The Goto command is used together with the Lbl command to jump (or branch) to another place in a program. When the calculator executes a Goto command, it stores the label name in memory, and then searches from the beginning of the program for the Lbl command with the supplied name. If it finds it, it continues running the program from that point; otherwise, if the label does not exist, it throws an ERR: LABEL error.
Label names can be either one or two characters long, and the only characters you're allowed to use are letters (including θ) and numbers 0 to 9; this means 37+37*37=1406 possible combinations. Of course, you should use all of the single character names first, before using the two character names. While you can technically have the same label name multiple times in a program, it is rather pointless since the calculator always goes to the first occurrence of the label.
You can position a Lbl command one or more lines before a Goto command to create a kind of loop structure. However, you have to provide the break-out code, since it isn't built-in. An If conditional is easiest, but if there is no code that ends the branching, then program execution will continue indefinitely, until you manually exit it (by pressing the ON key).
:Lbl A :... :If <exit condition> :Goto A // this line is skipped
Although the Goto command may seem like a good alternative to loops, it should be avoided whenever possible, which is especially important when you are first planning a program. This is because it has several serious drawbacks associated with it:
- It is quite slow, and gets slower the further the Lbl is in your program.
- It makes reading code (your own, or someone else's) much more confusing.
- In most cases, If, For(, While, or Repeat can be used instead, saving space and improving speed.
- Using a Goto to exit any block of code requiring an End command causes a memory leak, which will not be usable until the program finishes running or executes a Return command, and which will slow down your program down. See below for ways to fix this.
The Goto command isn't all bad, however, and is actually useful when a loop isn't practical and when something only happens once or twice (see below for examples). Just remember that you should never use Goto to repeat a block of code several times. Use For(, Repeat, or While instead.
Fixing Memory Leaks
One of the simplest memory leaks that occurs is using branching to exit out of a loop when a certain condition of an If conditional is true. If the loop is an infinite loop (i.e., Repeat 0 or While 1), you should take the condition from the If conditional and place it as the condition of the loop. This allows you to remove the branching, since it is now unnecessary.
:Repeat 0 :getKey→B :If B:Goto A :End:Lbl A Make Loop Condition :Repeat B :getKey→B :End
Of course, the only reason that this memory leak fix is possible is because of the If conditional (since the If conditional doesn't need a closing End command). When dealing with a complex If conditional, you will have to rework the conditionals so the branching has its own If conditional. Depending on how many commands there are in the conditionals, you might be able to just use an If conditional or you might need to use an If-Then conditional.
:If B:Then :Disp "Hello :Goto A :End Separate Into Conditionals :If B:Disp "Hello :If B:Goto A
This memory leak fix will work most of the time, but it isn't applicable when one of the values of the variables in the condition is changed by one of the commands inside the condition. The way to get around this is by using another variable for the If conditional that the branching uses. You initialize the variable to zero, assign the variable whatever value you want in the conditional, and then check to see if the variable is equal to that value in the branching conditional.
:If A=1:Then :3→A:4→B :Goto A :End Use Another Variable :Delvar CIf A=1:Then :3→A:4→B:π→C :End :If C=π :Goto A
If your program requires cleanup after it finishes, but it can exit from several different places, use Goto and place a Lbl at that point. This saves memory over repeating the cleanup code every time you exit. The usual considerations about Goto don't apply here: since you're exiting the program, all memory leaks will be gone anyway, and speed isn't much of an issue for something that only gets done once.
The code looks something like this:
:If K=45:Goto Q //user pressed CLEAR :... :If L:Goto Q // game over :... :Lbl Q :DelVar L1ClrHome
A common situation in programs is when a decision has to be made about where the program execution should go next. The obvious approach would be to use the value of a variable as the label name (i.e., something like Goto A, with A being a variable), but that doesn't work because the calculator doesn't interpret the label as a variable. So, the next best approach is to use If conditionals with the different values of the variable:
:If not(A:Goto 0 :If A=1:Goto 1 :If A=2:Goto 2
Another possible use for Goto is in program protection to break a program with an error without letting the user see where it happened. If the label that you want to Goto doesn't exist, you'll get a ERR: LABEL error, which doesn't provide a 2:Goto option. So, all you have to do is Goto a label that you know doesn't exist.
- ERR:INVALID occurs if this statement is used outside a program.
- ERR:LABEL is thrown if the corresponding label doesn't exist.