16. The statement GOTO

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The statement GOTO is the most famous statement of all BASIC languages. Many program theorists say that you should not ever use GOTO and they may be right. Even so, the statement GOTO is part of most programming languages and ScriptBasic is no exception.

Using the statement GOTO you can alter the execution order of the statements. GOTO statements use labels and the labels can identify program lines. The form of a GOTO statement is

GOTO label

where the label is a label, which is defined somewhere in a program.

Labels should stand on the start of a line preceding a command. There are two different label types in ScriptBasic. One is the conventional BASIC style label, a decimal number. Any line can contain an unsigned integer as a label at the start of the line. For example the following code is perfectly legal in ScriptBasic:

10 print "Hello word!!"
20 REM
30 print "This is line 30"

These types of line labels are available in ScriptBasic for compatibility reasons, and there is no need to label each line in ScriptBasic. The numeric labels can be present in any order and need not increase. This is not compatible with old BASIC languages, but there is little need for that. In those days these numbers were needed to help the simple built-in editor to sort lines and to allow the programmer to insert in new lines. (Did you program those days when the ZX80, ZX Sinclair Spectrum, Atari 800XL, Commodore, Enterprise computers were on top?) If you use these numeric labels the lines will be executed in the order as they appear in the source file and not necessarily in increasing label order.

The type of label, which is more modern and lets the programmer to write more readable and as such easy to maintain programs are the alphanumeric labels. These labels follow the syntax of any identifier of ScriptBasic, stand on the start of the line and are followed by a colon. Both numeric labels and alphanumeric labels can be used in GOTO statements.

10 print "hah" line input a if a = "n\n" then goto finish end if goto 10

finish: REM this is the end of the program

This program loops printing three characters and waiting for input after each print so long as long the user types a single n character. When the user types the n character the program executes the statement GOTO to jump onto the label finish.

The labels are subject to name space modifications as any other variable or user defined function names as described in the section Name spaces.

All GOTO instructions should reference a local label. This means that you can not jump out or jump into a function or subroutine code. You are free to reuse label names once in each function or subroutine. In other words if you use a label in a function you can use the same name as a label without worry in another function or subroutine as well as you can use it in global program code outside of all functions and subroutines once. All these labels are different ones although all may have the same name.

All the labels are local to the subroutine or function that the label is defined in. This is implemented in ScriptBasic via name decoration. If you declare a label named MyLabel on a line ScriptBasic automatically converts it to module::MyLabel'function, where module is the name of the actual module (this is main if no module is defined); function is the name of the actual function or subroutine without the current module name. If the label is in the global program code outside of all functions and subroutines the function name is empty string. In this case the label has the decorated form module::MyLabel'. The modules can reference each-others labels using explicit module name notation. However there is no possibility to reference any label, which is not in the same function or subroutine.

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