to pass parameters to subroutines and receive parameters from them when they return. See the second example, following.

The only legal way to enter a subroutine is with a gosub statement.

If your subroutines are at the end of your program, you should have an end statement before the first one to prevent the program from running into a subroutine without using a gosub. This will result in a Return without gosub error message.

The variables used in subroutines are not local to the subroutine and can be accessed from other places in your program. (See Procedures and Keywords.)

See also

Functions goto, proc, pop, return()

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